Hi all
My good old 110k mile 89 260hp 6 sp C4 w/w-51 had run so well I went ahead & had the motor re-built at 100K. Then at 110k I drove it to the jiffy lube to get it safety & emission tested & during the test motor died & wouldn't re-start. Had it towed to Tunex & asked them what they thought & could it be the fuel pump. Tech that worked on my car told me his buddy had the same car but it needed a new computer. Picked up car & they had replaced the fuel pump but said the problem was bad #7 injector they also replaced. Driving the car away from Tunex I noticed the speedo now read only zero & the motor drove rough & stalled when stopping. Had a trusted mechanic install new computer which made motor run better but I still get steady check engine light on hard accel that then turns off after a minute. Still no speedo most of the time. Every once in a while the speedo gives a low reading & then what seems like the correct speed for a few seconds but mostly zero reading at dash. Any tips as to if I need a new vss, vss gear, how to check codes? Thanks for any help.
Brad

 

The 89 used Multec injectors (gray body color). If one was bad, they all should have been replaced. The coils short out. They should measure around 16 ohms.

A plastic gear on the transmission output shatf drives the Vehicle Speed Sensor gear and when rotated generates a sine wave that goes to the dash cluster. The dash cluster processes the signal and displays the speed in the LCD. The speed signal is also converted to a 2k pulse per mile signal and sent to the ECM.

Why the speedometer stopped working after Tunex worked on the car makes no sense.

You can remove the VSS and attach a drill to it and see if the speedometer displays the
proper speed. There are two wires at the VSS connector. Yellow wire is the signal from the VSS to the dash. The Purple wire is grounded by the dash.

To pull codes from the ECM you jump pin A to pin B and turn the ignition to On.
You will hear the fan come on and a clicking noise which is normal.

The Service engine soon lamp will blink the number 12 three times, then if there are any trouble codes 3 times. When all codes have been displayed it will blink 12 again three times indicating all codes have been displayed. Turn the ignition off and remove the jumper wire when done.

Example 12 would be Blink short pause Blink, Blink long pause next code.
Example 24 Blink, Blink short pause Blink, Blink, Blink, Blink.

To clear the codes, disconnect the negative battery cable for a few seconds.

 

Thanks Hooked!
I'll check for codes today.
Thats a great explanation w/diagram & everything.
The Tunex my car was at is owned by a long time friend of mine that I've taken stuff to for decades. When I dropped off my car there was a trashy looking C4 parked in the back & the kid that worked on my car said it was his buddy's who'd left it there unable to afford parts. Said that car need computer work & etc. I picked up my car after three days & the trashy car was gone and my car had new problems but ran. The kid that worked on my car was fired for stealing parts off customer's cars. I'm wondering if he snagged my computer & VSS for his buddy's car. Before I buy a new vss was wondering if nylon /plastic gears on the vss strip & would maybe give a small reading from time to time? I will remove the vss & spin with drill asap. Will check hooked to dash still and check disconnected with ac volt meter.

 

It's possible the VSS was swapped and has the incorrect gear on it which isn't meshing with the gear on the trans output shaft.

Another possibility is that the VSS driven gear was destroyed by somebody who put the drive gear in the wrong place on the output shaft.

 

Neither of these are likely. The car's an M6 and there's not an install choice for the drive gear.

OP - your VSS in an '89 M6 is black and it would be very easy I'd think to tell if it had been tampered with.

VSS generally isn't sporadic, it either works or not and doesn't display "random" speeds. If the connector were loose at the VSS it could certainly be likely sporadic but when it made correct contact it would likely be consistent If you remove the VSS I believe you should find a purple 43 tooth gear. If you spin the VSS and get no speedometer, check it with a DMM or VM and expect minimal AC voltage. Set your meter for AC not DC.

You mention you had the ECM replaced. Do you still have the core? Ask the fellow who replaced it if everything in the ECM vicinity looked to be correct and installed properly. Thieves aren't generally very tidy and maybe it was obvious someone had been there. Shops have garage insurance, I'd maybe be asking. If your original ECM had never been replaced or tampered with you should be able to have it scanned for a PROM ID and if that ID turned out not to be appropriate for an '89 M6 car you could be on your way to sorting out the problems.

 

You think your computer landed in the tech buddy's car?

 

I missed that in the original post. I have seen VSS gears that were thrashed by having the driven gear in the wrong place on a 700r4 output shaft, which made me suggest that as a possibility.

He said something in post #3 that suggested that maybe the VSS had been swapped, which is why I suggest that maybe it now has the wrong gear.

A VSS is basically just an AC motor, so it's pretty rugged and hard to hurt one. It's much more likely to be a problem with the connector or wiring.

 

Sorry guys for not posting on this. Have been in & out of the hospital with some health problems/surgeries. I hope to be able to look at my car soon. Thanks for your responses!
Brad

 

Hey, where you located, there is a lot of us Utah Vettes guys who can help.

 

I had a similar problem, turned out to be a bad wire coming from the transmission..