G5 Club banner
1 - 15 of 15 Posts

·
Registered
Joined
·
13 Posts
Discussion Starter · #1 ·
I disconnected my battery overnight, came back in the morning, reconnected it, and tried to start my car.
It tried to crank a few times, then stopped. I still get power to the accessories, but there's no crank when I try to start it.
I figured it might be a short in the battery, since it was really badly corroded. Took it to the store, the test came back bad, I replaced the battery and the positive terminal. Cleaned the ground wire, and reattached. No difference.

I've tried the key relearn. It didn't work. The dash wasn't really showing the anti theft light beyond the usual quick flash, but maybe I've just never noticed that before?

I haven't checked the relays yet. I have checked the fuses. They all look OK, except the fuel pump fuse. It's not receiving power at any point. If the car is off, if it's in ACC, or if someone is turning the keys. I'm not sure how it gets power, but I'm guessing the ECM turns on the relay at some point during power on?

I had some doubts that it would help, given that things were fine until I disconnected my battery, but I tried adding the extra ground wire to the ECM. It didn't work.
I've tried cleaning and re-attaching both ends of the battery-to-chassis ground. Didn't help.

I've searched high and low on the net, and ... it sounds like "power steering. low fuel" is kind of a poor indicator of a specific problem, but I don't really know where to go next. I don't want to start randomly swapping parts on a 2008 low-trim G5. I don't know how to check if I blew the ECM. Is there a diagnostic procedure I can do to verify something like that? am I missing something obvious? Should I pick up a 12v wall wart, a multimeter, and check the relays?
 

·
Registered
'Gremlin' the 07 G5GT (913L)
Joined
·
164 Posts
I disconnected my battery overnight, came back in the morning, reconnected it, and tried to start my car.
I don't know how to check if I blew the ECM
I dont see how disconnecting a battery could fry your ECU
Does the Radio turn on? If not then check your battery connection, theres no power to the BCM. There should be 2 wires coming off the positive end, the smaller one is for the BCM. If your Radio does come on then I'd start playing with relays/fuses
 

·
Registered
Joined
·
13 Posts
Discussion Starter · #3 ·
I dont see how disconnecting a battery could fry your ECU
Does the Radio turn on? If not then check your battery connection, theres no power to the BCM. There should be 2 wires coming off the positive end, the smaller one is for the BCM. If your Radio does come on then I'd start playing with relays/fuses
yeah, I don't quite get it either. The radio comes on, as do all lights and the horn works. I've been meaning to pick up a multimeter for a while. This weekend I'll grab one and will test the relays. All of them except 1 are original as far as I know, so they're getting a little past their best before date. Will post an update on Saturday or Sunday.
Thanks
 

·
Registered
'Gremlin' the 07 G5GT (913L)
Joined
·
164 Posts
I did find a thread about what sounds like a similar if not the same issue you're having
Credit to Coby7 Wont Start, Low Fuel, No Power Steering
Pull the "ECM/TCM" fuse out from the BCM block and replace it even if it looks good. You are losing GMLan between BCM and Theft deterent module and ECM. Replace with a new fuse even if it looks good. This fuse is called ECM/TCM but actually it powers the "BCM" GMLan translator that communicates with all CPUs.
Gives ya a spot to start looking around atleast
 

·
Registered
Joined
·
13 Posts
Discussion Starter · #5 ·
I pulled it, but haven't replaced it yet. Same with the IP/IGN fuse and the Passkey fuses. Since they come in packs of like 10, I'll probably end up replacing several just in case. I've got my shopping list ready for tonight.

With gas prices being what they are, if I park this thing for a couple weeks, I'll have enough saved up to buy a decent code reader to make diagnostics easier.
 

·
Registered
Joined
·
13 Posts
Discussion Starter · #6 ·
I replaced several fuses. In the engine block, IP/IGN, the fuel pump, and ecm/trans. In the passenger compartment block, I replaced the ignition switch/passkey 3, HVAC/Passkey3 and the ECM/TCM. None looked blown, none tested bad, but what the heck.
I pulled and tested the continuity and function of the crank relay, the fuel pump relay, the pwr/tran relay. I didn't get a chance to check the run/crank relay. Will check that tomorrow in the afternoon.
There's voltage on pin 87 (I think it's 87? I'll confirm) on the fuse block for the above relays. I didn't have someone around to turn the key to see if a signal is getting to the relays during startup, but I've heard clicking when it's turned earlier, so at least some relays are activating.

There's a little green corrosion on the ground wire going to the battery. I don't know how much is too much, or if any at all is too much. The connector looks fine.

Either way, still no crank, no start.
 

·
Registered
'Gremlin' the 07 G5GT (913L)
Joined
·
164 Posts
Is it still throwing those warnings on the DIC after the fuses?
 

·
Registered
'Gremlin' the 07 G5GT (913L)
Joined
·
164 Posts
If its not throwing any actual CEL then I think its gotta be something electrical
 

·
Registered
Joined
·
13 Posts
Discussion Starter · #10 ·
I'm going to check the resistance between the battery and the ground tomorrow afternoon, just to rule out that bit of greenish wire as the source of the problem. If that's not it, if I have time, I'll pop out the ECU and will see if any caps are popped or if anything is scorched or something.
 

·
Registered
Joined
·
13 Posts
Discussion Starter · #13 ·
Replacing the ground wire led to some small progress. The "Low Fuel" indicator is gone.
I tried putting the key into the 'ACC' position, and jumped the crank relay and the fuel pump relay at different times. The crank came on and went off as expected. The fuel pump clicked on and I verified at least some current across the fuse using a test light.

I got a little curious and tested continuity on the engine fuse block. I should have taken notes, but I know for sure that PWR/TRAN and COOL FAN 1 had no circuit between pin 30 and the chassis. If I understand correctly, the ground for those comes through the ECM.

I'm leaning more and more towards the conclusion that I somehow managed to cook my ECM changing this battery. Will pull it this afternoon to take a peek inside.
 

·
Registered
Joined
·
13 Posts
Discussion Starter · #14 ·

Yep... sounds about right. That corroded ground wire might have undervolted the ECM and cooked it. I won't know for a couple weeks.
 

·
Registered
Joined
·
13 Posts
Discussion Starter · #15 ·
Went in today and pulled the ECM. I wanted to be a little thorough, so I checked the grounds on the harnesses... and one of the two grounds has no continuity to the chassis?! I'm not sure when or how that happened. Definitely seems like something to check out.

Does anyone know how to get info on what wires go to which pin on those? I guess I could chase them all down next weekend, but maybe somebody has a pinout diagram?
 
1 - 15 of 15 Posts
Top