Could Speedometer problems be linked to the ECM??

[JGrefenstette]

My speedometer and odometer have been failing at the same time, off and on for the past few months. since i bought the truck in early may. i have been able to just tap the gear reducer and it would come back on. but a few weeks ago it went out and stayed out. this was just before my ECM went dead. I replaced the ECM to try and solve my charging systems problem. that worked, and the charging system is good again, and the speedometer came back to life after i put the new ecm in the truck. this made me think that the speedometer was possible linked to the ECM going out. i thought that if it was my problem was solved and i have a speedometer again. but yesterday it went out. this was 4 days after driving it with the new ecm. I dont know if its just a coincidence that it came back after the new ecm, or if it was strongly conected.



here are my questions for yall:



Is the speedometer conected to the ecm in any way at all???



If so, does the speedometer dying again mean that my ecm is getting ready to go bad again, and its just eleminating the weekest link first?



If not, why would it come on emidiatly after i replaced the ecm?



(one last thing, when it went out, it was a few minutes after leaving a treefarm that i work on. i hadnt taken the truck there since the new ecm, and i was wondering if all the bouncing could have contributed to the failure of the speedometer agian. i left the farm and it worked fine while i drove 1-2 miles to the gas station, i parked, let it idle for 5-10 minutes while i filled two gas cans, then when i pulled back out on the road it wasnt working?)



any thoughts or comments would be more than appreciated!

thanks for taking the time to read my mess of thoughts and concerns.

 

[Philip]

Yes the speedo is connected to the PCM. It receives a signal from the VSS (vehicle speed sensor). Then the PCM converts that signal for your speedo reading.



The VSS is mounted on the transfer case where the rear drive shaft comes out. I think on the getrags it is a two wire plug in. On a auto it is three wire. The VSS is also mounted onto a adapter box. This box then screws into the drive setup in the transfer cae.



The drive adapters are a noted problem. The drive gears are plastic. They have been know to strip out. I have also seen a VSS that had rusted out and wouldn't grab the drive tang.



So start around the VSS and look it over good.



Also check the wiring going down to The VSS for chaffing or breaks.

 

[dpuckett]

I didnt know the gears inside the adaptors were plastic- the aftermarket one I got had metal gears, as did the OEM 2wd 90deg adaptors.



Check the wire connections in the pigtail for the VSS- they need to be completely round to make good contact. Also look inside the t-case output shaft and make sure the gear is intact and solid on the shaft.



Philip- the 93 autos were the only ones with 3 wire VSS connections. Mom's 92 has 2 wires, just like the rest of the 5speeds.



Daniel

 

[JGrefenstette]

ok. so does the fact that my speedo is out again, should i be concerned that my PCM is about to go dead again? seeing as the speedometer came back right after i put the new PCM in.

 

[Philip]

Intermintent speedo is a sign of a problem around the VSS or its drive system a lot of times. I wouldn't get to hasty about condemining the the new PCM yet.



Check out what me and Daniel suggested.

 

[JGrefenstette]

ok thanks. that makes me feel better. ill check it out like yall have said.



just to make sure i totally understand. your talking about the, what i call "gear reducer", that comes out of the side of the T-Case, and check the wiring for any wear marks or pour conections. and to take it completely off of the t-case and inspect it for worn gears or signs of failure.

 

[Philip]

That what you need to do.



Also look over the drive pin that comes out of the reducer box and into the VSS. See if its rounded off or the VSS opening is rounded out and not holding the pin properly.

 

[1stgen4evr]

"concerned that my PCM"



In my view PCM is an acronym for putrid crappy module. These trucks should have never been built with one. They seldom go out actually but have ongoing problems with connectors, wireing, grounds, you name it. And Dodge in their infinite wisdom located it BEHIND THE BATTERY of all places.



James

 

[JGrefenstette]

i think this mystery is exposing itself a little more every time i drive. since the speedo went out on thursday the 14th, it hasnt come back. well i took it out for a little sunday afternoon drive today to get some fresh air and make sure it was going to take me to work and back on monday. the speedo was out for the first 10 minutes, then i pulled off on this pretty rough and bumpy gravel road. i parked for a minute, and when i putted my way back up onto the pavment and got up to speed, the speedo was back! i am begining to think and hope its just a stuburn gear reducer.



what do these little symptoms sound like to yall. an old flakey gear reducer, or some bad electrical connection, along some sort of wire or possibly in the ECM?

 

[Philip]

What your seeing is the same problems Case500D had on his W350. We found the drive tang rounded off on his reducer box going into the VSS. The VSS socket was worn also. But the new reducer box cured his speedo problem.

 

[JGrefenstette]

where did you find a new reducer box?

 

[Philip]

We picked one up from a dealer.



Here is a link that has them listed also. About 2/3's of the way down the page.



727 Mopar Transmission from PATC



With shipping from that place the price came out about the same.

 

[JGrefenstette]

so am i looking for the speedometer "gears" or the dodge "ratio adapter"?

 

[Philip]

Most of the problems are the ratio adapters followed closely by the VSS.



First look yours over and see if it has problems. If it does then get another one.

 

[Dieselman359]

You mean to tell me that I have a 'like-new' reducer box. Oo.

 

[Philip]

Chris that problem was on the W350 you didn't get to take home. :-laf:-laf



Sorry

 

[Dieselman359]

Ahhh schucks:{