Speed relay

[tuultyme]

I have a 1970 280SL automatic transmission with the 0231 0116 0062 JFUR6 distributor; vacuum retard.  One of the previous owners had added an Allison electronic ignition system. I do not have any manual or diagram for this system.  I replaced some of the vacuum lines to the distributor so that now the timing is retarded.  This retard is not removed via the speed relay above the required 2300 RPM therefore I have had to adjust the 800 RPM timing to 0* to accommodate this lack of advance at the higher RPM's.

I have been using the Haynes and BBB manual to test the emmissions controls; speed relay relay box, etc.  The 17*C coolant temperature switch works.  The 100*C temperature switch appears not to work.  I can get the two-way valve to shut off the vacuum to the distributor by direct connection to the battery but there is no power coming from the speed relay at any speed.  For that matter I do not record any voltage coming from the coil to the speed relay.

There is a wire going to the Allison control; but I cannot follow it further to the speed relay.  This  connection may have been removed when the electronic system was added.  Would anyone be able to work with me to get the speed relay/vacuum advance system to work properly.  I do not want to just start replacing relays until I know that the wiring is correct.  This later system seems to be a little complicated.

Also while testing the constant speed control, which is "functioning" all the time when the engine is running, there is a connection to ground even when the engine is not running.  I am not sure if this is related to the other problem but it is electrical.

Bruce; 268Blaugrun(green) 1970 280SL; IL

[jeffc280sl]

Hi Bruce,

I have been through the same exercise on my 1970 280SL 4 speed so I think I can help.  Do you have John Hassel's write up of the Crane/Allison installation?  If not send me your email address and I'll send you a copy.

The speed relay needs rpm data to turn the two way valve on.  This data is provided by a wire connecting pin 4 of the relay to the negative terminal on the coil.  Use an ohm meter to see if this wire is present?  Alternatively, if you have an electronic rpm gauge terminate the green lead in pin 4 of the speed relay connector.  You should get an rpm reading if the wire if there is a connection between pin 4 and the neg coil.

The 100 and 17 degree C switches terminate on pin 5 and 6 respectively on the relay opposite the speed relay.  Disconnect these wires from the switches and use an ohm meter to determine a connection.  Alternatively, take the connector off the 100 degree switch and ground it on the valve cover.  This simulates an over 100 degree setting and the two way valve should be on. To test the 17 degree switch simply remove the wire to it and make sure it is not grounded.  In this way you are simulating an under 17 degree situation and the two way valve should be on.  

Let's take this slowly.  Try testing the rpm input to pin 4 of the speed relay.  And then test the 100 and 17 degree switches.  Let me know what you find out and we can go from there.  These tests and circuits are independant of the ignition system.

Jeff C.
1970 280SL 4-speed

[tuultyme]

update.
Using a ohm meter I have connection between the 100*C switch and pin 5 of the relay, and connection between the 17*C switch and pin 6 of the relay.  Grounding the connection to the 100*C switch to the valve cover causes the two-way valve to close (turn on).  Removing the wire from the 17*C switch does not cause the two-way valve to close.  I did not have a connection between the negative terminate on the coil and pin e of the speed relay.  The PO had remove the Green/Black wire from the coil to a connection under the battery tray.  I replaced this wire and now I can get an RPM reading at pin 4.
After hooking this all back up I tested the voltage at the two-way valve; I get about .18 volts at low RPM and about .3 volts over 2300 RPM. The two-way valve is not closing at the elevated RPM.  Next?

Bruce; 268Blaugrun(green) 1970 280SL; IL

[jeffc280sl]

Hi Bruce,

Let's stay with the 100 degree switch for a moment. It is normally open, so with a cool engine use the ohm meter between ground and the terminal on top of the switch to see if it is open.  At some point when the temp is above 100 degrees use an ohm metter to see if the switch closes.  I replaced mine because I wasn't sure about the temp and when it would close.  This has a benefit of incresing idle via the two way valve when the engine gets really hot so I wanted it to work.

I made a mistake concerning the 17 degree switch.  Sorry about that. Under 17 degrees this switch it is normally closed.  So check it with the ohm metter when the engine is cold.  Above 17 degrees it will open.  Ground the lead to the switch and the two way valve should come on.  Mine is always closed so its broken.  I have not found this part for purchase on line.  Let me know if you do.  This switch increases idle via the two way valve when the engine is cold.  I imagine many of these are broken and this may explain low idle when the car is cold.   I have adjusted CO and idle speed with a warm engine so the vacuum retard is present.  Imagine idle speed when cold with vacumm off via the two way valve.  This enhances the action of the fuel injector pump thermostat which enriches the fuel mixture when the engine is cold.

Now for the speed relay.  You have rpm data on pin 4 so that's good.  You should have +12 V on pin 2 when the car is running.   Check that.
The speed relay is pretty independant at this point.  With +2400 rpms at pin 4 of the speed relay the speed relay must take +12 V from pin 2 and put +12 V on pin 1.  Make sure there is a connection between one side of the two way valve with pin 1 and the other side with ground.  With under 2200 rpms at pin 4 the speed relay should remove +12 V from pin 1.   If you look at figure 3.99 of the Haynes manual you can see what I'm trying to say. In summary you know the two way valve is working.  If pin 1 of the speed relay is wired to the two way valve, +12 V is present on pin 2 (ignition on) and rpm data is present at pin 4 the wiring is correct.  If the two way valve is not on above 2400 rpms the speed relay is broken.  I have not studied the interaction if any between the auto trans fluid pressure switches and the speed relay since mine is a 4 speed.  I can study the schematic if we need to.  I don't think there is any interaction however.

Jeff C.
1970 280SL 4-speed

[Bob G ✝︎]

can someone comment on how to test a speed relay for a 1968 280SL. It sits on a braket agaainst the left inner fener with a three pin plug attached. Is it important that it works on my car to adjust the timming . I now this was an early attempt to cut down on hydocarbons on decelleration by cutting off fuel.
I am however not sure of how it works and how to test it. Are these rebuildable ? and is there a source for used good working unites I can contact?

Bob Geco
1968 280SL
11304400121906

[jeffc280sl]

Hi Bob,

On page 77 of the Haynes manual they spell out how to check the speed relay on an auto and manual 68/69 280SL.  From what I can tell the speed relay does not influence timing retard and advance.  It seems to only control the cut off solenoid on the fuel injector pump and the venturi control unit.  The schematic does not show a reference to the venturi so I don't know about that.



Jeff C.
1970 280SL 4-speed

[tuultyme]

I checked the speed relay conncection it seams to be connected correctly.  I read a post by BenZ Dr. from awhile back that said that the fluid pressure switch on the automatic transmission affects this system.  Following the Haynes manual (section37 #32)I tested currect from this switch.  I light up a test light bewteen #1 and #8 on the relay box all the time there is no difference as noted in the manual at different speeds.  Is there someting wrong with this connection to the trans or the switch itself?
I still get up one volt at the two-way valve connecting when driving.

Bruce; 268Blaugrun(green) 1970 280SL; IL

[jeffc280sl]

The schematic for the auto shows that the speed relay has 6 pins vs 4 for the manual.  From what I can tell pins 5 & 6 are related to what the BBB calls a working current relay which controls the stopping solenoid on the fuel injector pump.  Pins 1,2,3&4 are wired the same in the manual and auto trans.    

+12V should be present at pin 2 of the speed relay when the car is on.  Pin 3 of the speed relay is ground and it is connected to one side of the 2 way valve via the current relay. I think this is how the speed relay is supposed to operate.  Pin 4 receives rpm data from the neg coil terminal.  When it reaches 2400 rpm pin 4 circuit switches +12V (which is provided by pin 2)on to pin 1 which is wired to the 2 way valve.  

A test to try is as follows:

Jumper pin 1 and 2 of the speed relay.  This simulates a 2400 RPM reading by pin 4 circuit.  Your two way valve should be on when the jumper is in place.  I think this vacuum/retard operation is the same in both cars (auto and manual.  If the two way valve does not come on take the car for a drive and see if the fluid pressure switches come in to play and the valve comes on.

Jeff C.
1970 280SL 4-speed

[tuultyme]

Followup on my speed relay.  As the speed relay for a 70 with the A/T is no longer available; item #0005452432 Ray Paul (SLClassic)suggested I send it to Beckmann Technologies in North Carolina.  They repaired it for $155 got it back yesterday put it back on the car and drove to a local cruise night.  I can hear the the timing advance about 2300 RPMs.  As instructed in the Haynes manual always test the speed relay function to the two valve with a volt meter not a light bulb.

Bruce; 268Blaugrun(green) 1970 280SL; IL