Speedometer calibration

[KevinP]

Is there anything that I need to do besides changing the gear in the transfer case to get my speedometer to work correctly? I changed the gear to a 37 because I went from 3.07's to 4.56 gears and went up to 35" tires. As it is now, the speedo always reads 0 when I stop but no matter how fast I go it goes to 40mph and stays there.

[Kurt]

Is there anything that I need to do besides changing the gear in the transfer case to get my speedometer to work correctly?

Change the gear and clock the sender correctly so the gears mesh. That should have been all that was necessary.

As it is now, the speedo always reads 0 when I stop but no matter how fast I go it goes to 40mph and stays there.

Never heard of that happening as you described.

So even if you are doing approx 5 mph does it reads 40 or does it read 0 until you hit about 40 and then it jumps up?
Assuming the latter my only guess would be that the speedo sensor isn't clocked properly in the tail housing.
Second thought is that possibly the sender is flakey.
Third thought is that there could be a wire harness issue wire(s), pinched, shorted, open.
Fourth thought is that you may not have seated the new gear all the way into the sender.

Are you running the same speedo sender as before your work?

[KevinP]

The only thing I did was change the gear and clock it. The sender is the same one. The speedo did act funny when I put the 31" tires too but I thought once I changed the gear it would be ok. After work tonight Ill see if I can take a video of it and show you what it does.

[Frank]

We also did a SYE change to the T-case. FjR68

[Kurt]

Which SYE kit was used? Brand?

If Teraflex it may be a couple of issues. (besides those mentioned above)
1) The snap ring that holds the output bearing in place has to be clocked properly so it doesn't interfere with the speedo gear.
2) A blue plastic speedo gear sleeve that's used on the output shaft is held in position by two snap rings. Possibly one or both snap rings weren't put on allowing the plastic sleeve to drift back and forth on the shaft.

[KevinP]

Advanced Adapters SYE is the adapter kit I used and I think I have all the snap rings in place. Frank helped me so Ill double check with him but I know we watched the step by step video while we did it.

[KevinP]

Here is what my speedo does.

https://www.youtube.com/watch?v=kE4JuyYDx0k

[Kurt]

Man that's an odd one...
To me that points to a possible problem with the sendor, wiring or the PCM.

First thing I'd try is disconnecting your battery for 10 minutes, re-connect it and see if the problem persists.

If you still have a problem I'd then pull the sensor and then carefully use a variable speed drill to spin the speedo gear.
You'll need to have the key in the RUN position. Vary the speed of the drill and see if you get the same behaviour. You might have to reverse direction of the drill to simulate forward speed.

The 3 wires on the sensor are Ground, +8 volts and the sensor output. The sensor output is a series of voltage pulses that go back to the PCM. The rate at which the pulses come out of the sensor are proportional to the vehicle speed. The PCM measures the rate at which the signal is pulsing and then calculates the vehicle speed and in turn the PCM drives the speedometer needle.
A fixed speed like the 38 MPH would occur IF the sensor was sending out the pulses at the same frequency regardless of how fast you are going. Usually the sensors work OK or just stop working.
Got a spare sensor to pop in and try?

A fixed speed of 38 MPH could occur if the wire into the PCM was picking us "noise" at a fixed frequency which was the same frequency as 38 MPH. (Again unlikely but still possible)

My last suspect is that the PCM has a problem. Got a spare one you can toss in?

[Frank]

Dam Kurt : Was you an Engineer in another life or what ? We will try the drill thing first. Its weird to see the speedo just fall from speed as you are slowing. It also just jumps to a speed after you are already about to shift to 2nd. The speedo has had an issue since he started driving it ,so Im sure its nothing we did with the SYE or axle change out. Thanks for the direction ! Why do we need these devices , we dont go that fast anyway. FjR68

[Kurt]

Nope... Electrical Engineer in this life