TAC/PCM troubleshooting

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May 28, 2015 | 11:33 AM

volvochimp

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TAC/PCM troubleshooting

Hi-
New to the forum and to LSx. I am having trouble getting a 2005 dbw lq4 swap running. It is a complete pull-out (confirmed running by seller) and I had harness modification and PCM unlock done by a provider on ebay.

My problem is that I get no rpm signal out of the PCM. Crank and cam sensors show signal into PCM on a scope but no RPM registers on scan tool while cranking. I also notice that my throttle doesnt open when pedal is activated - throttle body also makes clicking noises. PCM grounds test under .3V on drop test.

Identifix lists no RPM from PCM problem as bad PCM - fix is to replace PCM. My harness guy said that a bad TAC could cause no rpm also.

Both PCM and TAC for 2005 are pretty expensive. How should I proceed to troubleshoot this issue? I'm stumped. What should I do next? Is it true that PCM requires TAC module to register rpm? That seems unlikely but I am new to this stuff.

Thanks!

May 28, 2015 | 02:38 PM

DrX

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Is it also a 2005 truck pedal?

May 28, 2015 | 02:42 PM

volvochimp

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Quote:

Originally Posted by DrX

Is it also a 2005 truck pedal?

Yep! came with pull-out

May 28, 2015 | 04:34 PM

DrX

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Should be working if all matching parts and tune, wiring is correct and engine harness is grounded to block. TAC ground is normally tied into engine harness ground. Don't see how TAC would affect RPM reported.

May 28, 2015 | 06:08 PM

volvochimp

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Quote:

Originally Posted by DrX

Yeah - so likely PCM? Do you know if pedal is supposed to operate throttle plate with just key power on? That is not happening on mine.

May 29, 2015 | 07:38 AM

DrX

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Quote:

Originally Posted by volvochimp

Yeah - so likely PCM? Do you know if pedal is supposed to operate throttle plate with just key power on? That is not happening on mine.

Could be wiring/grounds, wrong tune in PCM. I am more suspicious of a wiring issue with a "modified" harness. Also the TAC and PCM need to both power up and power down together.

Are any trouble codes set? If there is a problem with the throttle system, it will be disabled(Reduced Engine Power mode) and there will be codes set.
Normally the throttle will be operational with the key in the on/run position. It will power down after 15 sec(battery saver mode) but will re-activate as soon as you touch the pedal. When you first switch the key on it should quickly self test(close and return to startup position). If tests fail it will trigger REP mode. Need key-off for at least 15 sec to clear REP, but it will return at next key-on if problem is still present.

All 2003-2005 truck TAC/pedal/PCMs should be interchangeable.

Last edited by DrX; May 29, 2015 at 07:45 AM.

May 29, 2015 | 10:59 AM

Pacer Racer

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All 2003 to 2005 TAC's will not interchange. The TAC has to match the OS. Seeing the post about the TAC and PCM needing to be powered up at the same time reminds me of an issue I ran in to. The guy had a battery disconnect and sometimes would forget to turn it on before trying to start the car. When he was turning it on he wasn't turning off the key, causing the Ing and Bat power to be applied at the same time. This cause the PCM to improperly boot and the not to run right. So make sure rhat that battery power is on before the ignition