Engine/Transmission (1994 - 1998) speedo quit

[MikeBennett]

I've recently aquired a stock 1997 3500 CTD 2wd 5 speed truck with almost 400K miles on it. It runs and drives fine, but I'm planning on pulling a friend's 10,000 pound boat up to a high lake on Friday and have been a little worried about the imfamous NV4500 5th gear nut. Not that I plan on ever being in 5th gear when towing the monster boat, but I would like the piece of mind that comes with knowing the nut is where it needs to be before I try and climb 7000 feet up into the Sierras with 10k behind me.



Anyway, pulled the back off the NV4500 last night and everything looked fine. Put it all back together, filled it with oil this morning and took it for a test drive. Shifts fine, but now the speedo doesn't work: worked fine before. When I removed the back half of the transmission, I just removed the speedo sending unit and let it hang there from it's wires. Wiped some of the oil film and road grime off it and put it back in this morning. I've had speedo drives in and out of various cars and trucks before, but this one's got me stumped.



After returning from my test drive, the first thing I did was to unplug the speedo drive from the vehicle harness and clean the contacts with electrical contact cleaner: still didn't work. Then I removed the speedo drive again , expecting to see ruined teeth on the gear or stripped teeth or something but no, the gear teeth look fine, just like they did when I removed it. The teeth on the tailshaft looked fine also. I thought maybe I had cocked the speedo drive to one side slightly and caused the gear teeth not to engage the tailshaft, but the drive goes in very snug and there is no tilt or mis-alignment that I can see. I reinstalled the drive, being careful to make it go in straight, but it still doesn't work.



I did not notice the orientation of the speedo drive before I removed it the first time. The drive has four slots on it which seem to indicate in can be installed in any one of four orientations. I didn't think it's orientation would matter, because the drive gear looked centered underneath the transducer, but I could have missed something. Does it have to go in only one way?



Anybody been down this path before? I supposed the speedo drive could have gone belly up during my inspection of 5th gear, but that just seems too coincidental. Is there a way I can test the transducer to see if the gear is actually turning?



Many thanks,



Mike Bennett

 

[GAmes]

The adapter is not concentric, which allows gears of different tooth count to be used. You need to pull it again and count the teeth. On the side of the adapter are two range scales like 32-38,39-45. Mount the adapter with the correct range numbers at the 6 o'clock position wth the tangs of the forked hold down in two of the slots. If it still doesn't work you managed to break the drive shaft of the sensor. The square shaft should be about an inch long. If it is shorter the broken piece will be stuck inside the gear, so that is confirmation.

 

[MikeBennett]

Thank you very much, that makes sense. Went back out to the truck tonight and pulled it , inspected it, re-installed the sender unit in the correct orientation and it works like a charm!



Thanks again, this site is terrific!



Mike Bennett

Madera, CA