Shift Pointers
- Author: Pete Luban
How many times have you successfully completed a job on a vehicle only to have it come back or not leave with some unrelated problem? John Q Public doesn’t care about or understand your explanation of how it has nothing to do with the repairs you did, only that he has a problem and you were the last one to touch this vehicle.
This is such a situation.
A 1995 Mazda Millenia equipped with the 2.3-liter supercharged engine and the LJ4A-EL (JF4O3E) transaxle came in for a basic overhaul.
During the road test after completion of the job, the traction-control OFF lamp came on, as did the traction-control-system lamp and the Check Engine lamp. Holy mackerel, what the #%A* is going on here?
Scanning the engine computer resulted in retrieval of a code P0105, indicating a fault with the MAP (manifold air pressure) sensor. This shop is very thorough and therefore knew that this code was not present and the warning lamps were not illuminated when the vehicle came in.
It seems that during either the removal or installation of the transmission a vacuum “T” was broken. The technician found that this vacuum T is prone to failure because it’s in a place where heat is constantly present. In other words, it doesn’t take much to break it!
One of the functions of this T is to supply vacuum to the MAP sensor, as shown in the vacuum diagram in Figure 1. There’s the reason for the Check Engine lamp and the P0105 code. Well, that solves that part of the mystery.
So what about the traction-control lamps that are on?
If you take a look at the ABS/TCS code chart in Figure 2, you will notice that the only engine-management-system code that affects the traction-control system is the MAP sensor, and what’s the function of the vacuum “T” that was broken? To supply vacuum to the MAP sensor. Is this starting to make sense?
Figure 3 indicates that the 1K terminal at the ABS/TCS control unit receives a MAP-sensor input signal, which may cause a code 10 to be stored in the ABS/TCS control unit and therefore turn on the TCS indicator lamp. With a code stored in the TCS control unit, the TCS OFF lamp will illuminate to indicate a TCS fault.
Figure 3 is a view of the engine control module (ECM), which indicates that the 3W terminal receives the MAP sensor signal input. When the vacuum T broke, the ECM sensed a problem with the MAP-sensor signal and stored code P0105.
So, there you have it. Once the vacuum “T” was replaced, all systems returned to normal operation, the codes went away, the warning lamps went out, and all was well other than some unnecessary labor, because the vacuum T was not easy to get at.
Then there was the faulty shift solenoid on a Mazda 626 with a CD4E that initiated the torque-reduction system, but that, my friends, is another story.
Many thanks to Diane at Cotton’s Transmission, LaGrange, GA, for sharing her experience with me. A nice job of nailing the problem, Diane.
Until next time.
Pete Luban is a technical consultant for the Automatic Transmission Service Group (ATSG) and a frequent contributor to Transmission Digest.
