Low Idle, Code 34, and Brake Switch

Confused on this one. I had to replace the brake switch on my 1991 MJ 4.0 because the detents holding it in the keeper were worn and it kept moving out of adjustment. Replaced the switch, adjusted it so the plunger was fully extended at 3/8" as per the FSM. Brake lights worked fine. Started it up and the idle was very noticibly lower. Drove it around the block and when stopping in DRIVE (have the AW-4) the engine nearly stalls, idle is only 500RPM on the tach. Went back home, and checked the codes, and got a Code 34, Cruise Control Circuit fault. I don't have cruise control. But I know the brake switch supplies inputs the the computer and cruise control (if I had it). There is NO check engine light. Could replacing the stop light switch affect my idle somehow? Oh, and the TPS checks out okay with the voltmeter. No other problems, just very low idle. Thanks for the help.

I also sends a signal to you Transmission control computer to unlock you converter.

Okay, but will this affect idle speed? Tranny is shifting fine, including overdrive.

comanche91 said:

Okay, but will this affect idle speed? Tranny is shifting fine, including overdrive.

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I would think so if your converter is still locked.

What are the symptoms of a locked torque converter? I have no experience with this one. I can see on the schematic that the stop light switch applies a ground to the TCU when the brakes are applied, but the transmission seems to be okay.

Pull the battery for 10 minutes and let the computer reset. It will wake up and relearn the settings. The computer should then compensate and set the idle correctly.

comanche91 said:

What are the symptoms of a locked torque converter? I have no experience with this one. I can see on the schematic that the stop light switch applies a ground to the TCU when the brakes are applied, but the transmission seems to be okay.

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Low idle and high transmission temp.There is three wires on the plug, was there three pins on your new switch?
The brake light switch for manual shift is different than the automatic one.

Three pins, exactly like the old one. Mopar part. Thanks for the help sp far. Old man, I've had the battery disconnected for about three hours now. I'll hook it back up and try again.

Tried it again after everything cooled off. Started up, same instant very low idle. Before, on a cold start, the idle went up about 1K RPM, then quickly settled in at 600-800 as everything warmed up. Only thing I have done is change the stop light switch with an identical part. Instant cold idel is now ~500RPM. Checked the fault codes; blinks three times, then a 5, 5 to end. No more 3, 4 fault code. Realize I can pull the throttle body and crank up the idle screw from the bottom, but that's just a band-aid. Sure would like to know how this happened. Any suggestions? Thanks, Don

Guess not......

Tried adjusting, etc. and the connector is polarized - can only go on one way. I'm stumped. Guess I'll take the switch out and check it with the meter.

old_man was right on again

Well, I still do not know how replacing the brake switch affected idle, but old_man was right on. Cleared the codes and drove it about 60 miles and now it idles just fine. Bennie at Hesco told me the same thing; that the computer has to "re-learn" certain settings and it can only be done by driving it. It must get a mileage input as well as the other sensor inputs. Be hell though if you couldn't drive your rig. Is there any around this?? Or is it an pre-OBDII thing?