Speedometer Malfuntion

[dwalker151]

Ok, here goes an interesting one. The speedo in my work truck wasn't working so I put a rebuilt gearbox down on the transfer case. Now the blasted thing registers but the guage won't read over 40 mph! now she's old but she's not that old!!! Anybody got any ideas?



Thanks

Dean

 

[BushWakr - RIP]

dwalker,



When you say "rebuilt gear box" what are you referring to??? Do you mean the speedo gear assembly itself??



If so, one thing you need to check is the inside of the gear assembly has a square cut hole for the gear drive pin, which is square too. If the hole or the drive pin (shaft) is rounded it will read for a while, but will eventually be unable to keep up due to spinning.

Also, if you speed sensor has numbers on it, it two areas side by side with what looks like an attaching bracket, then the sensor must be positioned correctly to read properly.

To get a look at what I'm talking about, go to the link (pic) I have attached and check out the very last/bottom pic in the series... .

It will show what I mean by numbers and bracket etc. .



https://www.turbodieselregister.com/user_gallery/displayalbum.php?&userid=5168&albumid=489



Bob

 

[dwalker151]

By gearbox I mean the ratio-rite that the speed sensor screws on to. I have a 5-speed so it's on the back of the transfer case. I ran a test without the gearbox and the guage did the same thing it went up to 40 mph and stopped. I would assume that this would rule out a slipping shaft because it should change speed if I removed the gearbox.



The speedo never has worked since I bought the truck. The first thing that I did was change the speed sensor and I'm thinking just for a check I would screw the old one on and see if anything changes.



Dean

 

[BushWakr - RIP]

Dean, have you tried "pulling the codes" to see if there are any faults being reported by the PCM???



If you're not familiar with this, you turn the ignition key to 'on' then 'off' then 'on' then 'off', then 'on' again and wait... .

Watch the left side of the insturment panel. You will eventually see one of the lights flashing on and off... count the flashes. . eg:

two flashes, pause, two flashes. = 22

3 pause 4 = 34 and so on.

Give that a try and see if you have any codes showing. BTW, when you see 5/5 that means 'end of codes stored'.



Bob.

 

[bflood]

I had an erratic and low reading speedometer that was caused by a loose tail shaft yoke courtesy of an incompetent Dodge Dealer in Grinnel, Iowa. The tail shaft was moving in and out, on acceleration/deceleration, causing the speedometer gear to barely engage and it would slip and be erratic. After properly torquing and loctiting the yoke retaining nut the speedometer cleared right up (but the new transmission was now damaged, they also put the wrong gear oil in). After many $$:{ , and zero help from the Dodge dealer or Chrysler , I ended up replacing my new, but damaged, GETRAG (the spell checker suggested "GET RAG" - sounds reaonable ) with an NV4500.



This doesn't sound like your problem but still its the same subject.

 

[BushWakr - RIP]

Actually I was going to suggest that very thing next. Not having the 5spd I don't have personal experience with this but have read about this problem and fix a number of times before.

Maybe someone who has done it could give a more detailed explanation of "how to, what to watch for, what tools" etc.



Bob.