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1998 code P1518 TAC Module,Reduced Power,Won't Accel

by Jack
(Old Saybrook,Ct.)

TAC Module Wiring

TAC Module Wiring

Hi,my 1998 conv.is having a problem with codes P1518 TAC module and P1571 Traction Control.Can not accelerate much over an idle,sputters and just runs terribly.Did some research and saw that 97 and 98's had a problem with the TAC modules so I tried to order a new one and found that they were on a national back order so I settled on a (supposed)good used one.Upon removing the TAC module I saw that apparently it had already been replaced because it had the new,updated part #25319443.I went ahead and installed the one I had bought which had part #12578953 which is supposed to also work as well as a couple of other part #s. No change in problem. Tried cleaning engine and chassis grounds,checked connections at TAC motor and gas pedal and PCM. Checked TAC motor harness at TAC motor and at back of engine where it takes a downward turn-all seem to be OK. I then followed the Alldata troubleshooting chart for code P1518 checking voltages and continuity and all tested well until step 18 where it asks if there are 0 volts at the TAC module serial data circuit-I have about 4 volts like I had back at step 13 where they asked if there were 0-4.5 volts at both serial data circuits. It looks like Alldata is saying to replace the TAC module if there is not 0 volts. At this point I am not sure if I installed a bad TAC module or if I am missing something.Not sure what to try next.If I do need a TAC module I read that Dorman #599-959 will work if you reverse the wires at the TAC motor connectors.Is this true or does anyone else sell new TAC modules so I don't have to buy another used one? Thank you,Jack.

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Jun 10, 2019
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starstarstarstarstar 		TAC Problems
by: Chris Corvette Braniac

Jack,

So we were doing some work on your problem. Its hard for us to understand were you are with troubleshooting so we will do our best to explain. It appears you have a short in the wiring or maybe a faulty replacement TAC module. Please take a look at the wiring diagram. The wire you state has voltage where it should not. Please isolate the APP circuit (disconnect) the gas pedal plugs. Doing so will help isolate the location of the fault in the wire or the TAC module. If you still see voltage search the wiring for faults. Perform a continuity check in a loop isolating each wire segment. Most importantly inspect all plugs with the system. Corrosion on plug ends could be your problem.

The TAC module runs 2 identical systems to control the throttle body. A failure in any one will cause a fault. This is a cross check system. You can use this to understand which circuit is bad by comparing them.

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