Bouncing speedometer

[FM]

My speedometer goes back and forth like it's a seismograph. I'm guessing it's back feeding from somewhere but how to go through all that spaghetti and find it is a puzzle. Does anyone know how I can sort it out. I've searched but have not found any previous post on topic.



TIA,

Fred

 

[100 Proof]

That is typically caused by a speedometer cable that needs to be lubed.



100 Proof

 

[DPKetchum]

Don't think there is a speedo cable on these trucks. All electronic. Bet its a cluster/speedo head issue.

 

[rbattelle]

DP's right - there is no cable. Could be an intermittent fault in the speed sensor. Is the speedo bouncing between "0" and the correct speed?

Ryan

 

[FM]

I can be siting at a stop light and next thing you know I'm doing 100 mph then 30 back to 80 and so on. This truck is so fast you don't even feel it. Going down the road it's all over the place. Just wicked fast needle movement back and forth with no pattern I can tell.



Fred

 

[p-Bar]

Check the speedo sensor in the rear diff. You can pull it out and sometimes the metal filings cause it or it just may need to be replaced. Usually the ABS light comes on also.

 

[FM]

Thanks all. ABS light has not come on yet. I'll pull the sensor and clean off the face.



I thought I read a post a year or two ago about a circuit back feeding the speedo making it behave irratically but I've not been able to find it. Coarse there's always the chance that I'm just trippin on that one.



Fred

 

[FM]

I cleaned the sensor off but the speedo acted the same. Today I put a new sensor in and it still acts the same bouncing all over. It looks to me like its getting sporadic voltage from somewhere but I'm not sure how to trouble shoot this. The way I understand it is the sensor goes to the PCM and the PCM feeds it to the speedo so the options are the sensor, the guage, PCM, or a short somewhere. Does this sound about right?

TIA,

Fred

 

[rbattelle]

The way I understand it is the sensor goes to the PCM and the PCM feeds it to the speedo so the options are the sensor, the guage, PCM, or a short somewhere. Does this sound about right?

Sounds right to me. Now you've got a headache on your hands.



You might get somewhere if you pull the instrument cluster codes. Hold the trip reset button down (on the dash, not the overhead), then turn the key to RUN. The instrument cluster will perform a self-test (all the needles will run their full-sweep). After that, any instrument cluster codes will display in the odometer window.



Post those codes here... hopefully someone can interpret them for you.



Ryan

 

[FM]

Thanks Ryan, I'll try that tomorrow. Meanwhile, today I had to go to Wilmington to UNCW and back for my daughter to check it out. I set the cruise and it starts cutting out on its own. Just drops out. I resume and it picks up fine. Go down the road a bit and it cuts out again. Did this there and back. Don't have any idea if it's related to the speedo or not. ARG!!! The speedo, I can take it or leave it. It'd be nice if it worked and eventually it will but I drove my 63 for 11years without one so it's nothing new. The cruise though I'd really like to have for long trips. Come to think of it, the 63 didn't have cruise either. Guess I'm getting spoiled. Power that won't quit. Great mpg for an 8000# truck. Ball joints that... ok, 2 out 3s not bad.

Many thanks

Fred

 

[rbattelle]

It's interesting that the cruise isn't working either. That implies to me that the ECM isn't getting the proper speed signal from the wheels.

Going with that idea, then, you've at least isolated the problem to the wiring between the ECM and the wheel sensor. Although it doesn't necessarily rule out an internal ECM fault.

Let's see what the instrument cluster codes say. Also, when the cluster runs its self test and all the needles run the full sweep, watch the speedo carefully to see if it moves smoothly.

Ryan

 

[FM]

Well I just couldn't stand it so I ran that test. None of the guages moved smoothly. Smoothly being a constant rate of change of the needle position as it sweeps the face. But they all pegged before the speedo did and none of them started backing off until the speedo was about half way back. Of all of them the speedo moved the most like a clock hand with near consistant swings between stops. Then the od shows "C-codE" and next shows"00 00". I've got not a clue.



Fred

 

[rbattelle]

The 00 00 would indicate there's no known problems with the instrument cluster, which suggests to me the problem is with the speedometer gauge itself. (Although that doesn't explain the cruise control issue).

I don't think the speedo is replaceable by itself - you have to buy the complete cluster ($$$).

I'm not sure where to go from here...

Ryan

 

[dunderkofler]

I set the cruise and it starts cutting out on its own. Just drops out. I resume and it picks up fine. Go down the road a bit and it cuts out again. Did this there and back. Don't have any idea if it's related to the speedo or not.

It is related. When the speedo jumps the computer decides you're accelerating too fast (probably spinning the tires) and kicks off for safety.



Mine had the same problem at about 34k. After several warranty trips where they threw parts at the problem to no effect they finally discovered two bad (I'm guessing insulation rubbed through) wires in the harness to the pcm. After replacement of the wires I've had no further problems (now at 108k).



Yours is the third 2003 that I've heard of with this malady. The other was a DTR member by the name of BigAl52. In his case "they discovered that it was the abs sensor in the dash losing a ground inside the sensor. They also found a faulty ground wire on the engine. " Copied from a pm he sent me after they finally got it sorted.



Good luck chasing this one down.

 

[nickleinonen]

the cruise control may get cut out on its own if the speed changes dramatically. the ecm/pcm thinks that the wheels are slipping [ice conditoin] it will do the same for engine speed increase [clutch in or trans thrown into neutral]


just skimmed the wiring schematic. . looks like the speed signal is processed by the abs module before the signal is sent to the pcm...

 

[FM]

I do a lot of guessing so here I go again, the pcm or ecm does not read what the speedo guage is doing but sends it the signal so if the guage is doing it's own thing with the signal it should not cause he curise to cut out? I'm also guessing if it was the abs sensor in the dash loosing ground the instrument cluster diagnostic would through a code for that? Likewise I'm guessing if the abs module is bad it would throw a code also? So I need to see if there are any codes?



But in the case of a skinned wire, the pcm or ecm just sees an increased voltage and does as instructed? Is visual inspection of the wires the only way to find this or is there a contact I can put a multi on that would show the voltage spikes that match the speedo surge?



Thanks All,

Fred

 

[p-Bar]

How about checking the plug that the sensor plugs into? What is weird that usually the ABS light will come on when the speedo does whack out since the ABS uses the speedo sensor also.

 

[dunderkofler]

I'm guessing any code thrown by the abs system would require a drb-3 or similar to read. I know that both of my readers will only see the emissions codes.



As to checking the wires I'm unsure how they did it. They did not unwrap the harness that I'm aware of. If they did they should have removed the bad wire instead of cutting and capping with heat shrinkable tubing. You might be able to backtrace the wire while running but I would be afraid of harming/killing the computer.



I just went out and looked at what they did on mine. I made a mistake in my previous post. Pin 11 on the 50 pin connector (rear), white with orange tracer - the vehicle speed signal, is the only wire they replaced. One of the parts thrown at it was an abs module, but it wasn't until they replaced the wire that the problem went away.

 

[rbattelle]

is there a contact I can put a multi on that would show the voltage spikes that match the speedo surge?

You'd probably need an oscilloscope.

You could just bite the bullet and try replacing the wire between the sensor and the ECM. PITA.

Ryan

 

[FM]

I guess I'll look again for the obvious. I looked at the sensor conection again today saw nothing wrong. Don't have a drb-3 or an oscilloscope. May have to suck it up and take this one in but not yet. May replace the wire or beat on it with a large hammer depending on my state of mind at the time.



Thanks Again

Fred