Idle Increase Solenoid Wiring

[toms]

Patient is a 1971 280sl with automatic transmission.

I am trying to diagnose the following symptoms:

-rough idle in gear
-hard warm start
-hard down shifts


I have determined at a minimum that I have a bad idle throttle
switch (this is the switch with two terminals on the side of the venturiintake manifold). Got that on order.

I have verified my idle stop screw and linkage are set correctly. I suspect that I may not be wired correctly at the idle increase solenoid.

This solenoid has a metal plate attached to the round solenoid. Attached to that plate is a bakelite block with 4 terminals. The first terminal has a wire that goes to the safety neutral switch. The 2nd and 3rd terminals have the wires that emerge from the solenoid. The 3rd terminal also has a wire that attaches between the 3rd and 4th terminal.

For a picture, see the reference below (once at the site-click on attachement for picture). Unless the entire bakelite bar is hot, I don't see how the solenoid is getting power nor how it is grounded.

I wonder if someone can verify this wiring connection to determine if this wiring is as it should be.

Thanks in advance for any help you can provide!


Tom Sargeant
Apprentice Mechanic


For picture:

http://www.mercedes113.com/discussion/5724

[stevebfl]

You definitely need to put the throttle switch on before going too far. Your hard downshifts are undoubtably caused by the lack of the idle position on the double lifting solenoid (trans modulator control).

I doubt that the hard starting or rough idling has much to do with the idle compensation solenoid. I forget what triggered the solenoid but it is also probably tied to the idle switch and the gear position. I would be very sure about your mixture in dealing with the two symptoms. Lots of stories on inline injection.

[toms]

Steve,

Thanks for the response. I can't fix the idle throttle switch until I get this wiring figured out. When I forced a closed circuit on the idle throttle switch, the wire connecting the neutral safety switch to the first screw of the idle increase solenoid got real warm (read smoked). This is because that first screw was not a terminal to enerergize the solenoid, but an attaching screw and essentially ground.

So I think I need to move that wire over to the next screw which actually is a terminal that would energize the solenoid, not ground. Question then becomes is the other wire bridging the 3rd and 4th screw. Is this just grounding the solenoid to complete the circuit?

Also, you wrote:

I would be very sure about your mixture in dealing with the two symptoms. Lots of stories on inline injection.

Can you explain this?

I am grateful for your comments and any other thoughts you might have.

Regards,

[stevebfl]

I think I would start by isolating the wires from that solenoid and see if the rest of the system works. You can hear the double lifting solenoid work with key on engine off as you close the idle switch circuit.

My partner has a gourgeous 71 280SL and I can look at the wiring there if necessary, I lost my wiring diagram years ago and its a simple enough circuit that I have always been able to work my way through the problems with a knowledge of what is supposed to work.

The point about mixture is that itsa very long story. One that I have writen much about. I would check the archives and form a question and I'd be glad to go over it. Probably the most significant diagnostic tool for mixture is a little knowledge.

The air and fuel are controlled separately by the same linkage and provide a very simple diagnostic technique. With the car at any steady rpm the relative effect of mixture is easy to see. Take the link loose that goes vertically down to the pump lever off at the cross shaft. Take the engine to 2000 rpm (doesn't matter the exact rpm just something steady above idle) while holding the link to the ball that it was previously attached to. Now with the engine at steady speed move the link up and down till the engine runs the smoothest. This will then be the best mixture. If you move the link in the increased throttle position (more fuel) and the engine runs better the set mixture is too lean. If you add fuel and it runs slower then the set mixture is too rich. very quick, easy evaluation.

Do this same manuver at idle to really evaluate your condition. The only problem at idle is that you can add fuel but since the pump lever should be on the idle stop you can not move it to a leaner mixture. So.... you take your link and add fuel to the idling motor (since they are usually too rich this should cause the motor to slow or run worse). To see how much too rich the mechanism is to now leave the pump lever alone and just move the throttle (to add only air). If things are right adding throttle should raise the rpms only a few hundred before dieing out lean. If by adding air the engine winds up running great at 1500 or above then you have a way too rich system; one that has enough fuel to support the higher rpm. Remember engine speed is determined by the amount of air. The amount of fuel is either right or it slows the motor. If a motor can raise speed by just adding air then the motor is way too rich.

[toms]

Steve,

Thanks for the insights. I did a search on fuel and air mix and have learned a bit. Once I do the test as you describe and determine that an adjustment is necessary, is that done via the idle screw (large slotted screw with the spring) that has the hose that attaches to the air intake filter housing?

Also, I noticed that gas is seeping from the bolt that holds the gas filter housing in place. I have a crush washer and new filter and seal and plan to replace this weekend. Could this be causing my "hard warm start" problem?

Also, I just replaced my idle throttle switch that was bad and this did fix my hard down shifts-chalk one up for the amatuer mechanic!

Regards,

[stevebfl]

The screw with the spring around it is the idle speed screw which of course adds air (as air is the critical element in engine speed).

If you do have a mixture problem after the test above go to the archives and look for articles I have wrote (and others) concerning adjustments. As I pointed out, the mixture should only be adjusted with the help of an exhaustgas analyzer. One can get into real trouble with the internal pump adjustments without gobs of experience AND an exhaust gas analyzer.

[toms]

Steve,

Thanks for the suggestion. I did a search of the archives, reviewed my manuals and followed the information you provided. I detemined (as you expected) that I was running really rich. I adjusted the air throttle adjustment and the fuel adjust on the back of the FI pump (not the internal adjustments, but the external screw). I used a vacuum pump instead of the co2 analyzer as the manual allows. The fuel and air mix is such that I idle around 750-800 rpm and the rpms advance only slightly before falling back when I disconnect the linkage and apply air only.

The problem now is that, under the load when the car is in gear, my rpm drops and enters a rough idle.

I think I am now back full circle to needing to resolve the anti-stall (constant speed) solenoid wiring.

Thanks again for all of your help.

[stevebfl]

If I understand, you used the thumb screw at the bottom back to adjust the mixture.

It sounds like you have started some of the problems I was talking about. The absolute first thing to check when there is a mixture problem on that car is the cold enrichment thermostat. It you are rich because it is not fully off and you correct the mixture with the thumb screw you have altered the relationship of mixture control to rpm. The thumb screw adjusts the tention on the lightest of three springs in the flyweight governor. If only has affect at idle and is very sensitive to rpm. I suspect that the mixture is again wrong when you put it in gear because of the rpm change. Have someone sit with the brake on and try the same tests after you are in gear.

The thermostat is easy to test. At the back of the pump there is a verticle cylinder with water hoses attached. At the base is a airfilter. This filter filters the air that is let in for idle speed increase when cold. The thermostat is inside the cylinder. The t-stat pushes a single piston up and down. Part of the piston is an airslide (that controls the additional air). Beneath the airslide the same motion works on the links of the governor mechanism to increase mixture for cold/warm running.

Remove the airfilter (22mm wrench). With the car cold, start the motor and place ones finger over the hole the filter came out of. It should almost kill the motor. After the motor is warm, do this again. Now there should be no more than a 50rpm change as the slide shoud have closed. If this is not the case then you have found the problem.

Disassemble the t-stat and the airslide below (just a couple screws. There are two common problems. First is a dead t-stat. Second is a frozen airslide (coolant leaks from the t-stat corrode the assembly). If it flunks the test and the airslide is free replace the t-stat.