Hello,



My 1996 LT1 firebird is having an issue where the power feed from the PCM (Circuit 465 - Green/White Wire) to the relay is dropping 12v for short intervals and causing a bucking as it only drops for a few seconds and I assume that since this is a M6 car once it powers back on the engine starts running again.



The section in the pick from the PCM to the factory connector C220 has a few splices in it so I ordered new terminals from Efi Connection and replaced that section of wire and I am still having this issue, I suspect the PCM is dropping the voltage just not sure about how to determine what the root cause is. The car is cammed and tuned with a 95 PCM and also came with a second 1995 PCM which I loaded the tune on and still had the same issue. I back probed connector C220 and that is when I found the voltage dropping, which is why I replaced that section of wire that is outlined in green on the pic.



If I supply 12v to that wire with a key on source I do not have any issues but the pump runs all the time with the key on. I tried Scan9495 and bought an eBay tech2 scan tool and neither show me a fuel pump enable signal. VATS is turned off and the starter relay is grounded and I am using a non chipped key.

 

Couple clarifications:

Are you saying the PASS-Key II (VATS) system requirement for the 50Hz fuel enable system has been “programmed out” of (turned off in) the 95 PCM?

And as part of this you have not rewired the fuel enable signal from the 96's BCM, which replaced the 95’s theft deterrent module?

And Scan9495 shows neither a “1” in column AV “Fuel Enable (VATS)”, nor any indication of DTC 46 in column BE?

Just a note, not sure if it’s important- There is a subtle difference at the fuel pump relay between 95 and 96, mainly with regard to the function of the fuel pump prime connector.

 

From what I understood and sounds like I am wrong the Fuel Enable (VATS) was for the signal from the BCM to the PCM and honestly I did not look at that while driving as I assumed I was looking for the signal from the PCM to the pump and thought that one did not matter since it was "tuned out"
and the VATs box in the tune has been unchecked and I have not rewired anything for the BCM , not sure if the previous owner did as I have only had the car since the first week of may this year. I was told the starter relay was grounded and vats was disabled so a non chipped key could be used.
Even though I used to be a tech years ago I am inexperienced with this system so any clarifications or re-education is welcomed.

 

I went back out and reconnected the SCAN9495 and it does show a 1 in the vats field but no 12v at the C220 connecter on the DK GRN/WHT wire when cycling the key

 

Thank You for your info.