97 Cobra electrical problems?

[iamosolo]

So a buddy and I finally got the motor replaced in my car, and I'm having a slew of electrical gremlins. Before I start, I would like to make it known that I bought the car NOT running.

First, the car would start then die immediately. It was then brought to my attention that the previous owner didn't give me a pats key. Got new keys, programmed them myself, problem solved.

Now that it will start and "idle", my gauges are all over the place, if they're working at all, all interior/exterior lights are REALLY dim, and the car will die as soon as I release the e-brake. Oh, when I say die, I mean the whole car, no interior lights, exterior lights, just the battery light on the cluster.

It should also be noted that when I got the car, one of the plugs for the anti theft box in the trunk was unplugged. I thought "surely that can't be right" so I plugged it back in, but the factory alarm goes off as soon as I open the door, unless the anti theft is unplugged.

I have checked all fuses. I also checked voltage at the battery, and with the engine running it's approx. 11.7 and varies little regardless of engine speed. Engine off, key off, is around 12.7 volts.

I have also had both the battery and alternator tested, and they both passed.

 

[AaRoN]

You've definitely got a problem in the charging circuit. I'm assuming you have a DMM. Switch it to AC volts and attach the leads to the battery while the car is running. You shouldn't have more than 0.5v AC. If you do, your diodes in the alternator are shot. Let me know what you get.

 

[iamosolo]

Well assuming I set it right (V~ on a range 200 which is the smallest I have) I got 30.7?!

 

[AaRoN]

What kind of meter do you have?

 

[iamosolo]

This...

 

[AaRoN]

K. Looks like you set it to the right setting. Your diodes are shot. They're allowing the wrong side of the AC ripple into your car. That amount of AC voltage will do all kinds of crazy things to your car's electrical/electronics systems. Replace your alternator.

Edit: Did the reading say 30.7 V or 30.7 mV?

 

[iamosolo]

:dontknow: I set it at the smallest range of AC voltage which says 200, and the screen displayed 30.7 so your guess is as good as mine.

Before I go spend $200+ on a new alt. I'm a little suspicious of my connections, as I'm not the one who hooked them up. The main wire coming off of the alt. splits into two slightly smaller wires about 6" from the end, both of which connect to the little post on the side of the fuse box in addition to the + battery cable. Is this right?

 

[AaRoN]

Check it one more time. If it's 30.7 volts, it'll read 30.7V. If it's .307 volts, it'll say 30.7mV.

 

[iamosolo]

So my alternator can still be bad even though it "passed" when tested at my local auto parts store?

 

[iamosolo]

Something I just realized... My car has a smaller crank pulley, larger water pump pulley, but the alternator pulley appears stock. All the underdrive sets I see for sale have all 3. Would this cause my alternator to spin faster, thus providing too much voltage, and possibly damaging it?!

 

[AaRoN]

You should never see above 0.5v AC at the battery. Regardless of alternator make, pulley size, etc. If it is in fact allowing 30.7 AC volts pass through, the alternator is junk (the diodes anyway). It'll still pass the crap Autozone tester.

 

[iamosolo]

Replaced the alternator, still have crazy gauges, e-brake still kills the car when released, and still getting the 30.7v AC. However, I'm now getting the 14.3v DC when running, as opposed to the 11.x I was getting before.

 

[AaRoN]

It has to be 30.7mV. There's no way you're getting 30.7v on a 12v system. My bad. I didn't even catch that earlier. You're alternator was definitely on it's way out though.

 

[iamosolo]

Looks like a missing ground wire was the problem. Found it in a box of extra parts, and it solved all the problems!

 

[AaRoN]

Awesome.

 

[AaRoN]

Looks like a missing ground wire was the problem. Found it in a box of extra parts, and it solved all the problems!

That missing ground is why you were reading 30.7v. The circuit was backfeeding looking for a ground. Since that one was missing, it went back through to the battery. It's likely that your battery's life has been excessively shortened.

 

[Cobracarlo]

which ground just curious as I remember to always replace it when my motor is out.

 

[iamosolo]

which ground just curious as I remember to always replace it when my motor is out.

It goes from the driver's side frame rail to the block, between the oil filter and motor mount.

 

[Cobracarlo]

oh i know that ground.