Hooked battery cables up backwards, now tach reads 1/2 normal... wtf?

[jeffesaurusrex]

I had to jump my car because I left the glovebox open. When I jumped it I accidentally hooked the cables up backwards. Everything seems fine except now, the tach reads exactly 1/2 normal RPM. It isn't wavy or anything, but at idle it reads around 400RPM and when bouncing off the rev limiter it's at a tiny bit over 3000.

Anything I could have blown that would cause this?

[SamE30e]

Maybe the coding plug? Does everything else work?

[jeffesaurusrex]

Everything else works, no blown fuses. One other weird thing happened though... the brake fluid indicator came on. It went out after I turned the car off and back on again though..

[Surtr]

I did that once (I've since learned to keep the red and black markings on cables very visible). I don't know if it's coincidence or not, but ever since my clock is finnicky. Sometimes it';ll just show random garbage, sometimes it'll switch to Celsius and back. From what I've read of other people doing this, you got lucky. You can blow starters, alternators, your dash, ECM, etc.

Also, brake fluid indicator? Do all E30s have those?

[jlevie]

Reverse polarity on the electrical system can wreak havoc with the car's electronics. In this case I'd say that either the cluster or DME is partially fried, at the least. You may find other problems later.

[jeffesaurusrex]

This may be a stupid question but what is the DME?

[Surtr]

This may be a stupid question but what is the DME?

The ECM.

[jeffesaurusrex]

Wouldn't the car run badly? I've been driving it for a while now, no problems other than that tach and the keyless entry doesn't work all the time now.

[jeffesaurusrex]

So I had to push start it this morning and when I got off of work tonight, the battery had gone bad. I replaced the battery, now it starts fine but when I replaced the battery, the tach stopped working entirely. It continued to work 1/2 normal up until I replaced the battery, then just quit altogether.

The car also idled a bit rough after I changed the battery. Before the new battery, it wasn't a problem. It seems to idle smooth then dip down to a rough spot for a moment, then idle smooth for a bit again and repeat.

I have no freaking clue what this is... Any tips? I'm now trying to sell the car, completely unrelated to this, but I'd like to get it all fixed before I let it go.

[Kershaw]

the car has to relearn fuel trims if you unhook the battery, right? if the roughness goes away after a few drive cylces i wouldnt worry about it.

[dnova89]

I had to jump my car because I left the glovebox open.

seriously? did you leave it open for 5 million years? i've left my tailights/front turns on by accident for like 6-7 hours and when I came back it started right up lol. that sucks, maybe your battery was super weak to begin with

[jeffesaurusrex]

I think what actually killed it was I have subs, and I turned the car on for a second before I started the engine and the subs were running. Because I have all LED bulbs in the trunk, glovebox and dome lights I don't think they would drain it.

I did get a new battery for free however, because I work at autozone and the old one happened to be a Duralast. Also, the ECU has relearned whatever it had forgotten because it now runs fine again. The tach however, still does not work at all. I've never seen such a thing. All other gauges work fine and the SI lights are not on.

[Atreus]

you tried swapping clusters?