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1990 Isuzu Trooper V6
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Discussion Starter · #1 ·
I just picked up a 1990 Trooper with a known issue. The "Check Trans" light comes on intermittently. I've done some research on my own and figured out I could pull the DTC from the computer. I was able to pull the DTC codes, I got code 12 (as expected) and code 33. As far as I can tell Code 33 indicates that the transmission control module is reading a force motor current draw greater than 1.5A indicating that the Force Motor (EPC) circuit is shorted to battery voltage. In your experience, how reliable is the computer at pinpointing check trans codes? For my first attempt do I pull the auxiliary pan and replace the force motor?
 

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I just picked up a 1990 Trooper with a known issue. The "Check Trans" light comes on intermittently. I've done some research on my own and figured out I could pull the DTC from the computer. I was able to pull the DTC codes, I got code 12 (as expected) and code 33. As far as I can tell Code 33 indicates that the transmission control module is reading a force motor current draw greater than 1.5A indicating that the Force Motor (EPC) circuit is shorted to battery voltage. In your experience, how reliable is the computer at pinpointing check trans codes? For my first attempt do I pull the auxiliary pan and replace the force motor?
I can confirm that DTC 33 indicates the current in the Pressure Control Solenoid (PCS) (aka Force Motor) control circuit is greater than 1.5 Amps. The current is highest when the truck is stationary at idle, the higher current causes the PCS fluid port to close reducing the hydraulic pressure. At idle the current is about 1.1 amps. As the truck accelerates the TCM changes the duty cycle of the PCS control signal resulting a reduction in current. At Wide Open Throttle (WOT) the circuit current is approximately 0.1 amps. The Throttle Position Sensor (TPS) is the widget that is controlling the PCS circuit and they do wear out. I recommend you try replacing the TPS before replacing the PCS. Replacing the TPS is a lot easier and less expensive than replacing the PCS.
 

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1990 Isuzu Trooper V6
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Discussion Starter · #3 ·
I can confirm that DTC 33 indicates the current in the Pressure Control Solenoid (PCS) (aka Force Motor) control circuit is greater than 1.5 Amps. The current is highest when the truck is stationary at idle, the higher current causes the PCS fluid port to close reducing the hydraulic pressure. At idle the current is about 1.1 amps. As the truck accelerates the TCM changes the duty cycle of the PCS control signal resulting a reduction in current. At Wide Open Throttle (WOT) the circuit current is approximately 0.1 amps. The Throttle Position Sensor (TPS) is the widget that is controlling the PCS circuit and they do wear out. I recommend you try replacing the TPS before replacing the PCS. Replacing the TPS is a lot easier and less expensive than replacing the PCS.
Thanks Buster 28! That's very helpful and good news that there is an alternative first try that doesn't include opening the auxiliary pan. I will look at sourcing a throttle position sensor and installing it to see if it resolves the issue. I'll update you on what I find.
 

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1990 Isuzu Trooper V6
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Discussion Starter · #4 ·
I can confirm that DTC 33 indicates the current in the Pressure Control Solenoid (PCS) (aka Force Motor) control circuit is greater than 1.5 Amps. The current is highest when the truck is stationary at idle, the higher current causes the PCS fluid port to close reducing the hydraulic pressure. At idle the current is about 1.1 amps. As the truck accelerates the TCM changes the duty cycle of the PCS control signal resulting a reduction in current. At Wide Open Throttle (WOT) the circuit current is approximately 0.1 amps. The Throttle Position Sensor (TPS) is the widget that is controlling the PCS circuit and they do wear out. I recommend you try replacing the TPS before replacing the PCS. Replacing the TPS is a lot easier and less expensive than replacing the PCS.
I'm amazed at how well you know your stuff, even from my short description. I ended up pulling the TPS and was surprised that it came off after only taking off one bolt. It looks like at some point the other mounting bolt broke off, you can see the remains in the mounting threads (see picture on the right). I would imagine if the TPS pivots or moves when throttle is applied then the computer would sense something amiss. Right now I'm thinking I'll pull the whole throttle body and retap that hole so that it can be mounted with 2 bolts like it was intended. If not I would have to source a whole new throttle body which seems like they run $50-$100 used. I'm guessing the TPS is not original and was changed at some point and during the change the bolt broke.

I also went ahead and tested the TPS. The resistance between the 2 outside connectors was always 5.22K regardless of throttle position. The resistance between the outer and inner pins ranged from 1.31K-6.55K, depending on the throttle position. I believe that's equivalent to a voltage run of 0.83-4.17V, which seems reasonable. I realized afterward that I didn't check the sweep on the sensor though which could be the issue.

I think at this point I'll try replacing the TPS with a new one from advanced auto parts and getting it mounted correctly with 2 screws. Then I'll go from there and see if DTC code 33 shows up again.

Gas Auto part Engineering Nut Metal
 

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Discussion Starter · #5 ·
Wanted to add an update on this thread, I installed a new TPS and boy what a difference. My trooper isn't near as sluggish, it has some get up to it now. The old TPS was definitely bad. It seems like it took some time for the computer to adjust to the new TPS. I have had "Check Trans" light come up since but I just turn off the car and then turn it back on and it's gone. I haven't driven the Trooper a ton yet, my plan is to give it some more time maybe the computer needs time to figure out the new TPS. I think there's a good chance with some time the Check Trans light will stop coming on the problem will be resolved because the old TPS was definitely bad. I'll keep you updated on this once I can vet it out some more.
 

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Discussion Starter · #6 ·
I also wanted to add a link to this related thread, seems to be the same issue with a great detailed description by Buster28. David doesn't say explicitly what the problem was but maybe it was a bad solenoid in his case based on his final post?

 
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