Technically Speaking
- Subject: Diagnosing a complaint of no forward movement
- Unit: Mitsubishi F4A51
- Essential Reading: Rebuilder, Diagnostician
- Author: Wayne Colonna, ATSG, Transmission Digest Technical Editor
A 2001 Mitsubishi Galant with an F4A51 transmission came into a shop here in Miami with a complaint of no forward movement. The complaint was verified, and the unit was replaced with a rebuilt transmission that matched in gear ratio. After a thorough road test the car was returned to the owner.
The very next day he was back with the car, complaining that the original problem had recurred but the transmission now seemed to be working properly. The shop made arrangements with the owner to leave the car, which was road-tested multiple times over a couple of days and never exhibited the problem of no forward movement.
The car then was returned to the owner, only to come back the next day with the same mysterious complaint, yet the owner was able to drive the car to the shop. This time the shop decided to rebuild and install his original transmission. Again, it returned the following day with the same complaint. What a nightmare!
The car then was parked outside the shop. Later that day the weather deteriorated and dark clouds rolled in, bringing torrential rain for which Miami is known during hurricane season. One of the shop’s technicians went outside to bring the car into the shop, and when he turned on the headlights and put the car in drive, it didn’t move. So he shut the car down, went inside and reported the problem.
A second technician then went out and started the car himself, and after he put it into drive without the headlights on, it moved forward. Neither technician had yet realized that one had the headlights on and the other had them off. But since the first technician had finally verified that this problem truly existed, the second technician decided to take the car on a road test.
When he turned the headlights on to drive it on the main road, it didn’t move forward. “What happened?” he thought to himself. So he turned off the headlights before shutting down the car with the selector lever still in the drive position, and the transmission engaged. “Aha!” he said to himself, so he turned the headlights on again and the car went to neutral. So the fix is to never drive at night! Well, it was a good try.
Now that the technicians had discovered that the mysterious complaint of no forward movement occurred only when the headlights were on, the next step was to determine why. As it turned out, one of the taillights had a compromised ground. When the lights were turned on, voltage sought a ground path through the backup-light circuit.
In Figure 1 you can see that when the selector lever is placed into the Drive position, the park/neutral switch supplies voltage to terminal 102 at the TCM (see figures 1 and 2). When the headlights were turned on, voltage seeking a ground path through the backup-light circuit would supply a signal to the TCM at terminal 108 as well. Then the TCM would think the transmission was in both drive and reverse and, as a safety feature, would energize the underdrive solenoid. This prevented the underdrive clutch from engaging and put the transmission in neutral. This also would explain why the vehicle would work well in reverse, since the TCM would see no signal other than reverse.
So it seems it was a bad taillight ground that trunked the system – or is that trumped?