throttle switch

[wwheeler]

My throttle switch on my '68 automatic M130 has been giving me trouble for three years since I have owned my car. It will work fine for about 3-4 months and then I notice the transmission downshift becomes harsh. I take it apart and clean and is good for another few months. When it goes bad, the resistance through the switch increases 10 fold. Again when I clean it up, the resistance is low and works fine. I have even tried one off of another car and it does the same thing.

I am wondering if my adjustment is incorrect. I have it set just so the switch contacts open when the throttle is just lifted off of the extended throttle solenoid (used with automatics). Is this correct?

I am thinking that I may be burning the point contacts when driving at lower constant speeds where the throttle will continuously open and shut right around the point where the switch opens and shuts. Would it be better for the contacts to  open when the throttle is a little more open? All of the wiring tests are good and the transmission double acting solenoid is about 12 years old.

Thanks for the help!

[jacovdw]

Hello Wallace,

Here is an abbreviated extract from the BBB that I have posted elsewhere on the forum a while back. I will have to check at home later if the rpm specification for the W108 280SE's is the same.

According to the BBB, you need an inspection lamp (or test lamp), a rev counter and an engine at operating temperature.
Disconnect the two wires from the throttle switch and connect one cable to ground and the other via the test lamp to the positive pole of the battery.

For transmission type K4A025 (as used in the 280SL pagodas) the maximum cutout speed for the idle switch is 1600 rpm.

Now with the engine running (idle speed) place gear selector lever in "P" or "N" (make sure parking brake is applied) and increase the engine speed slowly. The test lamp should extinguish at the latest when the cutout speed (1600 rpm) is attained.

If not, slightly loosen the two mounting screws of the switch and rotate until the test lamp extinguishes at the appropriate speed. Tighten mounting screws and recheck.

It is possible that arcing over the contact points (too closely set) in the switch is responsible for the fouling and hence the increase in resistance that you have found.

[ja17]

Good advice Jaco.  Yes the max speed is 1600 rpms, the minimum can be a bit lower 1100 to 1200 rpms.  The engine idle speed in gear should be around 700- 750 rpms.

[wwheeler]

Thanks for the info. I guess I passed up the process in the BBB. Those specs make perfect sense and will set it tonight.

My guess is that mine was set at about 1000 RPM; too low. I wish I would have thought about arcing the points earlier. The same thing happened to a brand new switch I had. Now they are NLA.

Thanks again,

Wallace

[Naj ✝︎]

Is it possible that the solenoid is sticky and drawing too much current? 

Naj

[wwheeler]

Naj,

That is a good thought. When I have pulled the trans tunnel cover off and observed the action of the solenoid, it seems normal. When the switch contacts are clean or when I bypass the switch, the solenoid snaps right to position. It doesn't seem to lag or be lazy. I am not sure how you would go about testing the amperage draw?

I would take the solenoid off and observe it's actions on the bench, but that is a lot of work.

If the switch contacts are clean (resistance low) and just the fuel pump is running, I can hear the trans solenoid work when I move the throttle. However when the contacts are dirty, the solenoid only works some of the time. Of course the voltage supplied is lower when the engine isn't running.

Will and should the solenoid normally work when moving the throttle with the fuel pump running and not the engine (at this lower voltage)?     

[jacovdw]

Wallace,

Since the internal resistance of the windings in the solenoid is constant, it follows therefore (according to Ohm's law) that current and voltage would play a part in the function of the solenoid. Meaning that a lower voltage would result in a lower current flow through the windings (current = voltage/resistance).

Without going into too much technical detail, the strength of the magnetic field produced by the windings is in turn related to the amount current flowing through the windings.

That is why the solenoid is a little sluggish when the ignition is on, fuel pump running and engine off (the overall voltage in the system is lower).

With the engine running, the voltage increases and so does the available current and hence the solenoid perks up.

So, the short answer to your question is: yes.

I would suggest that you try to readjust the switch after cleaning it and see if fouling of the contact points happen again.
If it does, then we can look further.

[ctaylor738]

What about wiring in a relay to power the solenoid, to reduce the current through the switch?

[jacovdw]

Chuck,

I don't think the problem is that there is too much current going through the switch under normal circumstances.

Three possibilities springs to my mind:

1) incorrect adjustment of switch resulting in arcing as the switch stays partially open at incorrect times.
2) binding 3-position solenoid or associated linkage causing excessive current drawn through switch.
3) a leaky seal on the throttle butterfly spindle considering that the engine breather is in close proximity.

If the solenoid is binding (or the associated linkage for that matter) then it is like trying to stall a DC electric motor by braking the shaft. As the motor is stalling, more current will be drawn and hence also heat generated.

As far as I know, these switches are not items that are routinely replaced during a yearly service. If correctly adjusted and all other components of the setup functioning as designed, then these switches should last a very long time with minimal interference.

[wwheeler]

Thanks for all of the replies. I have readjusted the switch with a tach and it works well for now. The solenoid moves easily and there is no oil in the switch, just contact dust on the points.

One thing that has always bothered me is the movement range of the solenoid. For kick down, the movement is slight (1/8") and that function always works. The idle side of the solenoid has much more movement maybe as much as 1/2". Could this be out of adjustment for the stroke lengths to be so different?

[ja17]

Hello Wallace,

Most likely the difference in travel  is in a function of the mechanicallimits  of the solenoid one one end and the mechanical limits of the modulator on the  other end.

Take care,

Joe