91 Vert Popup Headlight Battery Reverse Polarity
 

Morning, Group!

This post is about damage caused by reversing polarity to the car's battery during a jump start. Disclaimer: I did NOT cross-wire the jumper cable leads. The person attempting the good deed was not knowledgeable in jump starting car batteries.

Roughly 2yrs ago, my car's battery died and required a jump. The person connected the leads to the supply car properly then unwittingly cross-wired the leads to the FC.

I suspected that the terminals on both cars were opposite of each other. To mean, the Pos (+) on the supply car was on the left while the Pos (+) on the FC was on the right. Therefore, the Pos (+) lead was connected to the Neg (-) terminal on the FC's battery and the Neg (-) lead was connected to the Pos (+) terminal on the FC's battery. Yes, I know this is incorrect but I never got the full story.

After the supply car supposedly charged the FC's battery, the good-doer attempted to start the FC. As soon as the ignition was switched to the ON position, a loud *CRACK* was heard then the headlights immediately popped up. The ignition switch was set to OFF immediately after the cracking sound.

It was unclear if the battery was replaced or not however, the FC was able to start after this incident. Again, I was not present during this evolution and I suspect that I was given the highlights of the story.

Shortly after this incident, the FC wouldn't start. It would crank but wouldn't kick over. That's a discussion for a separate post. For now, keep in mind that this incident happened 2yrs ago.

So this weekend, I dug into the problem. With the battery disconnected, I manually cranked down the headlights. That process was unremarkable. As soon as I reconnected the Neg (-) lead to the battery, the headlights drove up. I played with the headlight switch on the dash and it had no effect. I also checked the headlight relay on the nose of the car and it checked good. With the relay removed and disconnected the battery, I manually cranked the headlights down. Then I reconnected the battery and kept the relay disconnected. The headlights drove up again. So I dug around the wire diagram and noted the Body CPU had a headlight retractor relay function (see pg T-44 in main FSM). Furthermore, the loud cracking sound could only mean a solid state component blew.

I removed the Body CPU from the FC then opened up the case. What I discovered inside was great! The photos should speak for themselves. That poor transistor bore the brunt of an unwitting mistake. The battery cross-wire could have caused more damage or created a fire. So I consider to be lucky that this was the only damage. Things could have gone horribly wrong elsewhere!

I am currently getting a replacement CPU so the car can become whole again. Hooray for the FSM, good sense of troubleshooting skills, and stressing the importance of proper jump starting techniques to the good-doer.

The posts on the battery are different sizes so the "next time" you will be connecting the NEG to the SMALLER POST..

Yes, you are correct. The battery posts are different in size. The pos and neg leads on the car will not fit if the battery is turned around. They wouldn't physically reach unless the wires were extended or replaced the OEM wires. This particular problem involved cross-wiring the jumper cable leads from the supply battery to the dead battery.

Another note is that on the 2nd gens the NEG has a Yellow Stripe on the stock NEG Cable.
That screws people into thinking that it is Positive when in reality it is not.

mister, thanks for reminding me of the yellow stripe! I don't recall seeing it on the Neg wire. Now I have to check the cable. The connector itself was replaced many years ago but don't recall if the cable was replaced.

Sure enough, the neg cable has the yellow stripe! Man, talk about being blind to certain details.

i blew both headlight bulbs, the main fuse, and the retractor fuse one day with a jump starter on my vert when it had sat over a year and the battery was stone dead. They def dont like dead batteries and obviously not being reversed

Let us know when you get it fixed. I'm curious if this is all of the damage.

After my last accidental run in with reversing polarity on a 2009 BMW, I make damn sure to check.

This was one of those 9pm adventures in the dark with a jumper pack.

..uh..is that he trick with the cables attached to your Yoohoo?...hahhaa!
Bzzt..no Babies..lol!

The wife and I are starting to talk about them.... I may have to give the ol' jumper pack thing a try again.:lol:

Here's an update to the condition of the circuit board. It looks like the board cannot be repaired without using advanced techniques. After desoldering the transistor and zener diode, an inspection of the eyelets and solder runs show heavy damage. Refer to the photos for a closer look.
https://cimg2.ibsrv.net/gimg/www.rx7...925c1ff4fa.jpg

Note the broken solder runs between Q1 and ZD3.


https://cimg3.ibsrv.net/gimg/www.rx7...083313d253.jpg

Close-up of burned up solder runs. Note the trace that runs from underneath the IC chip to the right lead of Q1 is nearly gone.


https://cimg4.ibsrv.net/gimg/www.rx7...3b4b08a2eb.jpg

Eyelet damage


https://cimg5.ibsrv.net/gimg/www.rx7...f2af238b47.jpg

Backside view of the CPU board. Notice the missing eyelets?


https://cimg6.ibsrv.net/gimg/www.rx7...279b2c5d47.jpg

Close-up of the missing eyelets. Two of them were removed during desoldering.

That is totally fixable. Since the eyelets are gone, put the part in the holes but bend the part towards something sturdy, even if you need to leave the leads a bit long, and hot glue them to the board. Now you just need to run jumper wires to bypass where the traces got blown off the board.

Thanks for the advice, JJ. I am also seeking help from a couple of buddies who repair boards like this.

I am actually shocked the CPU blew up like that. Usually the Main fuse and ROOM fuse go bang.

I wish I knew too. That wouldn't have happened if I was on scene.