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View Full Version : Bad ECU? Trailblazer SS, only 5.7 volts at fuel pump relay


Praxis
September 24th, 2014, 09:43 PM
Tuning a TBSS tonight, and the truck would shut down. Checking the Fuel Pump relay, I find that the trigger from the computer is only giving 5.7 volts.

Haven't checked the wiring yet, but anyone seen this, or could the ecu be bad?
bmc1025
September 25th, 2014, 12:19 AM
Check your grounds on the driver side firewall and anywhere else on the vehicle. I am almost positive the trigger for the fuel pump relay from the pcm should be ground not positive.
bmc1025
September 25th, 2014, 12:21 AM
Was the blazer acting up before you started tuning?
Praxis
September 25th, 2014, 04:45 AM
Was the blazer acting up before you started tuning?

Blazer is 12v trigger.

Yes it was, he had assumed it was passlock, so I disabled that.
bmc1025
September 26th, 2014, 06:07 AM
Are you getting 12v at all of the pcm and ign fuses when you are only getting 5 at the relay? If not check the ign switch out. If you are seeing 12v back probe the fuel pump relay signal at the pcm and preferably the 12v wire that feeds that part of the pcm. I'm on my phone so you will have to search for a pinout and diagram.
Praxis
October 1st, 2014, 12:32 PM
Tuning a TBSS tonight, and the truck would shut down. Checking the Fuel Pump relay, I find that the trigger from the computer is only giving 5.7 volts.

Haven't checked the wiring yet, but anyone seen this, or could the ecu be bad?

Ground issue. During the engine swap, they forgot the ground in the driver side knock sensor harness....

Attached, no problems now.