troubleshooting code 24 (VSS) on a 1987 R10 Chevy Suburban 350 TBI

[skizzik]

Can anyone tell me how to troubleshoot this code or provide me a troubleshooting diagram for this code? I think the VSS code is what's causing my Burb to occasionally stall at lights and my Haynes manual doesn't say much about the VSS and haven't been able to find much online.

My speedo doesn't work. It went out approx 2 years ago (it intermittently would make this whirring noise and the speedo gauge would bounce back and forth and then eventually stopped working altogether) and I hadn't fixed it because I didn't think I needed a speedo and I didn't know it was related to my stalling. From what I understand, the troubleshooting process is different if your speedo doesn't work compared to if it does work.

Can someone tell me where the VSS is in my truck? I've heard it's by the transmission, then I've also heard it's on the speedo, I've also heard some trucks have a VSS "buffer" and I don't know where that is. My TPS tests fine all throughout the range so I don't think it's the problem.

Thanks.

[RandyP]

I'm wrong more than right, but here goes.

I thought the VSS was the box that plugged in to the back of the speedometer, and then the other end of the wires went to the ECM.

In the photo the little green box is what I thought was the VSS, it has a sensor that is held in place with one small screw to the back of your speedometer.

[Jonboy]

Randy is pretty right. That is the VSS buffer; it emits a light that is broken by a black spot on the back of teh speedometer head, and counts the pulses. You really need the speedometer cable hooked up, VSS will affect idle, canister purge, TCC lockup, and can damage the trans (TCC slippage) and make the engine and trans run hotter as well.

[skizzik]

OK I tried replacing the speedo cable but it turns out that the problem is inside the transmission. The greenish plastic piece with teeth (that the speedo cable plugs into) that's near the transducer is worn out.

I've heard I'm going to have a hard time finding that part. I don't see it in my LMC truck catalog. Anyone know what that piece is called and where I can order a new one? (prefer not through dealership). I'll go to a junk yard if I have to but I'd rather have a new one.

[skizzik]

I've already talked to 2 mechanics and they both said "We can't get a piece that old. You'll have to talk to the dealership"

[skizzik]

It's the speedometer driven gear that needs replacing. Thanks to everyone for your help - couldn't have done it without you.

This thread can be closed.

[clinkerclint]

Sorry to bring up a dead post but my 88 chevy one ton/350/AT has the exact same issues. Stall at light throttle and the speedo either does not work or bounces. Did you get the stumble fixed?

[skizzik]

Originally Posted by clinkerclint

Sorry to bring up a dead post but my 88 chevy one ton/350/AT has the exact same issues. Stall at light throttle and the speedo either does not work or bounces. Did you get the stumble fixed?

Yes, I eventually realized that the computer was flashing a VSS error. When this happens, the engine doesn't know that the computer is moving and tries to maintain constant idle (I think that's how it works), which causes all the trouble. Here's what I suggest:

- is your speedometer working reliably? If not, you're probably getting a VSS error. I believe it's a code 24 of the service engine soon light.
- could be that the speedometer cable needs replacing, could be that your vss sensor is faulty or isn't reporting to the computer, or could be that the speedometer driven and/or drive gears are stripped and need to be replaced. This last option (the speedo gears) is what I'd investigate first if you're speedometer isn't working at all, as those gears do wear out over time. What caused my problem was a tranny mechanic put the drive gear back on the drive shaft in the wrong spot after rebuilding my tranny, which meant that the drive and driven gears weren't even touching.

On my suburban, the speedo cable plugs right in the back of the speedometer. If yours is the same way, you can unplug the speedo cable and watch it while you drive and see if the cable turns. If the cable looks like it's turning like it should, the problem is with the speedometer or the vss sensor itself.

Hope this helps.