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HOW-TO: Replace Idle speed contact switch S29/3

gerryvz

Site Honcho
Staff member
Here is the idle speed contact switch, which is a very common item to throw codes.

It has been known to cause issues relating to a rough/stumbling idle. A few folks have replaced their switches to remedy the occasional stumble at idle, but I actually think that the carbon tracks on the resistor plastic piece (from my recent post of the MBCA STAR article) is the real cause of this. In any case, this part can and does go bad, so it's good to have a reference and how-to for it here.

This switch is used with cars that have ASR.

It's sort of a pain in the butt to replace, but is definitely a DIY job. Here is the procedure:

A. Remove the knee bolster
- pull head light switch out with pliers (cover with a cloth so you don't damage switch)
- remove large bolt that holds the trim piece on
- remove trim pieces on both sides of steering wheel (pry up/out from the left most side on both of these)
- remove the 5 bolts under these trim pieces and 3 more on the bottom (8 mm)
- unscrew the parking brake release (3 phillips) and hood release (1 phillips) and detach these two cables

B. Remove black plastic piece above pedals
- remove one 8mm screw
- gently pry the piece out starting along the footwell vent

C. Remove floor mat, then remove accelerator pedal clip by pulling outward (you may also need to remove the carpeting to get to this clip)

D. Disassemble acceleartor linkage
- rotate the accelerator pedal ~200 degrees until it can be removed (1)
- detach spring (2)
- pry off the large clip (3)
- remove retaining clip (4)
- slide the assembly out off the hinge/rod

E. Unplug the idle speed contact switch (follow the cable)

F. Disassemble the plug to route the wires through the accelerator linkage

G. Replace the switch using the appropriate torx driver for the two screws (or a similar regular screw driver if the torx isn't available)

H. Put it all back together

Cheers,
Gerry

GSXR edit: Added screen shot of FSM showing factory identifcation number "S29/3" for this switch. Updated thread title as well.
 

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Re: Idle speed contact switch

Thanks Gerry for such a detailed explanation but what do you mean with carbon tracks on the resistor plastic piece?
 
Re: Idle speed contact switch

Some pictures to complement Gerry's explanation.

More coming after I finish the job. I got the wrong part from the dealer. They gave me part number A 124 545 01 14 instead of A 140... They said this one replaces the other one. I trusted them even when they look completely different (see picture) but in my opinion (correct me if I'm wrong) there's no way that part could go in there so I'll be returning it on monday and trying to get the correct one.
 

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Re: HOW-TO: Replace Idle speed contact switch

WTF!? Another electric component to maintain. What is this thing controlling - can it be disconnected or bypassed some way?

:ricer:
 
Re: HOW-TO: Replace Idle speed contact switch

WTF!? Another electric component to maintain

That's what I was thinking too, especially after replacing my ETA! I got a DTC 14 on pin 7 last week (indicating a bad S29/3) and I'm wondering if this caused my intermittent (now gone) idle surging problem. Looks like I'll have to start thinking about ordering one of these in my next parts.com order.
 
Re: HOW-TO: Replace Idle speed contact switch

WTF!? Another electric component to maintain. What is this thing controlling - can it be disconnected or bypassed some way?
The computers use this input to sense when the accelerator pedal is at idle, and it compares this to the position of the throttle linkage (from switches inside the ETA itself). You cannot disconnect or bypass it. This switch is not a very common failure, if it does fail, just replace it. I've never had to replace one on any of my cars.

:banana2:
 
Yup! If that code keeps coming back, I'd at least inspect the switch, and/or test it, and replace if needed. If the code doesn't return I wouldn't worry about it.

:checkeredflag:
 
I just changed my S29/3 Idle Speed Contact Switch. Does anyone know how this switch works? Is it a mercury switch? Any idea on how to test it? I have my old one and just wanted to see if it really was going bad. Thanks.
 
This switch is is for safety purposes. Its always in opposition to the M16/1s2 inside the ETA. Its in case you have your gas pedal not pressed and the car starts to accelerate/ETAs valve opens. You can see it in action when you move throttle linkage without pressing the gas pedal - ETA and EGAS are cut out and ASR light comes on. If the ETAs valve is opened, M16/1s2 switches to open and S29/3 switches to closed position and EGAS consideres it OK. S29/3 fault code can also pop up if your throttle linkage springs are missing or deformed.
 
Bringing this thread back to life - more so to show what I found on my C140 S500. I thought I had the ASR issues resolved but last night I started it up and the same thing happened. Asr lamp on and throttle body drops out.

Got the same code for this switch so I ordered a new one and tonight took it out. I wanted to see why this might have been bad. And discovered a broken wire inside the sheath. Note that this is the section of wire that moves slightly with the pedal. That is enough over many years to cause the wire(s) to break inside and throw the intermittent codes.

If these ever become NLA this might be a good way of repairing it - by carefully soldering on a new section of wire. I cannot see the switch itself failing half as much as the wire.

20200121_205403.jpg
 
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