Speedomoeter issue, but is it the ECM,BCM, or whatever they call it?

[AKMark]

Hello everyone,

I'm gonna feel stupid as I've been on many boards and even moderate a few and this is where I find my experience with wrenches lacking.

I picked up a 1991 V1500 Burb with a dead 6.2L recently. Rebuilt the motor as one of the connecting rod caps failed and led to crankshaft failure as well as a few other problems. Put the rebuilt motor in and runs like a champ, except that the speedometer doesn't work. I have also noticed the tranny doesn't want to shift into overdrive, but I think the kickdown linkage is out of whack as well, I will adjust that tomorrow. My first thought was the VSS on the T-case. I have a buddy with a 90 K-5 with the same sensor, so I swapped it, nothing changed. I would test my old one, but his K-5 is currently out of commission.

I pulled the cluster and found a really bad splice that went to one of the wires on the speedo, so I repaired it. Now instead of rising just above the lower side peg on the speedo, it rests on the peg. Did I make it worse? Right now, I'm leaning toward two different possibilities, first, the computer that rests by the brake booster. I believe the wiring goes to there (correct me if I'm wrong) and don't know if that is really functioning or not. My buddy offered his K-5's to try, but it's a 5.7L and even though they look similar, I wasn't sure if they were compatible. My second thought on this is I have a short, but not sure where as of the moment. I could start shooting wires, but that's something I don't want to do unless I have to. I did just realize I could potentially borrow his cluster to try it out as well, maybe I should try that.

Anyone have any ideas or suggestions to try? This is a really nice very loaded 1991 that runs great thanks to a fresh 6.2 and a tuned up injection pump. I cannot wait to actually take it out once this is fixed.

[AKMark]

Well a little more progress, the speedometer still doesn't read, but it no longer sits against the lower peg.

I did find the check engine light on, and I cannot get the codes to display, I slaved in another computer (the one near the brake booster) and still the same issue.

I think my transmission is fried. I did notice it about two quarts low (I haven't driven it farther than two miles or so and I added a large auxillary cooler) so I topped it off and shifting from 1-2 is really delayed, I have to floor it and eventually it will shift. 2-3 is okay, and it won't go into 4th. I made sure the kickdown cable is adjusted. The fluid in the tranny is slightly burnt. I was debating on changing the filter and seeing if I can breath new life into it, but at this point, a rebuild seems to be in my future.

[diesel65]

If I remember correctly there was a GM service bulletin about having a check engine light on and only pulling a code 12.
The fix was to replace the vacuum pump drive housing that has the engine speed sensor on top.
I think if you looked at it closely you may see the plastic case is cracked.

[AKMark]

The vacuum pump is fine, I just examined it while doing the engine rebuild.

[Robyn]

The little thin black magic box out by the booster is the controls for the rear wheel ABS only.

The VSSB is under the dash if memory serves as is the main computer.

Should all be over behind the glove box
I have a factory electrical diagram book on these trucks.
I will sort this out and get right back here.

[AKMark]

Thanks for the ideas folks.

[Robyn]

YESSSSSSSSSSSS the module out by the brake booster is the "Rear wheel anitlock controller mudule"

[AKMark]

That would explain why a 1990 K-5 with a 5.7L and a SM465 would have one as well.

I guess I can put that one back in my buddies Blazer.

[AKMark]

Well a few updates,

I found the TV cable was so messed up it wasn't moving at all. I replaced it and since I had a huge transmission fluid mess, decided to change the filter before I refilled it. I found the TV assembly return spring was very loose and not returning the cable back to the closed throttle position. So I took my handy pliers and bent the spring so that it was useful again.

Put it all back together, and smooth shifting from 1st to 4th. No problems except I didn't get a lockup, and my speedo isn't working, nor is the cruise, and my check engine light is still on.

So, I'm thinking that my ECM is bad. I've replaced the VSS once, so I'm thinking that isn't it. I do have another cluster I could try, but with the cruise not working, I don't think that's the issue.

I tried to pull the codes, but the light never flashed. Anyone have any ideas before I special order one of these things?

[AKMark]

The little thin black magic box out by the booster is the controls for the rear wheel ABS only.

The VSSB is under the dash if memory serves as is the main computer.

Should all be over behind the glove box
I have a factory electrical diagram book on these trucks.
I will sort this out and get right back here.

VSSB?? What is this object of wonder?

[AKMark]

I'm kicking myself, found both the ECM ground and the grounding strap swaying in the breeze by the firewall. Hooked them up and it's almost perfect. Everything appears to work except the cruise, and the check engine light flickers. I got a code 32 and 33, which I believe are vacuum/EGR related codes. I'll try to find a decent vacuum diagram and ensure things are routed right. I did notice the EPR valve line was no longer connected, but I can fix that one easily when it's cool tomorrow morning. That might be the reason I'm getting the codes and the cruise isn't working. We'll find out tomorrow, thanks for the help folks!