300 miles from home and tranny wouldn't shift

[someotherguy]

When MY speedo in the '88 K1500 went wonky, it had nothing to do with the VSS. The entire problem was the DRAC module in the instrument cluster going intermittent. In that the '88 had a 700-R4, I only lost the converter clutch, not various gears. Also lost cruise control and ABS, all of that relies on a signal from the DRAC.

I don't know where the DRAC is on a '95. Perhaps integrated into the computer.

1992-1995 it's in that little white plastic container behind the glovebox area. 1995 being a one-year-only setup, it's attached under the plastic tray the PCM sits on, so it's slightly more annoying to access.

Mine had a VSS crammed into the transfer case.

Point is, "speedo dead and no upshifts" can be something OTHER than the VSS. Don't replace it "just because". Diagnose first, replace later. This could be a failed wire harness, failed DRAC, failed VSS...and probably some other issues as well.

While I do agree, I haven't seen many scenarios of DRAC failure on 1992-1995. I have however seen a few examples of odd transmission behavior like you describe experiencing, on the 1988-1991 trucks. The memory is extremely vague as the trucks weren't mine so I was hearing about it secondhand, and quite a while back. Speedo would go dead, and.......I want to say the TCC would unlock. The person I'm thinking of was in an '89 C3500 so that would have been a TH400. He'd smack the dash (I know; I know) and it would behave again for a while. This told me the problem was in the cluster, or possibly oxidation on the cluster connector pins.

Richard

 

[NickTransmissions]

@cdivine - If you want to be sure it's the vehicle speed sensor and not something else (and you haven't already bought another one), you can use a multimeter and test the vss...get the rear wheels off the ground and rear on jack stands (chock the front wheels or put the vehicle up on all four jack stands) then put the vehicle in neutral. Set your multi meter set to low ACvolt setting. Un-plug the connector at the trans, place leads on the vss tabs then have some one else spin the wheels - you should get a voltage changing with the speed of wheels. Some times the pins will get dirty or corroded/oil soaked and VSS will stop working. All these sensors are what are called hall effect sensors and create their own voltage that gets transmitted to the ECM. The ECM then translates that voltage signal to a road speed number and sends to the speedometer gauge in your dash.

If you get no voltage change or reading at all, the VSS needs to be replaced. If you do then, to @Schurkey 's point, the problem is elsewhere.

 

[cdivine]

Excellent info from everyone. Yes the VSS plug was checked out. I would check voltage off the VSS like Nick suggest but tranny shifts now. It was a brief intermittent failure. Shut truck off for 1min and magically everything was fine again. I'll try a new sensor and if I ever have problems again I'll start tracing wires looking for shorts or other issues.

One more thing, the speedo on this truck has always been exactly 5mph under actual speed. Completely stock. Could it be the VSS has been reporting low voltage this whole time? New sensor goes in this weekend.

 

[NickTransmissions]

Excellent info from everyone. Yes the VSS plug was checked out. I would check voltage off the VSS like Nick suggest but tranny shifts now. It was a brief intermittent failure. Shut truck off for 1min and magically everything was fine again. I'll try a new sensor and if I ever have problems again I'll start tracing wires looking for shorts or other issues.

One more thing, the speedo on this truck has always been exactly 5mph under actual speed. Completely stock. Could it be the VSS has been reporting low voltage this whole time? New sensor goes in this weekend.

Run the test with both sensors. If voltage readings are largely the same between them, the fault is upstream either in the harness, ground or at the connection w/ ecm (or ecm itself). And if that's the case, keep your current one in the truck at all times.

 

[Schurkey]

I would check voltage off the VSS like Nick suggest but tranny shifts now. It was a brief intermittent failure. Shut truck off for 1min and magically everything was fine again. I'll try a new sensor and if I ever have problems again I'll start tracing wires looking for shorts or other issues.

One more thing, the speedo on this truck has always been exactly 5mph under actual speed. Completely stock. Could it be the VSS has been reporting low voltage this whole time? New sensor goes in this weekend.

Voltage is less important than the frequency. Yeah, there has to be some minimum voltage, but the speed signal is based on AC frequency, not voltage per se.

I've never seen a VSS go intermittent. I've seen wire harness problems that acted up as the harness flexed, leading to intermittent problems. My DRAC was intermittent. But I've never seen the sensor itself become intermittent. I suppose it could happen...but the odds are against it.

 

[MekongMan62]

Ha! I had the nearly identical problem with my 97 5.7 Tahoe a couple of years ago. Intermittently, for about a year. Was driving me nuts. Replaced VSS, replaced transmission switch on the side of the 4L60e. Didn’t help.

Turns out that it’s the ignition switch on the steering column! Contacts get a layer of carbon on them over the years, and the thing is wired to the transmission. Transmission goes into limp mode and your gauges die when the connection goes faulty.

Cost about $125 or so for the new ignition switch…think I got mine on Walmart.com. I replaced it about two years ago and the problem vanished forever.

I’ll bet that’s what’s up with your truck, look into it. Good luck