alfalfa4381

Ok, I have searched Google, this forum, and tried to find it in the 1000 page service manual - for months! Maybe someone can give the definitive answer....

I have a 1992 D150 5-Speed manual drive. The speedometer is my issue, but I have repaired the circuit board and I can run the speedometer and odometer from my bench by applying a 0 - 200hz 5v clock pulse to the green wire , with 12v applied to the red and black. But, It still does not work in the vehicle. With the speedometer cluster removed I attached my portable Oscope to the post where the green wire plugs in, and the black wire post to my Oscope ground. When I start the truck I get a lot of noise on the scope, and when I drive the truck the scope shows mostly noise with only 400mv . So here is where I need someone who really knows, to tell me, where is the sensor this pulse comes from? I have read about the VSS in the tranny (probably only for automatics), VSS in the transfer case, the ABS sensor in the rear end, etc. The Service Manual briefly mentions a Distance Sensor, which no amount of Googling brings up.
Since I see no VSS on the manual transmission or associated parts, I can only ASSUME that the sensor in the rear end is what triggers the Speedometer. I replaced that before I got into the cluster, simply because it was cheap and the old one is 32 years old.

The noise showing on my scope, along with no voltage from the trigger pin in the cluster, leads me to believe that I have a grounding problem. I also read that the ABS sensor in the rear end is wired to the ABS valve behind the left rear wheel (which I reconditioned and seems to be working fine), and then runs to the ABS Control behind the glove box. I have read on this site that the pulse signal runs through the PCMn and also that it does not.

So, what I hope you guys and gals can tell me is:

1. Where is the sensor for my particular truck truly located that sends the clock pulse to the cluster?
2. If it is the sensor in the rear end, I noticed that the sensor is a permanent magnet, so is there a plate or disk inside that has a steel piece that rotates for the sensor to count a rev? If so, does this ever present a problem? I noticed my truck, which I've had for only 6 months, has RTV on the pumpkin cover, so it has been serviced. Whatever is in there that triggers the sensor could have been deleted, damaged, etc.? The service manual does not show anything about what in the rear end triggers the sensor.
3. Lastly, is there anyone who can simply tell me the path this pulse travels from the sensor to the cluster?

If it truly is the sensor in the rear end, I want to jack up the truck, hook up my scope, with longer leads, directly to the sensor, unplug the ABS harness,and run the truck to spin the wheels and see what is coming directly off of the sensor. If I still have the same noise with no output, I will run a separate ground right there to see if the problem is solved. If not, I suppose the pumpkin needs to be carved!

I REALLY appreciate any help you can give me on this!

dguru

Here are your options.

89 has a cable 92 could be VSS.

alfalfa4381

Thanks Dguru. I clicked on the link, and the part listed is for Automatic Trans. When I filtered for 5-speed 2wd, Mopar states "No results for "distance sensor" in 1992 Dodge D150 Parts & Accessories"

pind

The VSS should be located on the lower keft side of the transmission tailshaft housing, in a 2wd application.

It will be just to the rear of where the extension housing bolts to the transmission case, and the wiring will run up to the top of the transmission where it joins a larger harness.

alfalfa4381

Thank you Dguru and Pind, I did find the VSS in the transfer case. I really appreciate your replies!!
I am working in my driveway, so a few weeks ago this fat boy had a hard time trying to spot it, now I see it is partially blocked by the crossmember.
Since my dash is apart, and I know the speedometer works on my bench, and it's no cost, I'm going to go ahead and cut the black wire near the sensor and ground it, and splice the or/wht wire to the green sensor wire at the odometer. That way the 32 year old PCM will be bypassed.
Then, if it doesn't work I'll pull the sensor.

dguru

Okay I did not see the 4wd part.

Could be the plug needs cleaning and blowing out. Wires could be broken. One side of the VSS is grounded. Could be bad ground. Are you sure the speedometer is getting power?

An easy test would be to use an ohm meter to test the ohms of the VSS sensor. I'm guessing about 400ohms. Then plug it in and test the wire closer to or at the speedo with one test lead grounded. Should get about same reading. If not trace wire for problem.

You may also want to inspect the gear by removing the one bolt and removing the drive unit.

pind

Yeah I got messed up with the d prefix. That tells me its a 2wd with an nv4500. W series would have it on the left side of the t case output.
either way it will be a two wire Hall effect sensor depending on build date.