allbusinessbuggy

Truck: 2001 Silverado LT 2500HD 8.1 Crew Cab, Short Box.
Problem scenerio A: Speedo may or may not work when first driven. If speedo works from the start, auto door locks will lock when 15 mph is reached, and stereo works fine and everything is FINE. THEN, speedo will drop off a couple of times before finally falling to zero. When that happens, the sound level of the stereo will decrease as if you turned it down two clicks. Weird right?
Problem scenerio B: Speedo doesnt work from the start and the '15 mph' auto door locks dont lock. I'm driving down the road for a while and the speedo jumps up to the proper speed I'm traveling, and at that exact same time the auto door locks finally 'lock'. Weird right?
Dealer laughed as if this is funny to them and to ME. Lost all sense of humor after 360 US dollars of worthless diagnostic.
Is there a single relay controlling power for all these items-VSS, stereo, and auto door locks? Confused and I'd say I'm out of diagnostic money for now. Please help.

suzuki-guy

the dealer should keep looking at it!! my dealer would charge one fee until they got it right!! Did you complain or just pay the bill? you cant let dealers or mechanics think you are gullable or they will take advantage of you!!! You need to take it back and complain sounds like a major wiring problem

ActiveMonkey

imo if i pay a dealer a diagnostic fee... i expect the correct diagnosis. No answer = stop payment on the check.

Mike Sigmond

Yea, I agree, stop payment. I had a recall on my guage cluster and got it all replaced for free. I dont know if that will cause all those other strange problems. Id contact the BBB if you dont get your $ back.

allbusinessbuggy

Thanks for the replies. Yes I did stop payment on the check. But I'm still at a loss of what the actual problem is and I wasted ALOT of time taking it to the dealer, so I thought someone out there has seen this type of problem before. Not taking to dealer again. HUGE waste of time and made me want to kill someone for that entire week.
HELP!!!

Mike Sigmond

I would contact Chevy headquarters about that one, Maybe they will recommend a dealer that will help you free of charge. Or just to simply report what you went thru. Im my mind thats bad buisness that the Rep for the car doesnt know whats going on. Like Bobs garage will have a better opinion, yea right.

Good Luck! I didnt find a phone # but here is the home offices address if you want to write them a letter.

General Motors Corporation
P.O. Box 33170
Detroit, MI 48232-5170

therewolf

Sounds to me like your bad ground strap is using the speedometer cable as a substitute electrical circuit perhaps. That juice still wants to get back to the battery, even when the normal wiring quits.

shawnvw

All of the problems stem from the VSS not working. While driving, the Speed Conpensating Volume on the radio turns the volume up by "two clicks". When the VSS drops out, the radio thinks the vehicle is standing still and lowers the volume.
The same thing happens with the locks. Your vehicle is set to lock the doors after your start rolling.....if there is no VSS, the vehicle doesn't know the truck is moving...once the VSS comes back, it see's you're in motion and locks the doors.

Here's a bulletin that might help you diagnose the problem a little bit....








The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Condition/Concern:

Vehicles equipped with the Allison LCT1000 Automatic Transmission may exhibit one or more of the following conditions:

DTCs P0721, C0237, P0500, C0055

Speedometer inoperative, erratic or drops out

Cruise control inoperative

Radio speed control volume (SCV) inoperative

Transmission shifting concerns

ABS light illuminated

The following diagnostic approach may be helpful in repairing any of the above conditions.
The testing should begin at the VSS sensor located in the rear of the transmission on 2WD models and in the transfer case on the 4WD models. An erratic signal caused by a loose or damaged reluctor wheel can cause any of the above concerns.

Recommendation/Instructions:

On vehicles equipped with the Allison LCT1000 Automatic Transmission, the VSS signal is sent to the Transmission Control Module (TCM) via Circuits 821 and 822.
The signal from the VSS to the TCM is an A/C signal that produces approximately 28-34 HZ per 1 mile per hour. This signal can be checked using a Fluke 87 (DVM) set on the A/C voltage scale with hertz function activated and connecting leads to Circuit 821 and 822.
The TCM converts the VSS signal from an A/C sign wave to a digital wave. The TCM sends out approximately 4.5 volts on Circuit 400 to the PCM/ECM. With the vehicle stopped, the DVM will read approximately 4.5 volts on circuit 400, but once the vehicle begins to move the voltage will drop to near 0 volts. To check the voltage with the DVM with the vehicle moving, the min/max function should be set to 1 ms, the max should be 4.5 volt and the minimum should be -4.5 volts. The reason the reading goes negative is because the TCM converts the A/C signal from the VSS to the exact digital wave, therefore when checking the HZ signal on circuit 400 the fluke meter can be set on the A/C or D/C signal with hertz mode active. Circuit 400 can be checked by backing out the terminal at the ECM/PCM connector and placing the red lead of the meter to Circuit 400 and the black lead to ground. The signal from the TCM on circuit 400 will be the same HZ reading as the VSS approximately 28-34 HZ per 1 mile per hour.
The vehicle speed signal to the IPC, radio, cruise control module, and body control is provide by the ECM/PCM on Circuit 817.

Mike Sigmond

Very Interesting, Its good to hear this from someone that knows what thier talking about. I am still amazed that the dealer you took it to did not know this and charged you for wasting your time.

allbusinessbuggy

Thanks for the help. You're right, Shawn, everything stemmed from the VSS. Told the dealer the problem was solved by a blind man with a computer (someone on an internet forum that had never seen the truck). Then told them I would never darken their doorstep again for any reason, I actually think they were happy about that. Thanks again.