Alternator Question

Red69t · 09-20-2021, 06:50 AM

Hello all - have a 69 911T with MSD ignition, but all else stock. I've had a few instances where my tach has bounced around randomly cruising, then acted normally. Last Saturday, I was driving, had to honk the horn and got no response. A few minutes later, the cart stayed on, but the tach dropped to 1k and there was no throttle response. Later in the drive, the car died and wouldn't turn back on - no response from turning the key. I was also low on gas, but the light was not on consistently, so I'm not sure that was the main problem, but when I added a couple gallons, it started up and was able to drive a couple miles home. I bought a voltmeter and put it in the cigarette outlet - generally speaking it was reading anywhere between 12.75 to 13.5 under normal load/idle and up to 14 under full throttle.

Would any of this indicate that my alternator is going or does the volt readings seem normal and perhaps it was a gas issue and my dummy light just wasn't reading all that well?

Thanks in advance - Tim

spoke · 09-20-2021, 01:44 PM

I get similar voltage readings from the cig lighter voltmeter. In general, any voltage about 12.6V up to 14.0V is good as it shows your alternator/voltage regulator is working.

About the gas, how low were you according to the fuel gauge? I never let mine get below 1/2 tank. These old cars aren't like modern cars where the computer tells you you have 10, 50, 100 miles to empty. Old cars just stop on the side of road when starved for gas.

snbush67 · snbush67's Garage

Alternator is probably ok. But the other stuff could be loose grounds or bad switches. The first place I would check would be the ignition switch, is it loose or worn?

Also check all the plugs under the dash to make sure they are plugged in completely, not loosely.

And check the wiring from the MSD to the tach make sure it’s not shorting out and that it is connected securely.

Red69t · 09-22-2021, 03:54 PM

Thanks guys. I find that my gauge is usually on “E” when I’ve only gone 100 miles on the tank. The red dummy light usually starts flickering on during hard bank turns at 140 miles, then stays on on a level surface if I have 170 miles on the tank. I was at about 140 when it kicked out so entirely possible the tank was empty but my usually rule of thumbs weren’t confirming.

Will check other wiring. I replaced the fuse box about three years ago and all wires there seemed in good connection when it died. Will look at the MSD wiring and under dash wiring next.

Tim

Red69t · 09-28-2021, 01:10 PM

So I took it out for a drive and filled it up with gas - all was fine on that outing. Grabbed the car to go for a ride last friday and about a 100 yards from my driveway it "died" with the car losing power/acceleration and the tach dropping, and then fired back up as I punched the gas a little bit. I immediately turned it around and put it back in the garage to assess further. I'm guess starting under the dash and looking at those plugs first is my best bet for troubleshooting?

A few years ago the car just randomly died on the highway and my fuse panel was melted, so i replaced that and all wiring there checks out fine, so I know that isn't the issue.