Sometimes I Wonder, ‘What Were They thinking?’ - Transmission Digest

Sometimes I Wonder, ‘What Were They thinking?’

Case in point is a 1992 Ford Bronco (Figure 1) with a 5.8-liter engine and E4OD transmission. The vehicle came to our shop with shifting issues. The owner said he was driving along and all of a sudden it shifted down and then slammed back into gear, and then a short time later it shifted down again but this time would not upshift. He thought it was stuck in low, although it did not act up on the way to our shop that morning. The owner had recently bought this vehicle with some known issues. There was a rear-ABS lamp on and the Check Engine lamp was not on.

Sometimes I Wonder, ‘What Were They thinking?’

R&R Tech

Subject: Shift problems
Unit: E4OD
Vehicle Application: 1992 Ford Bronco
Essential Reading: Diagnostician, R & R
Author: Randy Peterson

R&R Tech

  • Subject: Shift problems
  • Unit: E4OD
  • Vehicle Application: 1992 Ford Bronco
  • Essential Reading: Diagnostician, R & R
  • Author: Randy Peterson

Case in point is a 1992 Ford Bronco (Figure 1) with a 5.8-liter engine and E4OD transmission. The vehicle came to our shop with shifting issues. The owner said he was driving along and all of a sudden it shifted down and then slammed back into gear, and then a short time later it shifted down again but this time would not upshift. He thought it was stuck in low, although it did not act up on the way to our shop that morning. The owner had recently bought this vehicle with some known issues. There was a rear-ABS lamp on and the Check Engine lamp was not on.

This Bronco had been to another shop with the previous owner. We were able to acquire copies of the repair orders. Six months prior the original owner complained of slipping, hard to get into gear, play in the shifter and shifting hard from first to second gear. At that time the shop replaced the transmission range switch (MLPS) and updated the connector. About four months later the original owner took the vehicle back to the repairing shop with the complaint of slipping badly on takeoff. The shop performed a pan inspection and found minimal debris in the pan or filter. The shop serviced the transmission and did something that left me thinking, “How did they do that?”

The notes on the repair order said: “We set the computer where the system would erase its memory on key cycles. Vehicle may need a computer replaced.”

Now that it was here I needed to figure out not only what was going on with the transmission but also what was done to the computer. I started with printing out a schematic and a computer pin chart. I reached deep into my toolbox and got out a long-lost tool, my 60-pin break-out box (Figure 2).

Since there is virtually no data from the scan tool I wanted to see what the computer was commanding. Were the random shift problems commanded, or was it something from inside the transmission? As I was accessing the computer I noticed that someone had been here before. There was a wire splice and a yellow wire spliced to a red wire. Once I removed the computer connector I could identify the locations of the wires in question.

The yellow wire was from pin 1, keep-alive power, and the red wire, pin 37, vehicle power, which is ignition voltage (Figure 3). At first I was thinking, “What is going on here?” Then it all became clear; this is how they made the computer lose its memory every time the ignition was cycled. All of us in the shop were scratching our heads and asking ourselves, “Why would you go to that much trouble to kill the keep-alive memory but not figure out the problem and fix it?”

I put the wires back to their correct locations and hooked up the break-out box. I checked the powers and grounds first. When that all was correct I then connected another seldom-used tool, my LED box (Figure 4). I wanted to monitor the shift solenoids to see what they were doing during the malfunction. I also did a brief scan of the pressure-control solenoid while I was there. It looked proper. Off on the road test I went.

The transmission was working very well, shifts were smooth, timing was correct, and I was thinking, “Could the rewire be all that was wrong?” About two blocks from the shop all of a sudden it downshifted, then upshifted, and back and forth a couple of times. I was watching the LEDs, and the transmission was commanded to shift this way. It then shifted to first gear and would not upshift. When I pulled into the shop I grabbed the scan tool and checked for codes. I found P0634 stored in the continuous memory codes. These symptoms would be consistent with a faulty manual lever position (MLP) sensor. Knowing the MLP was replaced several months ago I was going to have to inspect all the aspects of the MLP circuit.

I inspected the sensor and unplugged it. That is when I saw three of the pins backed out of the connector (Figure 5).

I also took note that the wire’s insulation was bad and there were bare areas exposed on all the wires (Figure 6). I opted to replace the pigtail harness. I was sure this was the problem and was likely the problem when the computer was rewired. I replaced the pigtail, road-tested the Bronco and then rechecked the codes. It drove great; the shifts were very nice, TCC worked.

Just one problem: The P0634 was stored in continuous memory. Maybe it did need a computer. Before I could confidently recommend that I had better make sure everything I did was correct. As I was retracing my steps my buddy asked, “Are you sure the wires were in the right spot on the MLP?” I told him I put it back together exactly how it was. That wasn’t a good answer. I pulled out some old information that described the connector, the wire colors, circuits and positions (Figure 7).

Sure enough two wires were not in the correct locations (Figure 8). I made the correction and drove the Bronco, and now no more codes were stored and the Rear ABS lamp was no longer on.

This is my take on the situation: First of all I’m sure the MLP was at fault and causing the shifting issues when the vehicle was looked at last time, but when the code would not clear and the vehicle didn’t always shift correctly the conclusion was that it must be a computer at fault.

I see this scenario more and more and ask myself, “What were they thinking?” or maybe better yet, “Why weren’t they thinking?” There have been many times when I made a diagnosis and performed the repairs and did not get the right results. Then I had to go back, retrace my steps and see whether I missed something, did something wrong or misdiagnosed it. I don’t like when that happens, but it’s all part of the game.

Randy Peterson has been with Certified Transmission for more than 20 years and has done it all – building transmissions, diagnosing cars and running a store.

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