Prof Requiem

Greetings all, I have the honor of restoring two '80s Porsches, an '86 944 NA 5-spd that I have been working on since January and an '81 928 Auto I got in May. The 928 has a bunch of issues (knew that going in - clearly a project car), but here is the pertinent info.

The car turns on fine and runs great, fires right up every time (so long as battery is charged). The issue is that when you drive it for more than 3-5 miles (once it heats up) it starts losing power (indicated by a drop in the dash voltmeter and a decline in the oil pressure gauge) such that if you let it idle at a stop it will idle down to zero. The battery is not dead usually and will restart the car (in fact all the gauges jump up when the engine is off and then decline when it is running again) but neither is it charging.

So, repairs thus far - replaced the timing belt, replaced the alternator (more accurately, had them replaced as I had my mechanic do both of these - I almost did the alternator myself but glad I didn't as the bolt had been stripped and was a bear to get out so I hear). New alternator but still no system charge. With my Powerprobe hooked up it reads 12.3 volts with key off, 11.9 with key on, and no change when the engine is on (should jump up to 13-14v) even revving it registers no change in voltage. Even with the new alternator the system is exactly the same as before (this problem appeared day one - didn't show up during test drive as I didn't drive it far enough for it to do this, but as soon as it made it home it turned off).

Today as I was checking the voltage I cleaned the ground connection at the battery and where it bolts to the trunk chassis, seemed pretty corroded but didn't make a difference. I peeked up under the car and on visual inspection noticed an unhooked wire near to the alternator, but as it is currently in the driveway (not level) as my 944 is in the garage (fuel system overhaul) I haven't had a chance to get it jacked up to get a better look at the wire that is dangling to see if it is part of the alternator wiring.

Anyway, it will be a few days before I can get the 944 back on the road (waiting on parts...) but if I could get some insight into the problem and maybe a list of likely suspects for when I can get it in the garage, that would be great. Also it would be nice to know whether I should stop driving the 928 until I get this sorted. I don't plan on driving it much but it is hard to resist taking it out as I have only had it a couple months and most of the time it has been in the shop. So long as I am not stopped it runs perfect (I drool just thinking about how it drives...) - I have been dropping it into neutral at stops and keeping my foot on the pedal slightly to simulate idling. If this is dangerous to the system that would be good to know. Also if I am unable to figure the problem out then I was going to take it to a different mechanic that has more experience with older Porsches but was hoping to try to figure it out first if possible.

So, any help/advice is greatly appreciated and thanks in advance!

SeanR

First, welcome to Rennlist.

This could be a bad alternator but depending on the condition of the "blue" wire that goes from the alternator to the 14 pin connector (passenger side at the battery jump post). I'd check to ensure there is continuity between the small (blue) wire on the back of the alternator and pin 1 on the 14 pin connector first. This tells the alternator to start working. I have to ask though, did it charge when it left the shop that installed it?

Prof Requiem

Thanks for the reply Sean, no it has never charged right even after the shop put the new alternator in it. They are primarily a tire and brake shop so they were just doing the alternator as a favor, not their area of expertise. I can't get under the car until I fix my 944 though and get it out of the garage. I will check the blue wire continuity first thing when I get the 944 done so I can get back into the 928.
Just checking but from my symptoms is it dangerous to the car to drive it like this? Not that I'm planning on driving it much, but both our Porsches have been garaged for over a month except for the brief drive to and from the shop and I'm itching for a drive but don't want to do any damage to it. Mechanically it feels great, shifts very smooth with solid power, the only issue is when stopped it loses the idle down to zero. So when pulling up to a stop I've kept it from stalling by putting it into neutral, braking with my left foot, and keeping the gas pedal pressed just a bit to simulate idling. Any advice appreciated!
Thanks!

Prof Requiem

UPDATE
So I finished my 944 fuel system overhaul (running great) and have started back on the 928. I put in a new negative battery cable today and part of the problem is fixed - the alternator is charging the system now. I am reading 14v across the battery terminals now and it is steady, at least that's a good sign.
HOWEVER... the car is still losing power at idle. It seemed to be doing slightly better than before, but it would still idle down to almost nothing at a stop and when I pulled it back in after doing a 10 mile loop around my neighborhood it died as soon as I let off the gas. The gauges, particularly the oil pressure, which dip real low (warning sign) spring back up immediately as soon as the engine turns off. There is also an audible hiss, kind of a "shhhh" sound when the engine turns off and a similar sound is heard at low range rpms, like around 2k. Having only had the car a couple months and only worked on it a very little bit at this point, I don't know whether that is a normal sound for the car to make.
At this point my next step was going to be replace the battery as the one in it is clearly a very low end (says "ECONO" on it and is clearly remanufactured) and I'm not even sure if it's the right size. When I got my 944 it had a junkyard battery in it as well and it ran a lot better after replacing it. I'm also going to pull the car up onto my ramp wedges and check out the connections around the alternator, although that seems to be less of an issue now it's still worth a look.
Any help would be greatly appreciated!

Bilal928S4

Check all the grounds. Check the 14 pin connector on the driver side engine compartment. Check the vacuum system.

Alan

Its not dangerous - unless you count possibly getting stranded somewhere and some degree of damage to the battery by discharging it too much... Short trips for testing purposes should be no problem.

Certainly sounds like the alternator exciter wire - the blue wire - isn't hooked up - if it wasn't before the shop may have deliberately left it that way (makes sense if that's not their area of expertise...)

Alan

Alan

BTW - I think your idle problem is nothing to do with the alternator. More likely an issue with the idle control...

Critical thing is that you can restart - if power was so low that the car would stall - you wouldn't be able to restart.

Alan

jwillman

If idle is low alternator won't spin fast enough to charge. The alternator belt needs to be very tight on these cars as well, almost uncomfortably tight.

When you first turn on the ignition before cranking do you get the red low voltage warning light on the voltmeter? If not it indicates that the bulb is out or that the small resistor on the back of the gauge is bad. These have to work for the alternator exciter circuit to engage. Porsche went to a different resistance capacitor because of charging initiation issues, had to rev above 3000 rpm to start charging. see link below.

https://rennlist.com/rennforums/show...ght=alternator

Alan

This is true - however in this case it isn't ever initiating so its not an idle issue.
Those are possible however both would have to be bad for the alternator to never excite... more likely here (actually always) is a break (open circuit) in the exciter crcuit or it just not being connected to the alternator.

Either alone will start the alternator at sufficient RPM. The resistor change was made early, so most cars have 68 Ohms and a 1.2W bulb in parallel. Best test of the circuit is to ground the Blue wire at the alternator (or at the 14 pin connector) - the (no-)charge light should come on. If it does the bulb is fine, the resistor is very reliable it would be way down my list of things to worry about...

Alan

dr bob

Sounds like the new battery ground strap has cured the no-charge situation OK. Th eremaining issue is the low or no idle speed control.

Alan

I don't believe you fixed anything here - that it's working is likely coincidence, the battery ground strap can't possibly have fixed an alternator issue - it just doesn't work that way..

So likely it will come back soon and you actually have an intermittent OC problem somewhere...

Alan

Prof Requiem

Thanks for all the replies, I have been sick with the flu for the past week and haven't been doing anything with the car as you might expect.
Before getting sick though I did manage to get the car into the garage up on ramps so I could get a good look in there. I found a host of issues.
First, yes the red low voltage light does come on when key is engaged but not running.
Also, the voltage doesn't jump up to 14v until I rev it past about 2k.
Upon inspection, I can see the alternator belt sorely needs changing, it is tight but I don't think it it tight enough and shows clear signs that it is in need of replacement (cracking, there are bits of string starting to squeeze out the side) - definitely a must do.
There are also some odd wiring issues, it appears that the oil pressure sender (to be sure - that's the small cylinder between the oil pan and the alternator right?) has a bent terminal and one of the wires is not hooked up.
There is also another wire just kind of hanging there. Very strange. I'm not entirely sure what it is supposed to be hooked up to. I am still somewhat sick and trying to play catchup with my work so it will be a bit before I can get working back on the car, but I will take a picture of the situation and post it here when I get back in there. It would be nice to not have random wires dangling down onto the road.
Unrelated, but I also noticed in cleaning the engine compartment that the little two inch fuel hoses from the rail to the injectors are shot. At least one of them looks like it should be spraying fuel (thank goodness it doesn't) so I am going to garage the car for a while to get that situation fixed as well.
At least my 944 is running good so I'm not Porsche-less!
Thanks again for all the help everyone!

jwillman

My 79 had the higher resistance resistor (old style). When I ordered the new style - 68ohm if I remember correctly, and installed the alternator starting charging as soon as the engine was started

MikeinBloomfield

Where are you? Perhaps you could take a look under the same model year car, especially for the hanging wire issues.

A hissing sound to me, sounds like a large vacuum leak, which would maybe explain why its not idling. When my car had a leak at the injectors, I had to turn the idle screw all the way up just to get it to run and you could hear the air getting sucked into the wrong place. Doesn't explain the lack of power at higher revs I don't think.

Prof Requiem

Hey again, I am way out in rural north Georgia in the Appalachians, so it was somewhat of a fluke that I came into possession of both a 944 and 928. I doubt there are many of either in the area (although there are more 944s around the region).

The vacuum leak sounds like it might be it. I haven't even begun to mess with all that. I did a visual inspection earlier and noticed a couple connectors that weren't snug fit, but I couldn't get to them as they were recessed under the intake octopus. I have officially garaged the car and suspended the insurance for now so I can do some much needed work like replacing the fuel lines between the rail and injectors which are really worn, and I figured when I was working on that I would take the intake manifold off to clean around the engine real good and that would give me a chance to tighten all the vacuum lines. Lots to do, but that's kind of why I bought the car, these old Porsches are just so much fun to work on.