Expert: Robert Summers - 10/2/2008

Question
I have a 1999 C1500 in my shopThat started  with a fuel pumpp problem. The engine died no fuel pump would not operate. I checked the prime terminalin the power box. Nothing. I replaced the ECM with one from a salvage yard. Same ser number and the four letter code after this is is the same. Now the pump comes on but it will crank and then die. The code that comes up is P1635 / 5v reference signal. The local GM dealer says the new ECM needs to be reflashed. IT will crank and run with the old ECM if you supply powerto the Prime terminal I am thinking about dragging it to the local tech school where I just graduated from and use the TECH 2 to tryand solve this. I also forgotto mention that this is relative's vehicle. Any guidance would greatly appreciated.

Answer
Hi Blaine.... did you check to see if the fuel pump relay was ok....the prime circut is triggered by turning the key on and activating the fuel pump relay. pump should run for 3 seconds. check that the relay has no corroded terminals and has 12 v when you turn on the key. inj pulse is controlled by the ign module as the engine cranks it receives a referance from the crank/cam sensors. check that you have that referance to the ign module. also check that the oil pressure sensor is ok. if relay is bad it should default to the oil pressure sensor to turn on the pump once it has enough oil pressure...do not reflash the ecm as it can cause other diveability problems that don't exsist now... once refashed you can't go back to orig settings.... hope this helps