911T Won't Start -- What gives?

[Myami]

Hi Everyone:

I have a 1971 911T that is refusing to start and would appreciate any thoughts or suggestions.

History:

1. Last summer, I had a no-start issue and replaced the seven year old Porsche batteries with a pair of new Sonnenscheins. All was well for a few months.
2. But, as I drive the 911 less frequently, I would find it dead (no crank) after it sat for a month.
3. All was well again after charging the batteries back up and/or using my Noco GB40 for a jump.
4. Earlier this year, I suspected the clock was over-winding, so I disconnected the clock and after sitting for 6 weeks, it fired up no problem.
5. I sent the clock out for repair (it was over-winding) and left the clock plugged in after it returned from the shop.
6. After being fully charged and sitting for another month, the car is dead again.

Key Facts:

A. Upon finding the car dead (no crank at all), I hooked up the GB40 to the passenger battery. It cranked, but it was weak and wouldn't start.
B. I pulled the batteries out for a bench charge--passenger was completely dead; driver was 70%.
C. Reinstalled the batteries after the bench charge only to find that it wouldn't crank.
D. Hooked up the GB40 to the passenger battery and got it to weakly crank, but again, no start.
E. When attempting to start, the interior light flicker on then off. Not just a slight dimming.

My gut says the passenger battery is a dud (brick), but what gives?

Thanks in advance.

[steve911T]

Get rid of the batteries and put an AGM on driver's side. Spec a Miata size and go from there. Tape up the cables on the passenger side. Then get a good trickle charger and be done with it. You probably have a short somewhere. Had a AGM in my 70T (sold) for many years. Average AGM's, have lasted 8-10 years. Currently all of my vehicles have AGMs. Good luck.

[oldie44]

I had a similar problem many years ago with a 356 which was due to a current leak on one of the circuits. With a 1971, there should be no current flowing through any circuit with the exception of the clock when the ignition is turned off.
I suggest you pull fuses one by one and check for current flow with a mulitimeter. If any show a current draw, check/disconnet items to see if you can identify the short circuit. The clock should draw current intermittenly as it winds.
If all circuits are OK, look at the unfused circuits by disconnecting the battery and checking across the connection. One problem can be a defective diod in the voltage regulaor allowing reverse current when the motor is not running. Also, the wiper motor on the early 911s were not fused. I don't know about your 1971.
If your car has an alarm system or new sound system, that should be the first thing you disconnect to look for current leaks.

[oldie44]

I forgot to say you cannot measure a current leak with a mulitimeter. If you find a leak it will peg the meter and you should immediately disconnect or you might burn up your meter. You are looking for milliamp leaks.

[blucille]

Does your alternator light glow if you turn the key but don’t start it? The alternator light is required to get the juice flowing from the alternator.

Also, oldie44 has some good ideas but some potential voltage drains aren’t fused. Could be a bad diode in the alternator. Or it’s just not charging. Could be a bad voltage regulator. Bad battery cables or bad grounds to the battery that isn’t charging.

[Myami]

Originally Posted by steve911T

Get rid of the batteries and put an AGM on driver's side. Spec a Miata size and go from there. Tape up the cables on the passenger side. Then get a good trickle charger and be done with it. You probably have a short somewhere. Had a AGM in my 70T (sold) for many years. Average AGM's, have lasted 8-10 years. Currently all of my vehicles have AGMs. Good luck.

Thanks, Steve. I suspect it is my batteries. Bought a tested and both failed with the tester suggestion replacement.

[Myami]

Originally Posted by blucille

Does your alternator light glow if you turn the key but don’t start it? The alternator light is required to get the juice flowing from the alternator.

Also, oldie44 has some good ideas but some potential voltage drains aren’t fused. Could be a bad diode in the alternator. Or it’s just not charging. Could be a bad voltage regulator. Bad battery cables or bad grounds to the battery that isn’t charging.

The alternator light glows when the key is in Position 2 (fuel pump activated). It is not glowing once the alternator is operational and the engine is running.

I will say, however, that I had a alternator charging issue late last year--post battery replacement. With a voltmeter plugged into the cigarette lighter, I wasn't seeing a charge or anything north of 12.2 ish. I would charge it with a tender, drive it for 30 minutes and after shut off, the batteries were completely flat.

That prompted me to replace the VR (it was a new solid state unit) with my original Motorola unit. The voltmeter then showed 14+ and normal operation resumed. Until now.

[Myami]

Originally Posted by oldie44

I had a similar problem many years ago with a 356 which was due to a current leak on one of the circuits. With a 1971, there should be no current flowing through any circuit with the exception of the clock when the ignition is turned off.
I suggest you pull fuses one by one and check for current flow with a mulitimeter. If any show a current draw, check/disconnet items to see if you can identify the short circuit. The clock should draw current intermittenly as it winds.
If all circuits are OK, look at the unfused circuits by disconnecting the battery and checking across the connection. One problem can be a defective diod in the voltage regulaor allowing reverse current when the motor is not running. Also, the wiper motor on the early 911s were not fused. I don't know about your 1971.
If your car has an alarm system or new sound system, that should be the first thing you disconnect to look for current leaks.

Thanks, Olide44. A trace for draws is a good suggestion is something that I have been considering, but putting off in hopes of an easier solve. There aren't many electrical components in these old cars and I have not added anything of note.

A clue that would suggest all is well is the longer period in which the car was parked but the clock was disconnected. It started without issue. That prompted me to send the clock out for repair. John Bell confirmed the clock was misbehaving and would have been killing batteries regularly.

Alas, I will work to get my Sonnenscheins replaced under warranty and go from there.

[superninety]

There is nothing wrong with Sonnenschein batteries. I have had them in my 1970T for years. They fit and are high quality. I am really only using the driver's side battery, with passenger's side as a spare, and, important ballast. The car does handle differently without it.

Yours might be dead, completely dead, but I would bet something else is going on and caused the problem.