I've got a "new to me" 91 Jimmy. Has an issue where the speedometer acts erratic. Can't pin it to an exact speed, happens intermittently. Replaced both the VSS and DRAC, no change. dyeager535 recommended I log some data, see if its just the gauge or if the ECM is getting a bad reading too. According to the log attached below at around 2 mins, the speedometer dropped out and the ECM also wasn't getting any signal. Also, I've included a two links: first is what happens when it starts acting erratic (little to no speed reading) and the second video shows when I'm accelerating onto the highway, speedo hovers around 35 mph even though I'm accelerating. Once it shifts from 3-4, it shoots up to 55-60 and acts normally. Let me know what you guys think. No trouble codes either
CK5
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Erratic Speedometer - 1991 Jimmy
- Thread starter dj31499
- Start date
If both the speedo and ecm data drop out at the same time then you look at the common denominator. That’s the wiring to the drac, the vss and the tone ring.
There will not be any codes set. That system isn’t smart enough to do that.
My money would be on the vss sensor or the tone ring walking out of the window for the sensor. There are two snap rings that hold the the tone ring in place. It would be odd it would be missing one unless someone forgot to put the backside snap ring on the ring could move back on the shaft.
Should be pretty easy to pull the vss and see if the tone ring moves.
There will not be any codes set. That system isn’t smart enough to do that.
My money would be on the vss sensor or the tone ring walking out of the window for the sensor. There are two snap rings that hold the the tone ring in place. It would be odd it would be missing one unless someone forgot to put the backside snap ring on the ring could move back on the shaft.
Should be pretty easy to pull the vss and see if the tone ring moves.
I'll have to take a look at that tomorrow. Thanks for the advice.
This may not end up being related but I was flipping through the manual and found this (see attached). I haven't gotten any brake light codes, but I realized now that the brake light on the dash never comes on. Not even with the e-brake on. So I may have to track that issue down and see if that code shows up. I checked voltage on pins A and H which the manual says is for diagnosing the anti lock brakes and I'm getting about 9 volts (key on engine off) (says 8-15 volts is okay).
This may not end up being related but I was flipping through the manual and found this (see attached). I haven't gotten any brake light codes, but I realized now that the brake light on the dash never comes on. Not even with the e-brake on. So I may have to track that issue down and see if that code shows up. I checked voltage on pins A and H which the manual says is for diagnosing the anti lock brakes and I'm getting about 9 volts (key on engine off) (says 8-15 volts is okay).
UPDATE: Sorry for being offline the past few weeks. I tried to take a look at the tone ring, but I couldn't see much. Tried to get my phone in there to take some pictures but no luck.
I did fix the brake light and it is giving me code 6. Will try to diagnose with the trouble tree from the service manual.
I did fix the brake light and it is giving me code 6. Will try to diagnose with the trouble tree from the service manual.
I've attached a checklist of what I've gone through on the trouble tree and a diagram from the wiring manual to show what I did. Probed the green/black wire at the DRAC connector and at the pigtail end where it hooks up to the VSS. Got continuity on the multimeter, resistance was 0.5 ohms. VSS should have a resistance between 900-2,000 if it's good - mine read 1,200. I've already checked the voltage between A and H on the diagnostic connector, but I'm not sure where the ECU 6 way connector is (trouble tree says to check voltage between A and D). In the meantime I'll try to figure out where that is and test it, but if it checks out I'm not sure what else to do.
A and D are connectors on the RWAL Harness. Voltage there is 10V, so that checks out too. At this point is it worth it to remove the tail shaft housing on the transfer case so I can get a look at the tone ring?
kingsizexl
Registered Member
I had the same issue, the speedo would jump around, especially at about 35 mph and then drop out to zero. Very erratic. It wound up being a fairly easy fix. Make sure the wiring from your speedo sender at the transmission to the front of the car is good. I'm not sure if they are all the same but mine had the connector at the transmission sensor that ran about half way up the left side to another connector right about to where the automatic shifter linkage is and had another connector that then ran the wiring up to the engine compartment and into the firewall. The second connector was loose and as soon as I cleaned it up and made sure everything was tight, no problems since. Its only two wires and in the open, so very easy to trace/follow. I'd start there.
I think I know the two you're talking about. I think I checked continuity at both and it was good up to the DRAC. But I'll double check and clean them up real good and let you know what happens. Thanks for info, glad to know I'm not the only one that's had this issue!
Took a second look and look at what I found - right at intermediate connector near the transmission... Makes sense that I'd get continuity since the wire itself doesn't look like it's inbad shape but once you start driving I can imagine it not making the best connection.
kingsizexl
Registered Member
Yep, looks familiar. I think mine (and yours) might have been rubbing on the transmission and maybe shorting out which caused the needle jump. Not sure if you had this issue with it also but my fuel economy dropped to like 2-3 mpg and as soon as I fixed it, it jumped back up to where it use to be.
I've only had the truck for about a month so not too sure on the fuel economy. But it would make a lot of sense. If the ECM thinks you're not moving or moving slower than you actually are, the amount of the fuel it thinks it needs is going to WAY different. I could've lived without a working speedometer for now, but the fact it also plays a huge roll in how the engine runs is why I wanted to fix this.
Someone obviously tried to fix it before with that tape.
I've had decent luck depinning the connectors and using heat shrink tubing if you can force it over the terminal. As long as one wire is insulated, you can also, after insulating it, use a larger shrink tube section to cover both wires.
Won't make a perfect seal, but plenty of dielectric grease in the connector should keep it from getting worse. Usually the connectors arent in a place where the wires can ground out if they are exposed ONLY right up against the connector body.
I had a fuel pump relay wire like yours, but it wasnt grounding out. Eventually I just decided to eliminate it as a potential problem, got the right terminals, cut the wire back, and re-crimped it.
I've had decent luck depinning the connectors and using heat shrink tubing if you can force it over the terminal. As long as one wire is insulated, you can also, after insulating it, use a larger shrink tube section to cover both wires.
Won't make a perfect seal, but plenty of dielectric grease in the connector should keep it from getting worse. Usually the connectors arent in a place where the wires can ground out if they are exposed ONLY right up against the connector body.
I had a fuel pump relay wire like yours, but it wasnt grounding out. Eventually I just decided to eliminate it as a potential problem, got the right terminals, cut the wire back, and re-crimped it.
