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bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

At risk of jinxing myself, the car is running quite well, just that lean/flat spot in the 2-3K range, and it's not getting worse, if anything it's getting a little better.

Also, it doesn't really feel like it has extremely low compression. It feels fairly strong to me.
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1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

Aaand, I jinxed myself. Maybe.

I drove it one store, drove fine. Drove it to a second store, fine. Went to drive it home...not fine. VERY rough, popping, crackling, backfiring. Wanted to stall. I limped it home. Noticed that the cap over my fuel mixture screw had gone missing, and I know that causes a vacuum leak, but still. It was never enough to do that.

Went to restart, battery is very weak. It did start, but ran terribly again. Replaced the cap, drove it around the neighborhood...terrible, same stuff. Noticed the tach acting weird, and then it just died, as I was driving. Car still running (badly) but tach on 0. Parked it back at the house.

Question...could a bad battery cause this? I have seen mixed reports of bad batteries causing cars to run poorly.

Fifty50Plus · Posted: Tue Feb 01, 2022 10:26 am Post subject:

YES! Electric fuel pump...coil... Both need good amperage. Check your alternator for output and possibly ground strap.
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1979 924 NA race car
1982 924 NA race car - Sold
1982 924 Turbo almost a PoS
1981 924 Turbo a real PoS, new engine
1982 924 Turbo nice body, blown engine
1972 911 E race car - going to Vintage
Various 944s to become IT-S race car

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

I replaced the voltage regulator in the alternator a little while before the car went down for the head rebuild, and it seemed to be working fine. But there's no way a fully charged battery discharges in the time it did if it's good...probably even with a bad alternator. I guess I expected the alternator to be supplying the car if the battery was weak. Possibly the alternator is not as good as I thought...

924RACR · Posted: Wed Feb 02, 2022 10:08 pm Post subject:

Well, a dead battery (dropped cell etc) can kill the alternator...

That said, did you check or replace the ground for the alternator to the engine? I've ended up replacing both that and the main charging wire to the starter (since that's the current path, it joins to the battery at the starter).

Local parts shop should be able to check the battery for you.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

I checked it when I replaced the voltage regulator, and it seemed OK. That said, it's pretty old and I should probably just replace it.

I'm going to get a new battery. It's well out of warranty (by more than I thought, of course it spent a year just sitting in my garage while the car was down) and batteries here don't seem to last as long as they used to. I'll double check the alternator and associated bits once that's in.

924RACR · Posted: Thu Feb 03, 2022 5:34 am Post subject:

Sounds like a good plan. I assume (obvious but confirm) you didn't have any issue with the charging light on the dash being on, that kind of thing?
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

No, I did not have a charging light on.

I swapped the battery (battery hold-down stud broke in half, and then the "exact fit" battery from NAPA doesn't fit under the hold-down bit anyway, although it does just barely squeeze in between what's left of the stud and the lip on the other side...I'm going to have to come up with a better hold-down option than that though). Car started up and ran perfectly. I didn't have time to check charging voltage out of the alternator, but I will definitely do that next.

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

Well I suspect my alternator is not really alternating at all, lol. With the car idling, voltage at the alternator itself was a bit over 12V, basically battery voltage. Raising the speed did nothing. Unless I'm thinking about this all wrong, it sounds to me like new alternator time before I run this battery down.
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

Ugh, in a whim I just checked the gauge cluster when the ignition is on but not started and...no battery light. Now...I don't actually recall for a fact that it was on before. But if I'm not mistaken, a burned out bulb would mean the charging system won't work, correct?

How much of a pain is this going to be? Just one thing after another with this car...
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

924RACR · Posted: Fri Feb 04, 2022 9:48 pm Post subject:

Yes, it needs that little current draw, is my understanding.

Replacement shouldn't be hard to find, though...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Fasteddie313 · Joined: 29 Sep 2013 Posts: 2596 Location: MI

Start the car, jump the main hot coming off of the alternator to the exciter pole on the alternator (little wire) for just a moment.. Bet you will hear the alternator start taking power from the belt..

This will excite the alternator and get it started providing power again..
Do this and check the voltage again..

You can drive it like that after you excite it, but you will have to excite it every time after you start the engine..

I figured this out a little while ago when my exciter wire came apart and it took me a while to figure out how/where to fix it..
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80 Turbo - Slightly Modified

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

Ooh, that's a great idea, I will try it.

I found the little bulbs online so I've ordered a couple. I'm suspicious though...what are the chances that I was just messing around in there and something ELSE is wrong? Suspecting it was me somehow, I found the blue exciter wire attached to the alternator, but it goes into the same brown sheath that the main positive wire harness is in and disappears into the engine bay. It's supposed to connect up to a different wire but I haven't found the connector for that. I guess it's not so strange that a bulb would burn out after so many years, just questioning the timing.
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

Well, that didn't seem to work. Also, took the bulb out and a 9V won't light it, so it seems to be bad. Both things cannot be bad can they?

Also...that's not the size bulb I bought. It seems that particular bulb is larger. Dammit!
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

Got the right sized bulb, put it in, no light. I need to check the exciter line. I have a theory that a PO rewired this to give 12V to the exciter without going through the dash light, probably because it burnt out and they didn't want to deal with it. I say that because I can't really say I know that it was lit before, but clearly the alternator was charging the battery before.

My gauge lighting seems to be out as well....first time I have driven it in the dark and no lights for the gauges. Not ideal....that was definitely not the case before.
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)