m1guy

I have had an annoying speedometer problem for a while now. It works intermittently, and sometimes jumps around, causing my truck to shift in and out of overdrive.

Of course, my first thought was the speed sensor, and it looked original and well used. So I replaced it, and the speedo worked for maybe 2 days, then slowly started jumping back and forth, and eventually started moving enough that it caused problems with staying in overdrive.

I think the speed sensor is being driven properly. Sometimes the needle jumps around with the truck idling in park, so I think it is an electrical rather than a mechanical problem.

Also, when idling, sometimes my lights will get dimmer and brighter, like possibly the system's voltage is fluctuating. I wonder if that could be happening, and contributing to the erratic speedometer.

Anybody have any clues?
Thanks.

Nathan

I had a 92 cummins 2wd 5sp and it did the same thing with the speedo, I found out that the gear on the output shaft was actually worn out, it's plastic on the 5sp, my current problem with my 91 is that the speedo works fine it's just that my odometer quite working when I bought it.

timb

If you pull the sending unit off of the transmission you will see a square pin that drives the sending unit. Look and see if that pin is worn round. I had to braze mine up and grind it square again about 12 years ago and it is still working great. By the way, When you pull the unit off of the trans. the pin will probably stay in the trans. you will need to pull the pin out and check both ends.

IraPatterson

About the lights going dim at an idle, does your amp gauge go down low at the same time the lights go dim? Then does the amp gauge shot back up where it should be the lights get brighter again? My Truck has that problem, as well as other little things like watching the amp gauge bounce with the turn signal and other funny little things. A lot of it I think is just attributed to CCW (Chrysler's Crappy Wiring). However, you may want to check your battery (ies) as well. I run duals on mine and one of them is going bad which is half of my problem (the other half is I have not rewired my entire truck yet :P). Hope this helps.

m1guy

Actually, I did have the problem with the rounded shaft on the speed sensor. That was one reason I replaced it. I thought that was definitely the problem, and it seemed to be, because that was when the speedometer started working fine for a few days. But then, it started moving back and forth again (maybe 10 mph to each side of whatever speed I was going), and that got worse and worse until it started bouncing around enough to causeit to go in and out of overdrive again.

I even took the sensor off, left it plugged in to the harness, and put it in a drill to turn it. Even at a constant speed, the speedometer still moved back and forth. Plus, even when I have the truck sitting in park, the needle still jumps around sometimes. I don't think the sensor can possibly be turning when the truck is sitting still in park. So, that makes me think it is an electrical problem, where the speedometer is getting some kind of erratic signal, rather than a mechanical problem with the sensor drive gears.

And yes, my gauge also goes up and down when the lights get dim and bright. You know, that's a good point about checking the battery itself. Maybe I should do that before I go much further, since I still haven't got around to getting one, and the one I'm using now is 10 years old. But, it still cranks strong.

And, if the battery was the problem, I don't see why replacing the sensor (and sometimes just unplugging it and plugging it back in) would temporarily fix it.

sdstriper

I would pull the headlight switch out once and check the connector plugged into it. I have had a couple Dodge trucks where this was a serious fire hazard IMO. The switch had a near meltdown at some point and the connector and wires were brittle, flaking off and not the greatest conductors of electricity anymore. There is too much stuff on that circuit IMO, and then people go and add trailer lights etc on there too. If you have your trailer break unit wired thru the factory harness instead of seperate wires from it to the back of the truck, you are asking for trouble. Check the switch for a bad connection folks, may help the flickering and may prevent a melt-down.

Another easy dashlight improvement is to pull the instrument cluster. Take out all the little bulbs and toss the blue bulb covers that are mostly brown now from overheating, replace any burned out bulbs. Use some emery cloth to clean up the circuit board where the bulbs contact it.

sdstriper

The grid heater pulls pretty strong on these trucks. Is your dimming issue when it is cold outside and at start up only?