mcge8241

Ok I'm stumped on this. A few weeks back my 84 turbo mtx was on the highway and I had the headlights, radio, and heater on. Radio suddenly died and my voltage gage showed below 10v. I turned off everything I could and got it home. Car still ran, but as soon as I shut it off, the battery was completely dead. I replaced the battery and it seemed fine. Drove it to work and the voltage dropped steadily as I drove. Got it home and the voltage was really low.

I next replaced the alternator. Had it checked when I picked it up at autozone. Put it in, almost the same issue.

I've subsequently changed the ground cables and power cables to the alternator. I ran new, heavy gauge wires directly from the alternator to the battery. I checked the excite wire and it's producing 12v from the ignition. I ran the other 12v battery wire (in the same harness/plug as the excite wire) directly to the battery too.

Here's what it does. After fully charging the battery on an external charger, if I start it up the voltage is around 12.5v and, with no headlights or blower motor on, it will sloooooowly climb to 13.5v. The minute I put the headlights on, voltage drops to 12v or so, sometimes lower. Blower on too, 10.5v.

I've had this alternator checked twice, came back good both times.

RPM increases don't actually seem to help; the car will charge the battery at idle and sitting for a long time (from 8-10v to close to 12, sometimes if i let it sit a really long time it'll get above 13v). The voltage drops significantly faster when I'm on the highway as opposed to back roads.

I'm 100% stumped. New wires, new battery, new and good alternator...can't get the voltage up. I suspect that *something* is drawing a TON of current, and because of the RPM issue I'm thinking maybe the coil?

ANY help would be greatly appreciated. I'm about -this- close to driving the thing into a tree.

rogerz

did you check the alternator installed or was it a bench test. If a bench test I would take the car to a battery shop and have them check the system as it is installed and see what results you get. Is your ground strap grounded to the frame before going to the starter? Grounding thru the starter is not reliable. You didn't say the condition of your battery cables. If you have those one size fit all type that clamp on they can get corroded down into the wire. Look for corrosion on both ends of both battery cables.

ol55

Ditto above, especially on the installed test - Is belt tension good? What accessories are you running when you experience this?

I'm no pro- Larry

Stukinit

I have to second ol55 on that Belt Tension check, had a similar problem once on a different car of mine, turned out the tensioner hadn't been tightened down 100% and was letting the belt slip.

mcge8241

Cables and terminals are brand new. For grounds, I have one wire running to the frame and it tees to the body, and one running directly to an alternator bolt.

The tests were a bench test.

The more I think about it, I wonder if the belt slip might be contributing. I have the tensioner maxed out, but it seems as though the belt is stretched a bit. I pushed on the belt when it was off and it *seemed* tight, but I did the same thing when it was running (with the handle of a socket driver) and it deflected quite a bit. Maybe this sucker needs to be extraordinarily tight?

I'll run out and get a new belt tonight and give that a try. Can't hurt. Thank you all for the suggestions!
-Alex

FlyingT

If the cables and terminals are new, and connected properly, then it has to be a bad alternator. It should be putting out ~14V in order to charge a 12V battery. Your reading shows a bad alternator. The belt slipping could cause this, but should be making so much racket that you would fix that first. Shut off all the major power draws to lessen the load on the alternator, like the lights, fans, defogger, etc. Start the car and retest the battery with a meter again. If not ~14V, replace the alternator.

mcge8241

Well, the new belt (shorter) fixed it! All good.