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ECU fuse is fine when ignition ON/Crank. Blows when off
2011 Buick Lacrosse CXL 3.6L
Helping brother that's in another state figure this out.
Basically a snowbird car left in FL. Has been drove for a couple weeks than it stalled with a no start issue. Assumed battery/alternator. Starter about a year old.
Bought new battery while charging old battery. Old battery still good but was only reading about 5-6v
No start/crank with new battery. ECU fuse was blown. Quickly swapped out with fuel fuse and cranked nicely. (both 20a)
Now the odd parts...
When battery power is connected, ECU relay rapidly ticks. Rapidly ticks when opening door or trunk. Rapidly ticks when key turned to on position. Tick lasts for 1-2 seconds.
Ignoring the tick, with new fuses for the ECU & Fuel, the car cranks great.. can crank all day long. Fuses hold up as long as key in ON or Crank position.
BUT, the ECU fuse blows every time the ignition is turned to off position.
The rear window defogger uses same relay as ECU. So we swapped it and it still ticks in the ECU relay spot.
I had him search for ground wires. 4 total between battery and headlight. Some were only getting about 10v through. Had him clean them all and now reading same as battery.
Another ground wire on back of engine head. One more ground drivers side on stud of strut housing area. All cleaned up and showing same volt as battery.
I had him pull ECU relay to start testing power where relay plugs in. Only a partial reading as probe isn't reaching down far enough in fuse box slot. I'm not saying this is accurate because of it.. but when he did get it to touch, it showed between 3v-7v. We'll try again tomorrow when he can get a piece of wire or something to stick in there.
If relay is getting low voltage, that explains the machinegun rapid ticks. But why would it blow fuse.. only when turned to OFF position???
I had him search for ground wires. 4 total between battery and headlight. Some were only getting about 10v through ... All cleaned up and showing same volt as battery.
I'm not sure of what you are saying, here. Are you reading battery voltage on the grounds? How/where are the volt meter leads connected when testing the grounds?
"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer." I have learned more by being wrong, than I have by being right.
Positive to positive. ground to ground. I know it's not accurate as any other ground can complete circuit. Wire's are to newish/flexible with no rubs. No aging, stiffness, brittleness, rubbing etc. I'm just an old fashion mechanic with a hunch of fuse box and moved to start testing volts there.
After my last post I had him retest fuses with a digital circuit tester and probe in hole 1 of ECU relay. It was jumping 3-7 volts. It than spiked to 13v and 1/3rd of fuse box died. Now no power to center of fuse box but the rest tests fine.
Circuit board may be problem all along?
I've talked to 6 other old time mechanics. All with 25-48 years in the game. None of them have ran into fuse blowing when ignition turned off. It's always been that the fuse blows immediately, when turned on or when cranked. None of us experienced a blown fuse only when turned off.
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