95 Ranger 4.0 Speedo testing question

[wolfman1979]

Have a 95 Ranger with a 4.0L, OBD-II with 100+ pin EEC-V PCM.

Speedometer/odometer stopped functioning recently, and following the advice from here (http://www.therangerstation.com/forums/showthread.php?t=18170) I tested the resistance at the VSS. My meter showed 205 or so ohms, well within the normal range but no speedometer.

I thought to try to repeat the test on the other end of the wire coming from the VSS, where it enters the PCM. Thinking was, maybe I have a broken wire. But before pulling the cover off the PCM, I wanted to ask if this is even a valid test. Plus, it seems from the wiring schematics I've looked at that there is only 1 wire feeding pin # 58, and I'm not sure how I would test for continuity with only one wire. Would I ground to the chassis somewhere?

Any advice/suggestions for testing this circuit downstream of the VSS itself would be appreciated. I haven't been able to locate much info, just that the signal from the VSS seems to go directly to the PCM. My local mechanic charges for plugging in the code reader, so I'm trying to diagnose as much as possible on my own first, and it seems like this should be something I can test ... if I can figure out how!

 

[Brutus93]

Look at where the other wire from the sensor goes (one goes to pin #58 on the EEC-V, the other goes to what? Ground?) Check that out... it most likely goes to ground. If that is the case, I would throw one lead on any solid ground-point on the vehicle chassis (unpainted steel... not a bolt/fastener).

Just so that you know, most sensors use chassis ground as a reference. If it is a 2-wire sensor they are typically signal and chassis-ground. I can go into a deeper explanation of the electronics if you genuinely want to know. My best advice is to measure resistance from the pin on the connector that goes to pin #58, and ground. This means:

1) Disconnect your battery.
2) Unplug the engine harness from the EEC-V.
3) Set your meter to ohms and place one lead on a solid ground point, and the other on pin #58 on the engine harness connector. It does not matter which lead goes where when measuring resistance... it is not polarity sensitive. Any further questions please don't hesitate to ask.

 

[Brutus93]

...are you sure your speedo is not cable-driven? I forget now... Let me know...

 

[Brutus93]

Ok, just confirmed it is electronic. Please follow my directions and get back to me. You will not do any damage measuring resistance with the battery disconnected. You may do damage if you leave it connected. You would also likely blow a fuse in the meter. I would suggest disconnecting both the positive and negative leads... not just the negative. This is just to be extra safe. Disconnect the engine harness from the EEC-V just to make sure you are not measuring resistance off of the computer... because you will. You want to measure apples to apples. Also ensure you plug the connector back into the VSS. By following what I have written, you should measure very close to what you did directly at the sensor. If not, the wiring is the issue. Once again, let me know.

If everything looks the same at the connector as it is off of the sensor, then it may be between the EEC-V and the speedo... let me know.

 

[wolfman1979]

Got it, Brutus. That's pretty much what I needed to know, where to ground to. Normally, I wouldn't have given it any thought and just picked a nearby ground in the engine bay, but when fiddling around the "brain" ... well, I know my luck and wanted to ask around before accidentally screwing something up.

Will post results here shortly.

 

[wolfman1979]

Well, no dice. Got an open circuit it seems, a break in the wiring somewhere between the VSS and the PCM. That is, if I was using a good ground when testing for continuity at the PCM harness. I used some obvious existing grounds nearby, but still picked up no resistance from Pin # 58.

I'm going to break out the jack and lift the truck off the ground to make it easier to get at the VSS plug, and check it's immediate ground first before trying to trace the wire across the top of the transfer case.

To be continued...

 

[wolfman1979]

I do have one other dumb question, though.

If I do have to replace the vss wire, no problem ... if I can figure out how to get the damned cover over the EEC harness off and out of the way. As it is, the thing is held in place with a couple of integral clips and the main center mounting bolt which secures the harness to the EEC. Only problem is that the bolt doesn't seem to come out of the harness, and so the plastic cover over the wires can't be moved.

Any suggestions as to how this cover is supposed to be removed?

UPDATE: Never mind, scratch what I said above. I DO have continuity at the pin. Managed to get the cover up enough to pierce the wire directly, and had continuity. Then I realized my tester's probes were slightly too fat to reach the metal contact on the inner side of the harness plug. Attached a bit of bare wire and tried again. Viola, I have good resistance at the pin, right at 203 ohms like at the VSS itself.

So, I'm guessing my speedo itself is the issue, or the wiring from the PCM to the speedometer is busted.

Will let you know...

 

[wolfman1979]

UPDATE:

OK, so after a little bit of frustration and swearing, I managed to get the instrument cluster free. Replaced my burned out bulbs, then tested the VSS signal coming into the back of the cluster.

Surprise, surprise, I have continuity. 205 ohms on that circuit at the plug for the cluster, pretty much exactly the reading I got at the PCM harness pin and at the VSS itself.

So, then, what more is there that I might be overlooking? As near as I can figure, the speedo itself is at fault, and I need a new cluster (or just the speedo, but after seeing the back of the cluster it seems pretty unlikely that the speedo alone is a replaceable item). Anything else you can think of that might be going on and I can check before I run to my local U-Pull-it boneyard?