Author Topic: surging hesitating engine  (Read 16015 times)

Markus

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surging hesitating engine
« on: March 27, 2008, 00:20:48 »
Well I am at loss on my 71 280sl, with vacuum retard distributor.  I just had the speed switch and speed relay overhauled at Beckmann Tech. and I still have the same problem.  With a volt meter connected at the vacuum switch over valve, intermittent voltage is recorded, when driving, and becomes worse with higher engine rpms, as the mechanical distributor advance is canceled when the voltage drops to zero, and then comes back on up to 12.3 volts. This results in the car surging and hesitating, when the vacuum retard comes on and off intermittently. I also switched the " tin can working relay" to no avail. Has anyone come across this before?

jeffc280sl

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Re: surging hesitating engine
« Reply #1 on: March 27, 2008, 09:25:44 »
Hi Markus,

I have some experience with these boxes.  Mostly with the manual transmission SL's.  Is your car an auto or manual?  Do you have the Haynes Manual or any emissions documentation on the SL from MB?  I have quite a bit and will be happy to share.  Assuming you have an auto here is what the setup looks like.  Here is a write up I prepared for the manual transmission version.  There are really 4 subsystems in the emission control set up.  The auto is similar. Have you looked at the fuel cut off subsystem as causing the problem?


1970/1971 Emissions Control Systems
for 280SL w/Manual Transmission


1)Fuel Injector Shut off
    a)   Activation of the fuel shut off solenoid is controlled by    a ground signal from switches located as follows:
      i)Clutch Pedal released
     ii)Transmission in 3rd gear
    iii)Transmission in 4th gear   
     iv)Throttle at rest ( Throttle Switch is closed at rest.  Throttle switch is open when gas pedal is pushed in)
Notes:  Ground Signal from switch i), ii) and iii) is provided via the closed throttle switch to pin #1 of the 8 pin relay.  If any of the above switches are open the circuit is deactivated.
     b)   +12V is provided to the fuel shut off solenoid by pin #2 of the 8pin relay

2)4 pin Speed Relay Ignition Timing
     a)   Vacuum released by the two way valve will advance the distributor setting as follows:
       i)At engine speeds above 2400 rpms the two way valve is energized and vacuum advances the distributor setting.  This is accomplished via signaling between the terminal 1 of the ignition coil, the 4 pin relay and the two way valve
         (1)Pin#4 reads an rpm signal from the coil and above 2400   rpms  the speed relay connects power from pin #2 to pin#1 energizing the two way valve
      ii)At engine speeds below 2200 rpms  power to the two way valve is removed  and   distributor setting is retarded.  This is accomplished via signaling between the terminal 1 of the ignition coil, the 4 pin relay and the two way valve
         (1) Pin#4 reads an rpm signal from the coil and below 2200  rpms  the speed relay removes  the connection of power from pin #2 to pin#1 removing power from  the two way valve

3)17C degree switch and the 8 pin relay Ignition Timing
     a)   The 17C /62.3F degree switch is located to the rear of the cylinder head on the drivers side.  This switch is normally closed an it provides a ground signal to pin #6 of the 8 pin relay located near the battery.  At temperatures above 17C the switch opens.
      i)When this ground signal is present (at 17C or below) this switch will cause the 8 pin relay to provide a 12V to pin #8 and the two way valve will be energized advancing timing.  
     ii)When the temperature of the switch exceeds 17C the switch will open and ground will not be provided to pin #6 of the 8 pin relay.  Without a ground signal on pin #6 the 8 pin relay will not energize the two way valve


4)100C degree switch and the 8 pin relay Ignition Timing
      a)The 100C /212F degree switch is located on top of the thermostat cover.  This switch is normally open.  At temperatures above 100C the switch closes to ground and provides a ground signal to pin #5 of the 8 pin relay.
       i)When this ground signal is present (at 100C or above) this switch will cause the 8 pin relay to provide a 12V to pin #8 and the two way valve will be energized advancing timing.  
      ii)When the temperature of the switch is below 100C  the switch will open and ground will not be provided to pin #5 of the 8 pin relay.  Without a ground signal on pin #5 the 8 pin relay will not energize the two way valve.



Notes

Requirements for retarded ignition
i)   Engine temperature above 17C
ii)   Engine temperature below 100C
iii)   Engine speed below 2200 RPM
iv)   Two way valve not energized

Requirements for advanced ignition
i)    Engine temperature below 17C
ii)   Engine temperature above 100C
iii)  Engine speed above 2400 RPM
iv)  Two way valve not energized




Download Attachment: Emissions.jpg
49.75 KB

Jeff C.
1970 280SL 4-speed

Markus

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Re: surging hesitating engine
« Reply #2 on: March 27, 2008, 10:36:06 »
Thanks Jeff, you seem to be the one with distilled wisdom on these things. The car is an automatic, I have both the BBB, and the Haynes manual, and I have been looking at the same diagrams. Interestingly you mentioned that rpm is sensed from the coil at pin #4. I will check that next, working backwards from the 2 way switchover valve (OK), the working relay(OK), speed switches(just overhauled) Perhaps the rpm info coming from the coil is onerous, or the small black ground strap not making good contact. The automatic has a 6 pin speed switch, which also gets info at pin #5  ON@1350  OFF@1150. This was never connected on my car,(and no wires exist, so I wouldn't even know where to connect them, without someone elses car as an example) although, the oil pressure switches are on the transmission. Don't know if that could be a contributing function, but it should not impede the 2400/2200 side of things, which works, albeit intermittenly.

jeffc280sl

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Re: surging hesitating engine
« Reply #3 on: March 27, 2008, 11:28:15 »
Markus,

Here is what I think the wiring is for the 6 pin relay.

Download Attachment: 6 pin relay.jpg
35.55 KB

Jeff C.
1970 280SL 4-speed

jeffc280sl

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Re: surging hesitating engine
« Reply #4 on: March 27, 2008, 11:52:48 »
Markus,

Here is how I see the connectivity of the relays and two way valve

6 pin relay

#1 Controls the two way valve  (connected to pin #4 and #7 of the 8 pin relay)
#2 Power (connected to pin #8 of the 8 pin relay)
#3 Ground (connected to pin #3 of the 8 pin relay)
#4 RPM input from coil
#5 Controls the fuel shut off solenoid (connected to pin #1 of the current relay(#86))
#6 Starter Lock out and throttle control valve

2 way valve

1) Connection to Pin #1 of the 6 pin relay
2) Ground connection

8 pin relay

#1 Transmission Pressure switches
#2 Connected to pin #2 of the current relay (#30)
#3 Ground (connected to pin #3 of the 6 pin relay)
#4 Connected to pin #7 of the 8 pin relay
#5 100C switch
#6 17C switch
#7 Connected to pin #1 of the 6 pin relay and pin #7 of the 8 pin relay
#8 Connected to fuse #8 and pin #2 of the 6 pin relay

Current relay

#1 Connected to pin #5 of the 6 pin relay (#86)
#2 Connected to pin #2 of the 8 pin relay (#30)
#3 Ground (#85)
#4 Connected to the fuel shut off solenoid (#87)


Jeff C.
1970 280SL 4-speed
« Last Edit: March 27, 2008, 11:54:53 by jeffc280sl »

Markus

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Re: surging hesitating engine
« Reply #5 on: March 27, 2008, 16:14:32 »
Bonus on the info, I will begin checking each pin when I get back, in the mean time, Bob at Beckmann wanted the switch and relay back to double check them, since they eventually did find an intermittent problem with the rpm switch (solder joint cracked on the board)which originally checked out fine, maybe there's more.  Will keep you posted, thanks a bunch for the break down of schematic, it will help me test each component function.

jeffc280sl

  • Guest
Re: surging hesitating engine
« Reply #6 on: March 27, 2008, 17:23:54 »
Markus,

Here is some more of my thinking about the emissions system.  We know the two way valve is controlled by the 6 pin rpm relay in the automatics and the 4 pin rpm relay in the manual transmission 1970/71 SLs.

We should remember that the relay simply opens and closes the valve.  It has nothing to do with vacuum pressure.  Whatever vacuum is present from that created by the intake manifold is switched on or off.  This is the long way around to my point.  When the engine is under load, accelerating, the air intake throttle valve is wide open and I think there is no or very little vacuum pressure available for the two way valve to effect the distributor timing.  My theory is that under load this emissions subsystem does not influence timing.  Under load and above 2400 rpms the Rpm switch will open the two way valve but there is no vacuum pressure so it has no effect on timing.  It does however have an effect on timing when the engine is not under load.  The 6 pin RPM relay and the fuel cut off function on the 8 pin relay work together when the engine is not under load and when you are coasting.  While coasting at any RPM the intake manifold creates adequate vacuum pressure.  When coasting above 2400 Rpms the two way valve influences the distributor timing which burns fuel more effectively and reduce emissions.  When you put the engine back under load, even though the two way valve is open, there is little or no vacuum available from the intake manifold to effect the distributor timing.

What I'm saying is the driver should not notice any performance impact of the on/off, off/on functioning of the speed relay and two way valve.  

The same theory applies to the fuel shut off solenoid operation. There is a switch on the intake manifold that shuts down the fuel shut off function when the throttle valve is open (accelerating).  When the throttle valve is closed the transmission switches sense a coasting condition and shuts off the fuel to the FIP to improve emissions by reducing unspent fuel exiting the exhaust system.

I hope this write up is clear.  If not ask questions and I'll gladly improve the explanation.

Jeff C.
1970 280SL 4-speed
« Last Edit: March 27, 2008, 17:25:17 by jeffc280sl »

Markus

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Re: surging hesitating engine
« Reply #7 on: March 28, 2008, 22:17:55 »
I agree with everything you mentioned Jeff, and the problem is most noticeable at maintaining higher rpms. (ie 3rd gear and 4000rpm under partial load produces unmistakeable surging, that was much less noticeable in 4th gear and 2800rpm.)  It is equally noticeable at highway speeds, say 65mph and 3800rpm under partial load maintaining speed. When the surging starts, mechanical advance drops from about 35 BTDC to a max of 20 BTDC regardless of a further increase in rpm. Hence the higher the rpm under partial load, the more noticeable the surging. Interestingly, my IP only has one fuel solenoid, I believe it is only for starting, running, and off, which is why perhaps the transmission coast down, fuel cut off wires are not connected to anything. Unless they use the one solenoid for all functions. When I decelerate at lower rpms, there is alot of popping and burbling in the exhaust(unburnt fuel) which becomes more pronounced with an increase in idle mixture at the back of the IP.

hands_aus

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Re: surging hesitating engine
« Reply #8 on: March 29, 2008, 04:24:40 »
Markus,
Have you checked that the distributor advance feature is actually working properly?
It has been discussed online here. If the fly weights of the distributor are not oiled regularly via the shaft wick, the small springs that are used to reset those fly weights could become stretched or even weak.
This would cause all sorts of problems with the way the engine works.
Dr Dan is an expert on the distributors.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto
Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL #114, auto, ps , 717,717
best of the best

jeffc280sl

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Re: surging hesitating engine
« Reply #9 on: March 29, 2008, 07:25:13 »
Markus,

The newer FIP only have the fuel shut off solenoid.  The wire to my solenoid is disconnected.

Here is a graph of the distributor for our car.



Download Attachment: sladvance.jpg
39.73 KB


Maybe you do have an issue with the vacuum retard or mechanical advance functions.  I would disconnect the fuel shut off solenoid and the two way valve and see how the car runs.

Jeff C.
1970 280SL 4-speed

Markus

  • Guest
Re: surging hesitating engine
« Reply #10 on: March 29, 2008, 09:34:31 »
The distributor was rebuilt by Will over at S&S, I was over at his shop the other day, and he mentioned lets pull it, and test it again just to rule it out. I have run the car without the 2 way valve connected, and the max advance you get is 20BTDC at 3000-5000rpm. Connect the 2way valve (vacuum) and you go from 8ATDC idle (850rpm)to 37BTDC @ 3000rpm with switch over occurring nicely @ 2300rpm. The reason I am focused on the black boxes or the inputs going to them (working current and rpm info)  is as previously mentioned, the vacuum comes on, retarding the timing with a corresponding drop in voltage/current at the 2 way valve, intermittently above 2400rpm. Intermittent problems are frustrating as hell, I wish the damn thing would just fail as is!

jeffc280sl

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Re: surging hesitating engine
« Reply #11 on: March 30, 2008, 08:33:22 »
Markus,

The reason I sent the distributor curve is to see if your distributor is more or less sensative to the vacuum pressure than designed.  If you are comfortable with the distributor operation then it seems like you are on the right track with having your box returned to Beckman for testing.

BTW have you tested the 17C and 100C switches?  I'm interested to learn if the 17C switch operates.  I think it commonly fails and no one knows or can figure out its purpose.

Jeff C.
1970 280SL 4-speed

Markus

  • Guest
Re: surging hesitating engine
« Reply #12 on: March 30, 2008, 22:48:47 »
What a coincidence, I ordered, and just received from Will a new 17C oil temp switch, the 100C switch works (tested) I will also replace the trans. lock out switch on the firewall.  I'd like to change these things one at a time to actually isolate the problem, after I receive the black boxes back from Beckmann.
  The purpose of the 17C switch is (I believe) to cancel below 17C/63F the minute vacuum generated during cranking, (which is retarding the timing) so as to aid in cold weather starts.  The purpose of the 100C switch is to cancel vacuum as well, and advance timing when the engine gets to hot idling in traffic. The vacuum retard cancels, resulting in idle rpm going from 800 to 1000-1200. The theory being advance ignition timing, which results in increase rpm, helps cool the motor which is very close to boiling.

jeffc280sl

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Re: surging hesitating engine
« Reply #13 on: March 31, 2008, 08:47:20 »
Markus,

As you say "The purpose of the 17C switch is (I believe) to cancel below 17C/63F the minute vacuum generated during cranking, (which is retarding the timing) so as to aid in cold weather starts." As or more important then a starting aid is when the two way valve is energized, and closes, the vacuum retard cancels and the idle rpms go from 800 to 1000-1200.  I think this compliments  the fuel enrichment via the FIP when the engine is cold.

I need to remind myself that the two way valve is normally open.  Energizing the two way valve closes the valve and cancels vacuum to the distributor.

Jeff C.
1970 280SL 4-speed

Markus

  • Guest
Re: surging hesitating engine
« Reply #14 on: April 16, 2008, 08:41:48 »
I am happy to report that the surging "seems" to be fixed. After re-installing the switches from Beckmann,(they did recalibrate the speed switch potentiometers again), a new 17C switch, and new transmission lock out switch, the car is now running as advertised. Drove 600 miles, Dallas to San Antonio return, over the weekend with the MB Club, and the car performed magnificently! The car burned 1 qt 10W oil.(engine still breaking in) Any comments on what is considered normal oil consumption in these cars?  Received lots of compliments on the restoration, people just love these 113s.

hauser

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Re: surging hesitating engine
« Reply #15 on: April 19, 2008, 09:10:25 »
I had this very same problem once and then I did a Crane install and it's never missed a beat since.

1969 280sl 5 spd
Gainesville, Fl.

Markus

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Re: surging hesitating engine
« Reply #16 on: April 20, 2008, 13:05:18 »
One interesting observation, now when I start the car on cool mornings it fires almost immediately, and then idles @ 1500rpm for about 8-11 seconds, then drops to 900rpm. This confirms to me that <17C, the 17C switch is canceling vacuum retard and advancing timing to aid in the start, shortly after reaching 17C the switch opens and vacuum is applied to the distributor and the idle drops, due to vacuum being applied and retarding the timing. I suspect the 17C may have been the culprit all along, as the car did not go through the above cycle previously, although the old switch tests out fine?

jeffc280sl

  • Guest
Re: surging hesitating engine
« Reply #17 on: April 20, 2008, 17:40:04 »
Markus,

I think your explanation is very good concerning the 17C switch.  Here is a brief explanation about timing in general.  It is followed by my explanation for timing in the 70/71 280SL with manual transmission.

I'm very open to corrections and suggestions to improve the explanation.  Please contribute.  Once we have something close to finished I'll add it to the technical manaual.

                 TIMING AND VACUUM RETARD 101
 
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.  For the most part these advance and retard operations are accomplished by the distributor itself via the rpm sensitive centrifugal weights inside the body of the unit.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the auto cam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing".   Vacuum retard has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum retard drops out entirely; it has no part in the "total timing" equation.  When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark retard, and when the throttle plate opens, manifold vacuum drops, and the vacuum retard can returns to zero, returning the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum retard doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor auto cam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum retard, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum retard is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

 At steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum retard is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway.  
 
              1970/71 280SL w/Manual Transmissions

In addition to the distributor the 17C degree switch and the warm running device (WRD) also come to mind.  When the SL engine is cold the fuel injection system enriches the fuel mixture.  When the engine is below 17C the cold temperature switch in the engine cylinder head provides a ground to pin #6 of the 8 pin relay located nears the battery.  When a ground signal is present on pin #6 the two way valve is energized and vacuum pressure is applied to the distributor retarding the timing.  The above paragraph tells us that  rich mixtures  burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.  As the engine warms the FIP leans out the fuel mixture and the 17C switch opens to remove vacuum retard on the distributor.    The 100C switch is also designed to affect the timing via the two way valve.  When the engine temperature exceeds 100C the switch closes and a ground is provided to pin # 5 on the 8 pin relay.  This causes the two way valve to be energized and the timing retarded.  This has the same effect on the engine as the 17C switch although for a different reason.  With the timing retarded,  fuel is burned more efficiently ????????, idle is increased and engine temperature is reduced????????????.  As the engine temperature cools below 100C vacuum retard is removed.   As stated above timing retard via the two way valve will be initiated when the engine is very cold or warm.  It will be removed when engine temperature is within normal operating range.  

Note:  In all cases vacuum retard is only available if the intake manifold pressure is high enough to control the vacuum can on the side of the distributor.  As the throttle plate is opened, under acceleration, the intake manifold pressure is reduced and the two way valve will cease to retard distributor timing.

On the 1970/71 models vacuum retard is also used to reduce exhaust gas emissions. In addition to the temperature controls described above the vacuum retard function via the two way valve is initiated in the 1970/71 SLs by the 4 pin speed relay located near the battery.  This speed relay works closely with the engine rpms and throttle/loading as set by the intake manifold pressure as follows:
At 2400rpms and above the speed relay energizes the two way valve.  If the engine is operating above 2400 rpms and is not under a load condition the throttle plate will close, vacuum pressure is sufficient and will be applied to the distributor to retard the timing.  If the engine is under a load condition the throttle plate is open and vacuum pressure will be reduced removing or reducing the timing retard function on the distributor.  The purpose of the speed relay is to retard timing when the engine is above 2400 rpms and not under a load condition or coasting.  When coasting and not under load distributor timing is retarded and fuel is burned more efficiently reducing emissions.???????  At 2200 rpms and below the two way valve is not energized by the speed relay.


Jeff C.
1970 280SL 4-speed

Markus

  • Guest
Re: surging hesitating engine
« Reply #18 on: April 21, 2008, 21:26:36 »
Nice piece on timing and how it applies to mixture Jeff, although you may wish to reconsider the function of the two way valve, how and when it applies vacuum to the distributor.  
1)  The 17C switch is in the block, right behind the knurled knob on the IP, used to adjust your idle mixture. (not in the cylinder head)
2)  If you remember this, and nothing else when it comes to the two way/bypass valve it will help you understand how the system functions, for emissions control on the 062  distributor.  Anytime the "two way valve is energized, the distributor vents to atmosphere"
(BBB)
, resulting in advancement of timing, by canceling the vacuum retard.
3) The two valve is energized, venting the distributor to atmosphere/canceling timing retard under the following conditions:
i) When the engine temperature is below 17C the switch is grounded/closed, results in advanced timing, and better starting.
ii) When the engine rpm is greater than 2400 rpms, the speed switch activates the working relay, and again cancels the vacuum retard, allowing full mechanical advance.
iii) When the engine overheats and the 100C switch closes/grounded the vacuum retard cancels, resulting in an increase of about 500 rpm which helps cool the overheating engine.
Your post, of 3-31-8 is absolutely correct:

"I need to remind myself that the two way valve is normally open.(to continous manifold vacuum)  Energizing the two way valve closes the valve and cancels vacuum to the distributor."

jeffc280sl

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Re: surging hesitating engine
« Reply #19 on: April 22, 2008, 06:04:22 »
Markus,

Thanks for reminding me of my note to self.  I need to think this through again after which I'll edit my earlier response.

Jeff C.
1970 280SL 4-speed
« Last Edit: April 22, 2008, 06:20:50 by jeffc280sl »