91 Electronic Speedo - Who's the expert?

[dhcomp]

Ok, so a few weeks after my trans was rebuilt, my speedo started jumping around on the highway. Brake light also comes on, im guessing from ABS computer freaking out.

If its cold out, i can drive for an hour or so before speedo starts jumping. If its warm, i drive for 15 or 20 min and it starts jumping. Doesn't jump constantly, but will jump around 10 to 15mph every 10 or 15 seconds. Wierd.

Figure my VSS in the transfer case is bad. Pickup a used one, same EXACT symptoms, so that must be bad. What else could it be? Does the ring it read in teh xfer case ever break? Had a bad/hard shift right before it started freaking out the first time.

Any thing else to check? I'm thinking maybe there is a short somewhere......

Thanks guys!

 

[dyeager535]

If you can hook it up to a scanner, do so, and take it for a drive.

The signal is read from the toner ring, and interpreted at the DRAC. The ECM and Speedometer/Cruise/ABS do NOT share the output from the DRAC (ABS at least is much higher pulse count), so if the ECM speed is constant when the issue is occuring, it's very unlikely to be the VSS.

Assuming you have working cruise, does it continue to operate when the problem occurs? (you could infer proper VSS/DRAC operation from this, but not as good as ECM speed input IMO)

At first guess, if Speedometer AND ABS are freaking out, I'm thinking DRAC, VSS, or wiring of those two.

 

[dhcomp]

If you can hook it up to a scanner, do so, and take it for a drive.

The signal is read from the toner ring, and interpreted at the DRAC. The ECM and Speedometer/Cruise/ABS do NOT share the output from the DRAC (ABS at least is much higher pulse count), so if the ECM speed is constant when the issue is occuring, it's very unlikely to be the VSS.

Assuming you have working cruise, does it continue to operate when the problem occurs? (you could infer proper VSS/DRAC operation from this, but not as good as ECM speed input IMO)

At first guess, if Speedometer AND ABS are freaking out, I'm thinking DRAC, VSS, or wiring of those two.

Guess i found my expert .

Not able to hookup a scanner. My cruise has been funky in the last year, so i can't really determine anything from its function/disfunction. Any good failure points to look for on the drac wiring? I am getting ready to mod my drac, but have not touched it yet. Any insight as to where the harness runs from the VSS, or good ways to figure out if its shorting? What would happen if it was shorted to ground breifly? The condition i am experiencing?

I appreciate you help on this. Thanks!

 

[dyeager535]

The DRAC wires from VSS are twisted together, AFAIK they are the only ones on the truck this way.

To be honest I've never seen how the wiring was routed on the '90-91 trucks, so I don't have any idea on potential failure areas. I wouldn't expect the reluctor ring to go bad, if it did it would probably take the tip of the VSS sensor off, and you'd probably see that.

The VSS setup is pretty simple on the trucks, just to give you an idea. VSS has two wires to the DRAC, DRAC has a power and ground, and everything else that comes off of the DRAC is a single wire output to whatever component needs it. ECM, ABS, Cruise, Speedo.

Being somewhat time and temperature sensitive, I'd almost wonder if it isn't the DRAC itself, heating up and something going wrong internally.

If you've got any wrecking yards locally you can wander, Astro vans are a great source for the DRAC's, and of course all the '90-91 R/V's. I do know that there were wiring variations inside DRAC's (mainly sensor output locations) but I don't know what determined that. My donor DRAC came from an Astro. My experience with wiring it up "wrong" (used what I think was an '88+ C/K diagram) was that the cruise output was double what it should have been, but that's an easy correction to move the connector pins to the right location.

 

[dhcomp]

Ok, so 2 twisted wires from VSS join into a 4 wire weather pack connector with 2 wires from the rear 4l80e speed sensor deal. Try to un zip tie them to inspect, accidentally nick one with the clippers. Pull this whole Y harness out to inspect. Turns out, 3 or 4 places of insullation are GONE on one VSS wire, and the part from the 4l80e rear speed sensor has insullation breaks in almost every twist. Great.
Patch up VSS part, replace the wire in teh other part (didn't want to source new weather packs or a harness right now, might later), and cover all with electrical tape, then wire loom. Street tuner Red from kragen for those who care . Figure it will be easier to see what i've already gone through, plus see where it gets excessively hot or rubs.

Took it for a quick drive, speedo works, but not long enough to see if the problem is really fixed. I will report back. The 4 inches of wire from teh other side of the 4way weatherpack into the rest of the trans harness looked good, but i could only see a few inches before it went into a main loom.

 

[dyeager535]

Just an FYI, pretty much any of the GM vehicles that used the DRAC (Astro's, S10's, and C/K/R/V apparently) likely have the same harness, so easy to get "patch wiring" to repair or lengthen a harness. No idea if the 4L60E will have the same output wiring as the 4L80E, but I wouldn't doubt it.

Never seen one in as bad a shape as you describe, almost sounds like it had a lot of heat near it.

I won't pay for new connectors, stuff this "new" in the wrecking yards is typically still in great shape, and oftentimes the replacement connectors cost more than the sensor itself.

Sounds like you've got a handle on the issue though.

Did you pull the trans? I'm a bit suspect of the wiring "coincidence".

 

[dhcomp]

Just an FYI, pretty much any of the GM vehicles that used the DRAC (Astro's, S10's, and C/K/R/V apparently) likely have the same harness, so easy to get "patch wiring" to repair or lengthen a harness. No idea if the 4L60E will have the same output wiring as the 4L80E, but I wouldn't doubt it.

Never seen one in as bad a shape as you describe, almost sounds like it had a lot of heat near it.

I won't pay for new connectors, stuff this "new" in the wrecking yards is typically still in great shape, and oftentimes the replacement connectors cost more than the sensor itself.

Sounds like you've got a handle on the issue though.

Did you pull the trans? I'm a bit suspect of the wiring "coincidence".

I know every 90's 4x4 or 4x2 used teh same VSS, so woudlnt' be that hard to pull a harness out of a yard......but only places that will let you take parts of harnesses would be pick n pull.....and i REALLY don't want to crawl under a truck supported on old steel rims. Not worth my life.

Guess i'm stuck trying to order one, or find a salvage place that will sell me just what i need.

The part of the harness that was really bad was the 4 way to rear 4l80e sensor, and yes, it was pretty close to teh exhaust.

At first i figured i was worried about trans heat....until i started thinking about it. Then i zip tied the harnesses as close to the trans as i could to keep them away from the exhaust.

Thanks for your help man. I'm going to pickup my boat tomorrow, so i'll let you know how it performs.

 

[dyeager535]

Good luck!

One thing I've noticed is that on the Astro's, most of the wiring snakes around the motor/trans a lot, and almost universally the "doghouse" is removed from the cab, so you can get your hands on a lot of stuff without getting under the rig.

Around here, if an engine is pulled, most of the time they cut the harness (darn it), so getting "bits" of wiring or connectors is usually no issue.

 

[dhcomp]

Never thought of that on the astro's, good idea. Really not a frequent visiter to salvage places though, lets see if what i did works.......if it works, then acts up again, ill hunt down a trans harness. Thanks!

 

[dhcomp]

Works fine. Guess if you have an elec speedo, check your wires!