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1991 Caprice 5.0ltr speedometer stopped working. I replaced the VSS and PCM but still doesn't work

Customer (name blocked for privacy)

Status: Closed Value: $5

Have you tried turning the vss by hand to see if the speedo will read at all? If it doesn't read anything then the problem is 'upstream' ie: wiring or control head etc. If it reads when you spin it by hand, check the drive gear in the transmisson to make sure it has not broken or 'Apple Cored' (worn out in the middle so bad that it can not contact the driven gear - the driven gear can be worn out too, there should be no indentation in the middle of it either.

Wes Harrison
Above and Beyond Auto Repair

Reply to Wes Harrison's Post: No I haven't tried that but I will. The gears appeared to be ok when I changed out the VSS. Shouldn't I be getting a voltage reading at the electrical plug in for the VSS? I am also not getting any voltage at the 5 amp fuse location for the PCM at the fuse box. The fuse is good. I had the iginition switch to on while performing the voltage tests. Should the engine be running? I will try to move the VSS and get back to you.
Thanks,
Frank Riley

the vss will create it's own voltage when you spin it. That's how it works (simply put). No the eng does not have to be running but the key needs to be on

Wes Harrison
Above and Beyond Auto Repair

Be very careful when checking for voltage with a grounded test lamp. You could draw to much current through the computer and fry it. When checking and back probing for reference voltages to and from the computer, use a high impeadence digital voltage ohm meter or a self powered test light. These tools will read values without acting as a dead short to the computer. A non powered test light is just like shorting a paper clip to the computer because it requires 12 v to work and the veh computer operates on a five volt reference signal.

Wes Harrison
Above and Beyond Auto Repair

Reply to Wes Harrison's Post: I tried spinning the VSS in both directions with the ignition switch on and the speedometer did not move.

Ok - be right back looking for the electrical schmatic for you to follow. Question - does the car shift funny? The reason I am asking is that the vss sends a signal to the computer for controlling the shift points and lock up. I would normally hook up a scanner to see if the vss signal is making it to the computer. If it is not shifting funny, it probably is reading ok. and the problem might be in the control head. Another Question - is the check eng lamp on? if not and the car is not shifting funny then it points towards the control head. On a 91 it is replaced as a whole unit. P.S. I'm kind of new to this and I couldn't transfer a schematic for you - sorry but will try again.

Edited by MastrTech on October 12 2005 at 8:10 PM

Wes Harrison
Above and Beyond Auto Repair

Reply to Wes Harrison's Post: Thanks for the info, but I already knew that, and only use a digital meter for these tests. What I don't know and was hoping to get an answer on is the fact that there is no voltage at the fuse panel at location #20 for the ECM; SIR (5 amp). I am getting voltage to and from everything else on the fuse panel. I tried moving the wires on the back side of the fuse panel near that fuse location to see if maybe connection was loose. The connection seems to be ok at this point.

Reply to Wes Harrison's Post: Sometimes the check engine light comes on but will go back off whenever I restart the car. This usually only happens when I use the cruise control. It shows a code 34 for EGR system problems.

Sometimes the direct drive doesn't release under normal driving conditions and will cause the vehicle to chatter or vibrate. I either have to let off of the accelerator or push down a great deal more for it to release or downshift.

Before I replaced the PCM or ECM it showed a trouble code for the TPS having low voltage. I replaced that sensor with a new one. I think that was a code 43.

A schematic would be great if you can. My book doesn't show enough to help.

Thanks,
Frank Riley

Wes, did you give up or are you still trying?

I didn't give up - I didn't feel good last night and I actually fell asleep, sorry. Hey with all the information you have provided I can tell you've learned alot. This is the best tip I can possiblly give you. Go to a public library and use thier online resouce computer - it has the AllData system available on it - you will be able to look up almost anything about almost any car and it's free to use (only at the library, if you want it online at home it's 24.95 and will only work for one vehicle). - also on the ecm/sir fuse, you may not have power at that point. I believe the sir system is the saftey inflation restraint -Air Bag, and there may not be any power at that ponit unitl it's needed. The codes you have been getting - non of them have been the vss, have they? That would indicate that the computer is recieving the sig from the vss but the signal is not making it to you (the speedo is not reading) or it should have set a code.
P.S. I tried to figure out how to transfer stuff this morning and I know I'm doing something wrong. I'll just have post a question and ask...

Edited by MastrTech on October 13 2005 at 11:10 AM

Wes Harrison
Above and Beyond Auto Repair

Reply to Wes Harrison's Post: Wes, Thanks for trying but so far the only info you have given me that I wasn't aware of is that you weren't feeling good. Sorry to hear that. I'm on my second day of a headache that won't go away. As soon as you are able to transfer the wiring schematic, I'll hit that green accept button.
Thanks,
Frank Riley

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Wes, here are the instructions on how to post images:

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If you still can't post the images, then let the customer know so someone else can post the schematics. The customer is more than willing to pay if someone comes through with the schematcis!

Regards,
Maureen
Moderator

Moderator

Thank you Maureen and I have Advanced post turned on now. Thanks again.

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Edited by MastrTech on October 15 2005 at 7:55 AM

Wes Harrison
Above and Beyond Auto Repair

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