Fluctuating idle with AC on and brake pressed - DEFINITIVE FIX!!!!
 

Well, looks like I finally nailed it down last night.



I've been dealing with this problem for 11 years, or just about a year after I bought the truck new. I never had any clue as to what was causing it, nor where to start diagnosing the problem. I tried various adjustments, and changing all the vacuum hoses, and never had real success. I could adjust it out with the idle screw and the AC idle up valve, but as soon as the weather changed the problem would either come back or the idle would drop so low that I had to turn it back up. I had begun to accept that I was just going to have to live with this...



That is, until I saw a link to this at another forum recently:







The geocity link describes a wire running to the ECM that tells it that you have the brake pressed which causes it to cut the flow to the fuel injectors when the idle is too high and you're trying to stop. That's what causes the up-down-up-down of the idle.



The guy at geocities says there is a wire from the pedal to the ecm, then the signal goes back out of the ecm to the brake lights. He wouldn't give colors or locations because according to him the wires are different on different years. His fix was to find the two wires, cut them both before the ecm, and splice them together bypassing the computer all together.



So I got my meter out and started testing wires. I was able to find a wire that I thought was the output from the ecm to the brake lights but I couldn't figure out which was the input wire. To make matters worse, at some point in my truck's life someone removed one of the plugs from it ( I have no friggin' idea who, I bought my truck new and have never fooled with it... )and broke the little release pin off so i couldn't get the plug out. I ended the first night with frustration because I was only testing the plugs from the back and couldn't find the other wire...


Last night I went back out and started by wrestling the broken plug out first so I could undo all three plugs and test them right. What I figured out was than the pin I thought was an output was in fact the input, and, I noticed that with all three plugs out of the ecm that my brake lights were still on which indicated to me that there weren't two wires to the ecm after all - AT LEAST IN 1995 MODEL TRUCKS!!! Your mileage may vary if you have a different year.



So, I snipped off the wire, wire nutted both sides and buttoned everything back up. And held my breath that I didn't screw something up.



This morning on the way to work it was the perfect weather when this problem would be really bad and at every single stop I had a nice, smooth idle that was just a tad high, but no hint of a fluctuation at all.

Problem solved.


Before you go cutting wires on your own ecm, make sure of a few things.

1. Make sure your ecm isn't as that other guy described with two wires.
2. Make sure you test the wire and are confident that it is the brake lead on your truck.
3. When you do cut it, be sure to leave enough wire on the plug side that you can splice it back together if it isn't the correct wire.

Man, it was so nice on the way in today to not have to take my foot off the brake or turn off the AC when I stopped. I have a feeling this is one habit that will be very hard to break.



That said, here's the pic of the plugs and the wires I cut. It's the green one with a white stripe that I put the two red arrows on in this pic.

http://www.ovrrdrive.made2own.com/up...ight_small.jpg


and if you need a larger version of the pic, click here:



http://www.ovrrdrive.made2own.com/up...brakelight.jpg

Congrats, ovrrdrive! :bigclap:

Or just turn down the idle speed:
- http://www.4crawler.com/4x4/TLCA_Tru...ech.04.09.html

Works for me.

If you read my post you would have seen that it worked for me too... For a week then the idle would be too low so I would turn it up and a week later have fluctuating. And so on and so on...

This is a sure fix. The ecm doesn't know when you have the brake on after this.

I say do whatever works for you. :great:

Has worked for me for 5 years now. Part of the reason I found this place searching for a cure. Cutting the wire also works, so try the idle settings first and if that don't work start cutting. :D

It is a matter of getting the normal and fast-A/C idle speed set right. I can exactly duplicate this problem by turning the fast A/C idle speed up and exactly fix it by turning it down. But the idea of cutting fuel to the injectors is that the ECU "thinks" you are trying to stop when you put the brakes on and is attempting to help by cutting fuel to the engine. Probably will work OK with the wires cut, though. That is whey I list both fixes on that web page.

:) damn guys, I thought this was common knowledge!

Seems to be the wrong way to "fix" something by cutting a wire. It was there for a reason.

Yes, and that reason was fully explained and undesired.

It's not wrong, if you own the truck, do what you want with it. This mod doesn't make the truck unsafe, it probably only uses a little more gas with a higher idle.

Cutting the wire disables a feature of the truck, if you don't like an extra feature because it's too hard to adjust it correctly, then by all means, get rid of it!

Heck! i say CUT CUT CUT, AWAY. I took out my Ecu, & she runs like a champ. Almost feels like shes not runnin at all.........

Old thread, but what the hey....

This is interesting. How did you manage this? Without an ECU, my vehicles will not run....at all.

I think he was being facetious... AFAIK the engine won't run without the ECM unless you remove all of the FI and sensors from it and put a carb back on.

I do wonder what he was searching for to find this old thread if he went back to carbed...