28 code

SEAN SILBERHOLZ

MEMBER
1993 CHEVROLET SILVERADO
5.7L
V8
4WD
AUTOMATIC
15,000 MILES

Hi, I have the truck listed above K1500. I got a 28 code. It is an OBDsystem. I am having problems with the transmission sometimes. I looked it up and it says it is the auto transmission range pressure switch error. Is their anyway to test this or should I just replace the transmission pressure switch? Thanks.

Tuesday, May 8th, 2018 AT 9:21 PM

1 Reply

STEVE W.

MECHANIC
15,233 POSTS

Yes, it is possible to test it and it should be as it could be the switch, the harness to the transmission or the PCM itself. Rather than write out the entire procedure it is attached below. The first step is to reset the code and see if it comes back. The rest involve the use of a test light and/or volt/ohmmeter to test the harness and connections. About half the time it's the harness that is the problem. The other half is split between a bad switch and bad PCM.
The chart shows the valid combinations of the switches. The code sets if A and C are at zero volts or if all three read zero for more than two seconds. That can happen easily in the harness if the wires rub through and ground out, especially where it comes up from the transmission and around the top of the block.

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