speedo troubles

[1991cummins]

ok boys...

my speedo has worked great for 260k miles. about a month ago it died on me... just looked down one day while driving and it didnt read anything. so i checked the easy stuff and said to hell with it for now

i just now started to poke around, pulled the sending unit out of the tranny and the sending unit was bad, didnt have any resistance across the two poles and put out nothing when spun by a drill... so i got a new one thinking it would be a quick fix, with no luck... the new one had resistance and put something out when on a drill... but nothing when connected to the truck,

so therefore two problems arose at the same time... i also hand tested the sensor and it was good... so the problem has to be in the wiring or the guage... i followed the wire and it seems run fine, no loose connections jump out at me...

any advice, i heard those gears in the gauges go bad with time... possible?

any help is great,
Derek

and sorry about the novel, i seem to do that sometimes

[Ace]

Mine stops working when I unplug the PCM.

[1991cummins]

ummm. what is the pcm and where is it located lol

[Ace]

It's the brain box up under the fender behind the battery. Controls the grids, WIF sensor/indicator, OD, cruise and a few other things on the later model 1st gens. Speedo is on there too for some reason, but I haven't yet figured out how or why. Probably part of the cruise control interface.

Couldn't hurt to just unplug it and try cleaning the contacts with some WD40 and re-seat the plug to see if that helps.

[1991cummins]

yah, but i dont have any of that electric modern day conviences... lol, just the grid and wif sensor... so i will check that i guess... even though i doubt it cause my grid gets warm...

i took the cluster out yesterday and was looking at the speedometer... is there any way to put a small current on it and check to see if it moves... cause if not i cant think of anything to do but just pull one out of a junker or something... because i cant think of anything else that will work...

thanks,
Derek

[Ace]

I'm not suggesting the PCM is bad, just possibly a bad connection in the plug. Sounds like you've checked about everything else.

My speedo used to do a little trick where it would suddenly stop working for no apparent reason for a short time or maybe a day or two until I hit a big bump in the road somewhere and it would start working again. It has not done this since I started messing with the PCM a few months ago.

[1991cummins]

mine did that for a while a while back, but that was because the probe was falling out of the tranny... so ya... but i will check the pcm... is it on a seperate plug, or in a cluster? how likely is it the speedometer meter itself is out, how likely are they to fail? i mean i know anything is possible... but some more than others...

[Ace]

There's a big, honkin' multi-prong plug with a bundle of wires coming off it held on with a 5/16 bolt right on the PCM itself.

[1991cummins]

mine did that for a while a while back, but that was because the probe was falling out of the tranny... so ya... but i will check the pcm... is it on a seperate plug, or in a cluster? how likely is it the speedometer meter itself is out, how likely are they to fail? i mean i know anything is possible... but some more than others...