Cars are pain
 

Hello! New forum member and new G owner.

skip to bottom for short version.

back story:
I bought a 2004 G35 AT Coupe with 145k on it for $500. The catch you may ask? No crank, no start. I towed this bad boy home and went to work diagnosing issues. Discovered that the ipdm was toast. Ordered a replacement, dropped it in and it fired right up.

The current issue:
In my new running G excitement I hooked a battery up to it backwards. There was no sparks or anything, but there was a small amount of smoke coming up from under the engine. I checked all the fuses and none of the main fuses blew, a 15A radio fuse popped and that was it. Now when I try to start it I hear a clunk, not a click, it sounds like it’s coming from where the transmission mates to the engine. You can also feel the clunk. And see the engine move when it happens, but it does not crank. The only other issue I can find is that my power windows don’t work now as well. It almost sounds like the starter is engaging but not getting enough juice to spin.

Short version:
Reversed polarity, now car won’t crank, hear and feel a loud clunk from rear of engine when trying to start it. Power windows also will not work.

all fuses checked
grounds ( that I can find) checked
tried different battery and jumping
Starter will crank if bridge wired to positive terminal on battery.
swapped out ignition & starter relay
spun crank bolt (not seized)

Anyone have any experience with this?

U checked the fusible links on the battery cables?

yeah, checked the fuseable link fuses, ipdm fuses, small engine fuses, and interior fuse box fuses.

If the starter solenoid is engaging but not actually turning the engine then you probably just melted the winding in the starter.

You said it will crank normally just bypassing the start circuit and going straight from the big wire to the solenoid?

Yes if I pull the starter relay in the ipdm, wrap a wire around the positive tooth on the relay, then touch the other end of that wire to the positive post on the battery the starter will crank.

side note:
The only video I can find that mimics the “clunk” is someone trying to start a seized engine. It’s identical to the sound I’m hearing, but I can turn the crank by hand so the engine isn’t seized?

Ok just to verify, with the start relay installed it makes that loud "THUNK" sound as the starter solenoid engages.

But when you bypass that relay from the energized side you can crank the engine directly from turning the key to the start position.

The only thing that relay does is provide an interlock so the CLUTCH PEDAL has to be pressed on the 6MT. On the 5AT that relay is controlled by the TCM to make sure you are in park or neutral.

Are you SURE fuses 71 and 78 in the IPDM are good?

Did you pull literally every single fuse in the fusible link, the IPDM, and cabin Fuse Block - J/B and visually check the fuse and the pins that it stabs into?

The starter solenoid is a lot louder than a relay, the relay makes a faint clicking sound, the starter solenoid is very loud and easily heard from the cabin of the car.

The clunk noise is only when I turn the key to the start position. If I bypass the relay I can only get it to crank by bypassing the ignition completely by touching the bypass wire to the positive post on the battery.

I pulled every fuse in the IPDM, but I will recheck 71 & 78 when I get home. I didn’t pull the fuseable link fuses I just took the cover off and visibly checked them, aswell as check with the multimeter in both sides of them.

the relays were making that faint click before I replaced the IPDM, this thunk is a lot louder and you can physically feel it.

the wire that was smoking when I reversed the battery was a positive wire that’s runs from the fuseable link under the heat shield and toward the transmission. Do you think reversing the polarity could of fried the TCM and that’s why it won’t let the starter crank from the ignition?

I’ll be climbing underneath to get a look at the starter and the TCM later today, I’ll report back with anything new. Thanks!

Rechecked all the fuses, all good. Fuseable links and pins look fine. I’m thinking it’s the TCM. If I turn the key to ACC so the fuel pump primes and then I touch the starter relay bypass wire to the battery then the car fires right up. Although I took it for a short test drive and it shifts just fine, so maybe it just roasted the relay?

the power windows still don’t function, and the speedometer doesn’t work, but the speedometer could be an old problem as I haven’t driven the car till today.

You probably melted the primary wire to the starter, I would start by removing and inspecting it, I'm betting you'll find a melted section in it.

Will give it a closer look tomorrow. If that was the issue then how am I getting power to the starter by bypassing the relay though?

Ahh yeah good point, probably not that wire then.

Swap the starter relay with another relay in the IPDM, I think it's the same as the headlight relays, then you can check it to see if it's working.

Worst care scenario (or temporarily just to allow you to drive the car while you troubleshoot) you could literally just bypass the relay circuit and go directly from the hot side of the relay (when the key is turned to start) with a plug-in spade terminal to the starter solenoid. The car would be able to start in any gear though so keep that in mind.

currently I’ve got an on/off switch wired into the relay, so I just turn the key to ACC and hit the switch and it starts lol. Will be driving it like that until winter.

This winter I’ll be taking the motor and trans out and swapping to a cd009 trans, and rebuilding the motor with some upgraded parts.

thanks for all the help, sometimes a fresh perspective makes all the difference.

One pin on the contact side of the relay is energized directly from the key in the START position. You could just wire directly off of that one to the starter so it sort of feels normal.

You should definitely check the other side of the relay in the start position though to see if there is voltage being applied to the control/winding side. If there is no power on the winding side of the relay then it's probably a bad ignition relay, or a bad IPDM.

Easiest troubleshoot is to just make sure that all those relays in the IPDM are actually working. I think they're all the same relay part number so they're interchangeable, headlight low/high, ignition, start, ECM, fog light, fuel pump, and throttle body. Label them all with a sharpie, swap them out one at a time with the headlight relay, make sure the headlights still turn on.

If either the ignition or the start relay fails the car won't start.