Reimu

When I start my car it runs strong
Multimeter on the battery shows a little over 13 volts (mind you my battery is a bit low as of now because of all the testing I've done)

However after a minute the voltage starts steadily dropping and the car starts running like crap. Shaking, backfiring a little, it's not happy. I switched alternators with a questionable spare I have and the same exact problem surfaced.

Could it be anything other than the alternator/voltage regulator?
I happened to notice the big wire on the alternator seems to be shorting out as well, I traced it back to the battery and from there who knows.

944Ross

My alternator was putting out 13.2v but dropping below that when A/C or headlights were on. The brushes were worn to a nub. You should see a solid 14.0 +/- almost all the time. I'd fix that short ASAP!!

V2Rocket

How old is your battery?
Even if the alternator isn't working properly if the battery shows 12-13v it should be able to run the car for at least 30 mins or more without a problem.
When the car starts to sputter and die watch the voltage on the battery...if it gets to the 10s then the battery is dead.

Reimu

Starts sputtering after it gets down to 12.5 or so
I usually cut it off at this point
Battery is fairly new

I was doing some things with my multimeter on the battery cable leads (off the battery) and I noticed that the resistance measurement I'm getting drops when I open the door or turn on the parking lights
I never bothered checking this before so I don't know if it's natural
With the car off my parasitic draw is quite low

Noahs944

Mine is 14+ at the batt when running. What is your idle rpm when idling rough? Low I suspect. Could it be the voltage is low because of a low idle which is because of... (something that many of us are trying to figure out on our own cars)?

marc abrams

A few things come to mind here:
The car should run fine till about 10 volts.

Are you seeing sparks when you are connecting the battery wire to the alternator, that's a bad alternator. You do know you should disconnect the battery when doing electrical work.

The only way to check for parasitic draw is with a amp meter.

With all that said, check all connections at the positive battery terminal. This a junction point for both fuel pump and battery charging, at lease for the early cars.

Arominus

Cleaned your grounds lately? especially the engine grounds? you may even need to replace that ground cable.

Reimu

The voltage doesn't change much with rpm

I don't know if the alternator would spark when I hook it up as I've only done it with the battery disconnected. I noticed the alternator-starter-battery wire was shorting somewhere when I checked it with a multimeter. It's not the main wires as I've removed them all to check. Parasitic draw was tested with my meter set to amps.

The grounds *look* fine and with one lead on the battery ground and the other on the shock tower I get very little resistance.

Paulyy

I've been trying to diagnose my issues with alt/battery. If i turn my aircon on mine also drops.
i removed the alternator and got it "testes" apparently was fine. new regulator on it too now.

I've cleaned all contact points. (alt grounds to the chassis via the bolts. so clean the contacts the aluminium to aluminium contacts too)

usually if you can start you car and battery slowly dies, alternator is back
if you struggle to start, battery flat over night, but car can drive. battery is bad.

mytrplseven

Do the easy stuff first. Take the battery down to Autozone and have them do a battery test. It's free and then you'll know.

Reimu

Had the alternator tested, it didn't pass. Bought a refurbished one, charged the battery up, and the same issue presents itself. Except this time instead of running good then stumbling and dying I could barely get the thing to run at all.
Oh well at least I'm eliminating variables I guess.....

Tested the tps, dme temperature, and afm from the dme plug and everything checked out

Jfrahm

battery chargers can be hard on the ECU

Reimu

Battery was charged out of the car


But here is an important update
I kept tinkering with things until it wouldn't start at all. Just turn and turn and turn. Checked what I could and it looked like it wasn't getting spark. Eventually I pulled the DME and removed the covers and noticed that the bottom strip near where the plug enters the board had broken.

I bought this DME recently and it's the best looking one I've ever owned, I think it was refurbished at some point. And since the car was somewhat running earlier, this HAD to have happened just now. I'm very reluctant to just repair the break and pop it back in without getting to the bottom of the issue.

So what's going on? I've mentioned this before but can someone please tell me if they get any connectivity from their battery power cable to ground? I also ohm out at the ignition coil to ground.

Reimu

I repaired the break to see what would happen. Car started and ran great for a 15 mile trip.
Tried taking it out again and suddenly it was getting really lean again and bucking, eventually dying altogether. I pulled the DME apart on the side of the road and saw this, I taped a few copper strands from a wire on it which was enough to get me the 2 miles home.

As I understand it this is a grounding strip. My grounds are all perfect though.
https://i.imgur.com/hRoR1Kg.jpg

marc abrams

Find out what the DMC grounds on than buss line. Looks like a short to positive like a melted wires or a bad engine to body ground and high current found a route thru the DMC.