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93 A518 overdrive issues.

Old 05-08-2017, 09:37 AM
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93 A518 overdrive issues.

Haven't posted anything on here in awhile. I pulled a another truck on my car hauler with my 93 w350 this past weekend and experienced overdrive problems. With my OD button pushed to cancel overdrive while towing it kept shifting in OD. I have a POT instead of a factory TPS on my truck. I pulled over and disconnected the POT thinking it would keep the trans from shifting in OD, but it didn't. The weird thing is it only decides to shift into OD while going up steep inclines. I would think that inclines would make a trans want to drop out of OD if anything. I was about to make the trip home towing the truck without any problems. This morning I connected the POT and drove to work without a trailer and my truck still kept shifting into OD while having it locked out of while going up hills.

I thought that the POT played a major part in the OD fuction, so I am confused it still goes into OD with it being disconnected.

Thanks in advance for any help.
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Old 05-08-2017, 02:04 PM
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The overdrive lock out switch is a momentary contact switch. The toggle that holds the setting is in the PCM.

That said, if you hit the switch, the light in the switch should turn on and off alternately. That light indicates the state of the OD lockout register in the PCM.

The switch circuit is a fairly dry circuit (low current). Any dirt in the switch, or frayed wiring could randomly toggle the PCM OD register.

If all that seems to check out, the fault is probably in the PCM itself. Given that you're describing shifting into OD when pulling hard and with the TPS properly adjusted, it's looking like PCM.

Another possibility is that the electronics isn't involved at all. 12V goes to the OD solenoid, and the other side is grounded by the PCM. If that wire is frayed and randomly grounding, it would engage the OD no matter what the PCM was doing. Again, high torque rocking the engine might just do that.
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Old 05-08-2017, 03:36 PM
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Thank you sir.

Here is a little history. About 8 months the actual light for the OD button quit working, but the button still worked. Once I hooked the POT back up this morning the OD button does allow the OD to engage and disengage on level ground. What's weird is the button has no effect on cancelling out OD on inclines.

Also, I converted my charging system to a external voltage regulator 6 months due to a suspected problem with the pcm.

Are there any other test I can perform to tell if it is the pcm? If it is the pcm should I purchase a new one or have mine reconditioned?

Is there a way to rule out the OD solenoid part itself?
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Old 05-08-2017, 03:45 PM
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Also, I have another question? How is it possible for the trans to shift into OD with the POT disabled? I thought the POT/TPS signal was necessary for the OD function to take place.
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Old 05-08-2017, 10:19 PM
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The OD solenoid will only fail off. I guess it would be possible for the internal OD solenoid going to the PCM to short to ground and operate the solenoid.

If you disconnect the orange wire from the transmission leaving the blue wire in place, and it still randomly shifts into OD, that would be the case.

The orange with light green tracer wire from the OD solenoid goes up to the 10 pin connector under the intake manifold, and from there to the PCM pin 55. A short to ground anywhere along that wire will shift the transmission into OD.

The signal wire to the light on the OD switch is black with an orange tracer, and comes from PCM pin 37. The PCM grounds it to light the lamp. With the lamp off, key on, there should be battery voltage on it. If there isn't the lamp is bad. If there is and the lamp doesn't work, either that wire to the PCM is bad, or the PCM is bad.

All of the problems you are having, including the alternator failure, could be caused by a cruddy connector on the PCM.

Hope it helps
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