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Automatic Transmission

A Transfer-Case Identity Crisis

So in your ever-expanding list of causes for stacked upshifts in computer-controlled transmissions, even if it is a two-wheel-drive vehicle, investigate the possibility of a four-wheel-drive-low command being the culprit otherwise known as a transfer-case identity crisis.

A Transfer-Case Identity Crisis

Shift Pointers

Author: Wayne Colonna, Technical Editor

Shift Pointers

  • Author: Wayne Colonna, Technical Editor

Good morning, this is Wayne from ATSG returning your call. How can I help you?

Yes, I have an early-shift problem with an E4OD transmission in a 1990 F-series pickup. I have changed the throttle-position sensor and the vehicle-speed sensor, and it still shifts quickly. I can’t read anything through my scan tool, so I am kind of stuck. What do you think?

Well, one other possibility to consider is that maybe the computer thinks you are in four-wheel-drive low while you are really in two-wheel-drive high. This would make the shift curve incorrect, causing the constant early, rapid upshifts into fourth.

That would be helpful, but this is a two-wheel-drive vehicle. I don’t think that’s it. What else do you think it could be?

Ah, not so fast now. Many times manufacturers run the same wiring harnesses for two-wheel-drive vehicles as they do for four-wheel-drive platforms. If this unused four-wheel-drive-low signal wire becomes inadvertently grounded in some way, even though you do not have a transfer case, the computer thinks you do and shifts the transmission accordingly.

Well, how do I know if this is happening to me?

Open the hood and look for the EEC-IV processor on the driver-side firewall (see Figure 1). Locate the No. 12 terminal in the 60-way connector (see Figure 2). If you find a wire in that location, you have a four-wheel-drive-low signal wire in your processor.

OK, let me go look. I am on the shop’s wireless phone and the truck is very close. Hang on a minute. Yeah, there it is; I see the computer. Where did you say that wire should be?
If you look at the connector, you should find numbers embossed into the connector to be used for wire identification. Look for the No. 12 terminal; it should be a light-blue wire with a black tracer.

Yeah, I found it; it’s there, all right. What do I do now?

Place the ignition in the ON position and probe into that wire with the positive lead of your voltmeter set to DC volts, and place the negative meter lead to the negative post of the battery. How many volts do you see?

OK, let me see, I have 0.78 volts.

There is the problem. The wire is grounded. Snip the wire and see how many volts you have coming out of the PCM.

I have 11.98 volts.

You have just fixed your problem, and you will see it on the road test. The computer no longer sees a current draw on that wire, which means it no longer thinks you are in four-wheel-drive low, and since you do not have a four-wheel-drive vehicle, you do not have to worry about that wire. It is grounded somewhere in the harness, but who cares? Take the open No. 12 wire coming from the PCM, tape up the open end and carefully tuck the wire away.

If this vehicle actually had a transfer case, the next step would be to see whether the 4×4 HI/LO indicator switch was malfunctioning. If it was not, now the wire from terminal 12 at the PCM to the switch would have to be either repaired or replaced.

Four-wheel-drive-low signals often are overlooked as the cause of stacked or rapid upshifts in computer-controlled transmissions. Obviously so when it comes to two-wheel-drive vehicles; why even look for a four-wheel-drive-low command? This problem is also prevalent with GM vehicles. There have been times with electronically shifted transfer cases that the button will send a four-wheel-drive-low signal to the computer without sending the command to the transfer case.

So in your ever-expanding list of causes for stacked upshifts in computer-controlled transmissions, even if it is a two-wheel-drive vehicle, investigate the possibility of a four-wheel-drive-low command being the culprit otherwise known as a transfer-case identity crisis.

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