Speed sensor testing.

[Xengineguy]

My 2001 m1078 a1 speedometer stopped working and locked into a gear. Tested the speed sensor and got 9.5
ohms at 70 degrees. My 2004 truck tested 271 ohms. Looks like I need an output speed sensor. The one located inside the transfer case.
Has anyone tested the solenoid for c7 resistance? Also in the transfer case. My two trucks booth test the same at 3.1 to 3.2 at 70 degrees. The Allison info I have for the 3070 claims it should be 30 ohms for H solenoid which is retarder or transfer case clutch.
If anyone has an Output speed sensor and maybe a c 7 solenoid I could use them. Thanks

 

[Ronmar]

The allison manual says 30 ohms for the retarder solenoid, but the 34-1 manual says the center diff solenoid is 3-5 ohms, mine measures 3.3 ohms…

i would pull the rear access cover and check the speed sensor resistance disconnected from/ independent of the transfer case pass-thru connector… an issue with that connector could also give you that lower resistance…

 

[Skyhawk13205]

My 2001 m1078 a1 speedometer stopped working and locked into a gear. Tested the speed sensor and got 9.5
ohms at 70 degrees. My 2004 truck tested 271 ohms. Looks like I need an output speed sensor. The one located inside the transfer case.
Has anyone tested the solenoid for c7 resistance? Also in the transfer case. My two trucks booth test the same at 3.1 to 3.2 at 70 degrees. The Allison info I have for the 3070 claims it should be 30 ohms for H solenoid which is retarder or transfer case clutch.
If anyone has an Output speed sensor and maybe a c 7 solenoid I could use them. Thanks

I have had an issue with that speed sensor, on mine it would fail and lock into current gear. Eventually it would read a speed and start shifting normally. All the ohm checks would come back within limits but it would still intermittently fail. I ended up replacing the sensor but I found a pin on the weatherpack connector was pushed and the wire was stretched. I reseated the connector and adjusted the wires and it works fine now. I am not sure why that wire moves.

 

[Xengineguy]

The allison manual says 30 ohms for the retarder solenoid, but the 34-1 manual says the center diff solenoid is 3-5 ohms, mine measures 3.3 ohms…

i would pull the rear access cover and check the speed sensor resistance disconnected from/ independent of the transfer case pass-thru connector… an issue with that connector could also give you that lower resistance…

Yes I pulled the speed sensor and measured it on the bench. It was 9.5 ohms. Measuring thru the pass thru on the second truck
the speed sensor measured 271 ohms which is in range. My center diff solenoid also measures in the 3 ohm range.

 

[Xengineguy]

I have had an issue with that speed sensor, on mine it would fail and lock into current gear. Eventually it would read a speed and start shifting normally. All the ohm checks would come back within limits but it would still intermittently fail. I ended up replacing the sensor but I found a pin on the weatherpack connector was pushed and the wire was stretched. I reseated the connector and adjusted the wires and it works fine now. I am not sure why that wire moves.

I will look at them carefully. Checked for corrosion and breaks none found. Also checked to make sure there was no shorts to other wires in the bundle and that the wires could carry current. So far the wires look great.

 

[Ronmar]

i believe the 3 ohm for the diff solenoid is correct. If the speed sensor measures 9.5 independent of the pass-thru connector, it sounds bad to me… as to the wire moving like Skyhawk describes, it could be hydraulics. Oil and big spinning gears in there.

mine is occasionally showing a speed sense error now, but it is because I am getting a lot of noise feedback from the 7.4Khz center diff-lock PWM control signal. Occasionally when I select mode, the speedometer will jump up to 10-20. Since the TCU is always doing some form of ratio comparison, if it sees output signal but no turbine signal it will throw a speed sensor code, 22-??. Same as if you unplug the speed sensor and try and move, turbine RPM and no output speed input = 22-??

On the A0 they put a R-C(resistor-capacitor) filter circuit on the speedometer circuit in the VIM and provided a filtered output thru a capacitor that we don’t use/isnt connected. But they did connect the speedometer return across the 100K Ohm resistor of the filter to ground.

I messed up in my application of that load resistor initially and as soon as I got my mode/diff-lock signal working i started developing 22-?? Codes, and went back and found my resistor was not connected. When re-connected it still does it but not as bad, so I am suspecting there is some leakage in that transfer case pass thru connector that is putting more 7KHZ noise on the speedometer line than the 100Kohm load/filter resistor can dissipate to ground…

 

[coachgeo]

I will look at them carefully. Checked for corrosion and breaks none found. Also checked to make sure there was no shorts to other wires in the bundle and that the wires could carry current. So far the wires look great.

What about the harness attachment on the outside? Folk have found issues with outside harness where it hooks to trans.... and others the harness inside the trans.

 

[Xengineguy]

What about the harness attachment on the outside? Folk have found issues with outside harness where it hooks to trans.... and others the harness inside the trans.

Yes that was suspect also. I tested the harness all the way from the inside of the transfer case to the bulkhead connector.
Not only resistance but made sure there was no cross connection between wires and made sure each wire would carry
enough amperage to light a lmtv headlight.