Check engine light just came on

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In that year Tom, they used a PCM (Power Train Control Module) which inputed into the ECM (Engine Control Module) which sets the "Check Engine Soon" light. This was done as a forerunner to OBDII. It is kind of a hybrid system. And you are right, it could be a 85 depending on the GVW weight of the vehicle. Sometimes it could be as simple as a bad ground or a corroded connector. From what you are saying, I would be very hesitant to condemn the transmission.
 
I agree with CW. Tom is he done with it for now? Have you checked and cleaned the ground on the left rear side of the engine? If not do so. Also unplug the wire going into the trans to make sure you have a good connection. The speed sensor is directional, meaning the pcm is relying on the output to tell the trans what to do. If you have a bad ground the trans won't communicate with the pcm correctly. I still say to check the alternator to be SURE it isn't putting out ac voltage. If it is your system will go CRAZY. What codes is it throwing with the scanner hooked up?

charlie
 
gwcowgill said:
Tom,  from what you are saying, 1. It sounds like you are describing a 4L80E transmission with an input and output speed sensor. 2. When you step on the brake pedal the brake switch tells the transmission to stop coasting and allow engine braking. 3. It actually souinds like you may have a PCM problem that may be causing your problem.  I just ran a description of what you are experiencing and my son agrees that there very well be nothing wrong with your transmission.
Click to expand...

Sometime in the next few weeks or months, If you are passing by, maybe have a look see??
 
Tom,  The mechanic needs to look at signal voltage on all the pins on the PCM. This can be time consuming but sometimes is the only way to track down a problem. I feel there is nothing wrong with your transmission especially to the point where a $3000.00 replacement is needed. The 4LxxxE transmissions were very reliable units.
 
Tom, I know this is a dumb question but, Did he check ALL the fuses in the fuse block? I was just studding my manual. I wish there was a way I could scan and send it to you. I don't think it is your trans. There is a part called a vss speed buffer. If it doesn't see the correct in put 12 v or the refference low volt it will cause a 2nd gear down problem. it is code 72, which is a soft code, won't turn on the light but he should have seen it with the scan tool. He should have been able to see the speed increase and decrease with the scan tool. If it doesn't show speed up and speed down it is the speed sensor or wiring going to it. Awful hard to trouble shoot 1500 miles away. If the trans shifts fine, I don't believe it is in the trans. You need to find some one with the correct manual to trouble shoot this. With it working ok cold but not hot tells me the temp thermister is working ok. The ground for the vss module is on the rear side of the left head. Have you tried turning off the cruise control? The info also goes to the cruise control module.

Boy GW you are a fast typer. I think we're on the same track.
 
No, I did not turn off the cruise control. I thought about it but forgot about it. I am going out tomorrow to dump my tanks, I will try to remember to turn it off.
 
Sounds like what we used to call a "cowboy", diagnosing and fixing your coach by parts changing. I suggest you get to a competent location.
 
Before you do, check all the fuses in the fuse panel and remove that black ground wire on the head. Clean the crap out of it. Do you have a meter for dc volts? I know how hard it is to get under the dash on these but I bet we can find the problem. I'm an old fat man, and have spent many hours under the dash on mine. You've got nothing to lose at this point.

Time to go blow some snow, later.
Charlie
 
92GA, the VSS buffer is very likely the part that is causing the problem. I don't have a manual here but do a lot from memory which is going down hill. It sounds as though we may be narrowing in on the problem. If I remember correctly, when at the GM school, they had quite a few problems with the VSS speed buffer.
 
Don't know how anybody could remember all this. Did you work on um? I had to buy a manual for the chassis because I couldn't find anybody that could help me around here. It has more than paid for it self. If Tom is willing to do some testimg and has a meter I think I can tell him where to take the readings nessesary to get this thing going. My money is on a fuse or a ground, or bad connection. It's going to be tuff though, beings Tom don't have a scanner.

Charlie
 
OK been studding for the last couple hours. I'm convinced the problem lays in the vss speed buffer module, book says it's located on the brake structure. You have an input and an output sensor that are controlled by a steel reluctor wheel in the trans. The sensors generate power, imput goes to the pcm and the output goes to the vss buffer module. The front sensor is the imput, rear is output. The pcm calculates the difference between the two and controls the trans. The vss module is the componet that sends the signal to the speedo. the vss module sends vehicle speed and trans output speed to the pcm. The speedo is NOT fed any signal from the pcm what so ever. Did he replace the output sensor? If so it has to be the wiring from the sensor/generator, or the vss module. The module can't cost that much, but has to be replaced with the exact part number. Almost all trans codes are considered soft, and will reset with the ignition key, off/on will clear them.

The vss module will have 7 wires going to it c7 grn/yel from sensor, c8  blk [engine ground]  c9 pnk/blk ecm-crnk  10 amp, c11 dk blu, c12  ltblu from sensor, c13 brn to pcm, c15 blu/blk to speedo.
Charlie
 
Thanks for the info Charlie. The whole problem here is that I have already sunk $400 into fixing this problem and I have gotten nowhere. I am not really excited about the idea of sinking a bunch more money into this thing. He did replace the output sensor.
 
Tom you can check the wires yourself, by pulling the connector off the sensor/generator, and the vss module. Put a paper clip in the wires on either end and measure with a meter. If you have have no resistance it has to be the module. They can't cost that much.

charlie
 
I really don't have any idea of what you are talking about. I don't know where the sensor/generator is located or what it looks like. Nor do I know what the vss module is or what it looks like or where it is. Exactly what wires are you talking about? You will have to explain it to me like I was a little kid.
 
OK the output sensor is on the rear of the trans, has a plug in with 2 wires, one is grn/yel the other is lt blu. If you unplug the sensor and unplug the vss module under the dash. Use a paper clip to jump the the wires at the sensor. Use a meter set to ohms and touch to the same color wires on the vss plug, you should have .02 ohms or less. If you have no reading you have an open condition, broken wires. If you have a real high ohms reading you have a bad connection or a chaffed wire. and that condition must be repaired. If you have the low ohms reading the vss module is bad. I'll go out in the morning and look at mine to get an idea what it looks like and let you know. 

Charlie
 
Well I have a bad back and I can't crawl under the RV or under the dash. That means I have to hire someone so it will cost me a minimum of $100 just to get someone out here to do the job. I really don't want to spend another $100 plus parts.
 
Tom,

Keep good notes on this and when I make it down there (hopefully in a couple weeks) we can test the vss and see.

Steve
 
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