When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Been a while since I've been on, but after many years I figured I'd take another look at why my speedo/cruise/odometer stopped working. I'm sure it's all due to 1 thing.
A good while back I had discovered some bolts backed out in my rear differential and got chewed up. In the process is did a little damage to the tone ring and scuffed some of the plastic off from the sensor. I put new bolts back into the differential, did nothing to repair the tone ring (it has some teeth that are damaged), and tried a new sensor to see if that was my problem. It didn't fix my speedo issues.
Is there another location where the speed is registered, or is it the rear diff that sends the signal? Is there some wires I should be looking at? The speedo was intermittent before failing, suggesting a loose connection somewhere.
I'd sure like to fix the problem since I'm the only one that memorized what RPM corresponds to what speed, the cruise won't work, the odometer hasn't registered probably the last 50k miles, and the grid heater continuously cycles because it never detects that I've reached some threshold speed to shut off.
LOL, just realized I had posted this 10 years ago here.
I'm still not convinced the problem is with the tone ring as the speed slowly died, and would bounce around like there was a loose wire somewhere. Diff has not let out yet, and all I did was replace the fluid and put the cover back on.
Now my headliner is falling down and the front windshield isn't sealed and pops out a bit when you close a door. It's still a useful rig though!
Yes, on a 98 or newer truck, the sensor on the differential is what runs all those things that aren't working. I would definitely consider that tone ring a problem. I looked at one years ago that also had a couple teeth knocked off the tone ring by a stray bolt in the differential. Looking at the waveform on the lab scope I had hooked up, I could see a series of bumps, with a couple of flat spots in the middle. I took the diff cover off, and that's exactly what the tone ring looked like- a bunch of bumps and a couple flat spots. Also have a good look at the wiring from the sensor to the frame, there's a couple spots where the wires are pretty vulnerable to gravel.
I can imagine how a bad wave form from missing teeth would give the speedometer driver fits - also anything else that is monitoring that signal, like the grid control function. I wonder if that was affecting transmission shift control in any way... In addition to what Torquefan says - if you have a multimeter, disconnect the sensor wiring at the sensor and the other end and do a continuity/wiggle test on both wires. If they are disconnected on both ends, you could also do a continuity check to ground to see if they are shorted - though I suspect you might be seeing other problems if one of those wires was shorted to ground.
Clearing this up will probably help your battery life, as the heater grids would no longer be cycling needlessly. One of the few things I remember from high-school or college chemistry is that the chemical process when a battery is discharged and recharged does not go 100% in the "recharge" direction - a tiny amount of sulfate is left.
An aside: good on you Torquefan for using a labscope on your issue. There are some things a scan tool just won't pick up and the only way is to look at the sensor/PCM signal or waveform with a scope or graphing multimeter. I'm just a DIY'er, but I got interested in waveform diagnostics through my brother-in-law (a very good automotive tech) and bought a used Snap-On Vantage and later a Modis so I could look at waveforms for diagnosing some problems.
I just came on here to ask if anyone knew what I should see if I threw a multimeter at this. Was wondering if the speed sensor needs to read a certain resistance. I'll definitely do continuity checks to verify the cable is good, and not shorted to the chassis. One of these days I'll get an O-scope.
Any idea where the wire connects at the front end? I don't think I've traced it that far yet.
... and crap, I just remembered I loaned out my multimeter to my friend 90 minutes away. I'll see if Dad's got one. If there's a break in the rain this weekend my plan was to do an oil change extravaganza on every vehicle and piece of equipment we've got. Since I'd be under the truck anyhow, figured I might as well fix the darn thing. Maybe I'll glue the headliner back to the ceiling too.
I'm sure it should have a certain resistance, but I don't know what. I'd look it up on my AllDataDIY subscription, but I have a '96 and that sensor is only used for ABS and the resistance may be different. I suspect the connection at the front end is the PCM connector. Sending you a PM with a link to a free downloadable factory service manual - sensor resistance should be in there, and probably info to help you locate the other end. You may wish to consider subscribing to AllDataDIY.com - its the shop info service but on an individual-vehicle basis.
04-04-2019, 07:10 PM
