ABS and transmission problems linked?

2001 2500HD 8.1L Allison 5 speed transmission. The ABS indicator and brake light came on during a few easy stops where the abs activated even on dry pavement and slow speeds. During the same trip and just a few miles later, the transmission kicks out of gear and the indicator flashes. Putting in neutral and back into drive the truck will move but only for a few feet before it does it again. Does the same in any gear, neutral "resets" it then it kicks out again. Only code I'm getting is a p1781 which I think is a transmission code but not sure. Could these two issues be related as they both occurred at the same time? I'm kind of at a loss right now trying to diagnose. Thanks

Service Bulletin

Subject:
Allison LCT1000 P1781 or P0802 Setting in the ECM

Models:
(01-05 Chevrolet Silverado) and (01-05 GMC Sierra) and ( 03-05 Chevrolet Kodiak) and (03-05 GMC Top Kick) The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.

Condition/Concern:
Some customers may comment that the Service Engine Soon indicator lamp is illuminated. Technicians may find DTC P1781 (model years 01-02) or DTC P0802 (model years 03-05) (Transmission Control Module - Incorrect Voltage Level on MIL Request Circuit) stored in the ECM. This condition will usually occur during engine start up. This code sets when the ECM and TCM do not communicate properly on the initial "hand shake" during power up. One module may be powering down during crank due to a low voltage situation.

Recommendation/Instructions:
Note: For 2001 vehicles refer to TSB 01-06-04-020 for the latest ECM calibration. Check the vehicle for a possible weak battery or a dead battery in dual battery applications. Also check for possible poor battery cable connections, poor grounds, UBEC connections, or possibly an ignition switch dropping out during crank.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.

All grounds cleaned and batteries checked. How do I check the ignition switch to make sure it's not dropping out? Service Bulletin

Subject:
Allison LCT1000 P1781 or P0802 Setting in the ECM

Models:
(01-05 Chevrolet Silverado) and (01-05 GMC Sierra) and ( 03-05 Chevrolet Kodiak) and (03-05 GMC Top Kick) The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.

Condition/Concern:
Some customers may comment that the Service Engine Soon indicator lamp is illuminated. Technicians may find DTC P1781 (model years 01-02) or DTC P0802 (model years 03-05) (Transmission Control Module - Incorrect Voltage Level on MIL Request Circuit) stored in the ECM. This condition will usually occur during engine start up. This code sets when the ECM and TCM do not communicate properly on the initial "hand shake" during power up. One module may be powering down during crank due to a low voltage situation.

Recommendation/Instructions:
Note: For 2001 vehicles refer to TSB 01-06-04-020 for the latest ECM calibration. Check the vehicle for a possible weak battery or a dead battery in dual battery applications. Also check for possible poor battery cable connections, poor grounds, UBEC connections, or possibly an ignition switch dropping out during crank.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.

All grounds cleaned and batteries checked. How do I check the ignition switch to make sure it's not dropping out?

I do not do diesels but on gas engines GM trucks 1990's V8 this ignition switch did cause erratic shifting problems many had success replacing it.. they had over 200K miles on the vehicle.. so if high miles change the switch.

When the TSB talks about checking the ignition switch for dropping out during start up, are they suggesting the electrical switch be checked/changed or the key and tumbler cylinder along with the switch? Mine does have 256k on it. I do not do diesels but on gas engines GM trucks 1990's V8 this ignition switch did cause erratic shifting problems many had success replacing it.. they had over 200K miles on the vehicle.. so if high miles change the switch.

2001 2500HD 8.1L Allison 5 speed transmission. The ABS indicator and brake light came on during a few easy stops where the abs activated even on dry pavement and slow speeds. During the same trip and just a few miles later, the transmission kicks out of gear and the indicator flashes. Putting in neutral and back into drive the truck will move but only for a few feet before it does it again. Does the same in any gear, neutral "resets" it then it kicks out again. Only code I'm getting is a p1781 which I think is a transmission code but not sure. Could these two issues be related as they both occurred at the same time? I'm kind of at a loss right now trying to diagnose. Thanks

The ABS activating is probably a bad/dirty ABS/Wheel Sensor. I know on my old '03, my ABS/Wheel Sensor was filthy on the passenger side. I was also exhibiting the same symptoms you were having while coming to a stop. Cleaned it up and it lasted another month before it completely failed. Fairly easy piece to replace.

Not sure about the transmission though. I don't think there is any relation to your ABS issue. Could just very well be a coincidence. Or may be the ABS engaging when it shouldn't is causing the transmission to act screwy. I would try and replace the faulty ABS sensor first and see what happens. One way to determine which sensor is going back is to hit the brake at a low speed, let go of the wheel and when the ABS engages, see which way the truck turns. So if it goes to the right when the ABS engages, then you know the passenger front sensor is bad/dirty. Otherwise, you'll need a Tech2 to scan the OBDII to determine which sensor is going into fault.

When the TSB talks about checking the ignition switch for dropping out during start up, are they suggesting the electrical switch be checked/changed or the key and tumbler cylinder along with the switch? Mine does have 256k on it.

It's the electrical portion of the IGN SW. These have 5-6 different internal contacts, one of which powers the Tranny. As suggested, it would be a good starting point to replace it.

When the TSB talks about checking the ignition switch for dropping out during start up, are they suggesting the electrical switch be checked/changed or the key and tumbler cylinder along with the switch? Mine does have 256k on it.

The ignition switch is just that an electrical switch not a lock cylinder. The ignition switch has no key hole or is @the lock cylinder. it is down low at the bottom of the steering column...

Ignition switch replaced....problem remains. Moving on to replacing/reprogramming tcm. Hate shotgunning parts into a vehicle

Ignition switch replaced....problem remains. Moving on to replacing/reprogramming tcm. Hate shotgunning parts into a vehicle

good you replaced the ignition switch . that does fail over 10 yrs of use or gets weak on passing electrical power not expensive as well.

so next is the tranny electrical connector as that also will create the code and the drop out of gear.

you other issue is the front hubs need sensor voltage testing. the abs sensors on the front hubs do get pushed out due to rust. you will get NO codes but the brakes will not work @less than 5MPH..

the other issue can be the hub seals are shot and the hub is rusted causing the magnetic sensor to get fouled and not produce the correct output voltage @ slow speeds.

the sensor voltage must be 350MV AC or more when you by hand spin the hub .. all it takes is one side to do it ....[again NO codes will throw ]

having multiple electrical problems is good brain exercise..........

good you replaced the ignition switch . that does fail over 10 yrs of use or gets weak on passing electrical power not expensive as well.

so next is the tranny electrical connector as that also will create the code and the drop out of gear.

you other issue is the front hubs need sensor voltage testing. the abs sensors on the front hubs do get pushed out due to rust. you will get NO codes but the brakes will not work @less than 5MPH..

the other issue can be the hub seals are shot and the hub is rusted causing the magnetic sensor to get fouled and not produce the correct output voltage @ slow speeds.

the sensor voltage must be 350MV AC or more when you by hand spin the hub .. all it takes is one side to do it ....[again NO codes will throw ]

having multiple electrical problems is good brain exercise..........
Neutral safety switch and both connectors to it have now been replaced. Same issue remains. Truck pops out of gear in reverse and drive and gear indicator blinks. If I accelerate forward a little harder it doesn't pop out but won't shift out of first gear. Also, the speedometer is not reacting at all. Time to take it to the dealership.

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