Technically Speaking
- Author: Wayne Colonna
- Subject matter: Bad ground
- Issue: Why did this fix work?
There are times on ATSG’s technical help line, when we discover a solution to a problem, we would like to produce a bulletin or a seminar piece to share this information but do not. The reason for this is, although we determined a solution, we can only guess as to why the solution corrects the problem. Here is an example of the type of situation we find ourselves in.
A 2006 Ford Focus 2.3L with a 4F27E transmission comes into a shop with a complaint of a harsh garage shift into gear (drive and reverse), as well as harsh lift throttle coast downshifts. All forward upshifts are perfect. There are no codes stored in the system. In some cases, the transmission had been rebuilt, which included a different valve body and solenoids in an attempt to resolve the problem. A defective mass air flow sensor had also been considered as a possible cause among other sensor considerations, none of which corrected the problem. A clue as to where to begin to look came when a shop was willing to make a line-pressure check with a gauge. It was noticed that when the brake was applied, line pressure became slightly elevated. This pointed the tech to look into the brake switch and brake-light circuit. A bad ground in the trunk of the vehicle was discovered and repaired. This resolved all the harsh garage shifts and coast downshift bumps and clunks.
The problem here is we have no confirmation of exactly why a bad ground can cause this condition other than a guess. There would be ways to confirm what we think is taking place that would require making checks with a problem vehicle. In shops that we encounter with this problem, once the ground repair fixes the complaint, the car is quickly delivered to the owner. What is thought to be occurring is, with a bad taillight ground, the brake light circuit is looking for a ground path through the inhibitor switch circuit affecting PCM operation. But it’s only a guess. But we do know that fixing this ground eliminates the complaint. To find this ground circuit the liner needs to be removed from behind both the tail lights across the entire back of the vehicle (Figure 1). This will expose both left and right grounds for inspection and repair as seen in figures 2 and 3.
