I'm seeing some issues with my truck giving a little 'jolt' as I maintain speed just below the shift point between 3rd and 4th gears. It's a 1991 with the E4OD tranny. If you're not accelerating, it'll try to drop into 4th at about 32mph. If you try to stay steady on a flat road at 30, it'll sort of chug like it doesn't know what to do.

I'm thinking speed sensor, even though no codes are coming up. I had thought that the speed sensor was in the rear diff, but the chilton manual says that's the rear anti-lock brake sensor? They say the speed sensor is on the tranny itself. I've seen several posts with similar issues replacing the sensor on the rear diff.

Any idea which one of these I should be chasing?

 

the speed sensor is the same as the ABS. its the only electrical connection that i was informed of on the read end.

 

Yeah that speed sensor is technically the ABS speed sens
tho the EEC, speedo, ABS, and I think the cruse control uses it.

my 95 had a squashed wire back at the pumpkin every time I would hit a bump my speedo went to 0 engine and ABS popped on if it was during a shift
that freaked out and if I was breaking at the time the pedal would thump hard.
Almost caused 3 accidents.

Now for the VSS and TSS the EEC uses both signals for shifting and TC lock up times.

 

'87-'91 the sensor in the rear end housing is the RABS sensor. '92-'96 it is both the RABS and VSS. This is because '87-'91 have the mechanical speedometer, so on the speedometer cable where it attaches to the transmission there is a VSS. You will see the wires plugged in to where the cable attaches.

 

Sweet learn something new all the time.
Now for those years does the EEC/ECU use both VSS and TSS
for TC lockup and trans slippage?

 

I've tracked down the locations of both sensors, and found both on Advance and Oreilly's websites. Is there a clue as to which one (if either) would cause the truck to buck when it's close to a gear change? I would guess it's the one in the tranny, but my guesses are usually wrong.

 

Honestly, I'm not sure why the '87-'91 trucks wouldn't use the axle sensor as a VSS, but there's a second one on the tranny for a reason. Maybe they didn't think of it until later, when the speedometer cable was eliminated. I'm pretty sure the one on the tranny gives input to the EEC for shifting and the one on the rear is for the ABS only, but to make certain see which sensor has the grey wire with a black stripe. That's the wire that carries the signal to the computer. I don't have diagrams for the ABS so I don't know what color the wire for that is.

 

Maybe I was jumping to conclusions (or maybe I've got another problem)...

I got a memory code 542 tonight - my book says "Fuel pump secondary circuit failure - PCM to ground"

Any thoughts? Maybe a loose or almost broken ground wire somewhere between the PCM and fuel pump? Any other common causes of this one?

 

...been thinking about this.

I recently removed the front tank completely - left the wiring harness zip-tied to the frame rail - could that be causing the fuel pump code? If so, that might save me a lot of exploration underneath the truck, but puts me back to where I started on the VSS.

Should I jumper the connector or switch somehow to 'wire around' the missing front tank? The chilton manual shows that on single tank trucks, the selector switch is jumpered somehow?

 

As long as you don't select the removed tank then you don't have to do anything with the switch. Leaving it in the position of the remaining tank is the same as jumpering it. If you have moved the switch then you could get that code. I'm not sure what else to check for that code, I get the 2 digit same code on my truck all the time. I'm thinking that since my fuel pump is starting to fail then that's why I'm getting it.