Low Idle Voltage
12-02-2014, 06:01 PM
#1
Intermediate
Thread Starter
Low Idle Voltage
Need a little help. I've been going round and round for a while on this one and did a lot of searching here, but haven't found resolution to the below.
Car starts fine within the first few revolutions, idles to about 800 and voltage immediately goes to about 13V. after a minute or so the rpm drops to 650 +- 50 and voltage goes to 12V and master warning flashes and generator warning light comes on. Checked voltage at jump post and see about 12.1V. check voltage at battery B+ and see 12.3V. checked alt D+ and see 10.1V.
Shut down car and disconnected battery ground cable.
Ohmed out negative battery cable = 0.1 Ohms
Ohmed battery positive to alternator B+ = 1.0 Ohm
Ohmed battery positive to disconnected CE H8 = 0.1 Ohm
Ohmed disconnected CE H7 to disconnected alt D+ wire = 0.1 Ohm
reconnected H plug and continued:
Ohmed battery positive to disconnected alt D+ wire = 27 Ohm
which I assume is the resistance of the generator light bulb plus the 63 Ohm resistor, this resistor was removed and checked a few weeks ago and found to be within an Ohm or two of it's labeling.
I pulled the Paris-Rhone (65 Amp?) alt and cleaned it up a bit, was actually quite clean. It looks to have a fairly new voltage regulator installed. the brushes do not quite look like they have bedded in yet. Checked the three power diodes under the shield and they allow current only one direction as they should.
I checked the infamous 14 pin engine connector - it looks to be in good order, no corrosion and no frayed insulation.
Reinstalled everything with same results. At about 900 rpm, the voltage hits 14 and holds there when revved higher, but falls flat to 12 or so at idle. The 4 rib belt is quite snug.
A new H8 1000 CCA battery was installed a few weeks ago. (FPO had a smaller battery that rattled around in the box!)
I interpret these results as the excitation circuit being in good order, as well as the rest of the wiring. I have only had this car a few months and it has always acted this way. The car acted the same way with the previous alternator installed. The previous was the Bosch unit. (swapped in the Paris from another car) I believe the other car had fine voltage at idle with this alternator.
I guess what I'm wondering is - Is There a Problem? I wouldn't think a car should behave this way. My 944 never did, but it had much less accessories. Now certainly, all the above was unloaded - no fans on, no radio, no lights. Just power to fuel pump, engine computer, ignition. You know - bare minimum. I'm running out of ideas
Car starts fine within the first few revolutions, idles to about 800 and voltage immediately goes to about 13V. after a minute or so the rpm drops to 650 +- 50 and voltage goes to 12V and master warning flashes and generator warning light comes on. Checked voltage at jump post and see about 12.1V. check voltage at battery B+ and see 12.3V. checked alt D+ and see 10.1V.
Shut down car and disconnected battery ground cable.
Ohmed out negative battery cable = 0.1 Ohms
Ohmed battery positive to alternator B+ = 1.0 Ohm
Ohmed battery positive to disconnected CE H8 = 0.1 Ohm
Ohmed disconnected CE H7 to disconnected alt D+ wire = 0.1 Ohm
reconnected H plug and continued:
Ohmed battery positive to disconnected alt D+ wire = 27 Ohm
which I assume is the resistance of the generator light bulb plus the 63 Ohm resistor, this resistor was removed and checked a few weeks ago and found to be within an Ohm or two of it's labeling.
I pulled the Paris-Rhone (65 Amp?) alt and cleaned it up a bit, was actually quite clean. It looks to have a fairly new voltage regulator installed. the brushes do not quite look like they have bedded in yet. Checked the three power diodes under the shield and they allow current only one direction as they should.
I checked the infamous 14 pin engine connector - it looks to be in good order, no corrosion and no frayed insulation.
Reinstalled everything with same results. At about 900 rpm, the voltage hits 14 and holds there when revved higher, but falls flat to 12 or so at idle. The 4 rib belt is quite snug.
A new H8 1000 CCA battery was installed a few weeks ago. (FPO had a smaller battery that rattled around in the box!)
I interpret these results as the excitation circuit being in good order, as well as the rest of the wiring. I have only had this car a few months and it has always acted this way. The car acted the same way with the previous alternator installed. The previous was the Bosch unit. (swapped in the Paris from another car) I believe the other car had fine voltage at idle with this alternator.
I guess what I'm wondering is - Is There a Problem? I wouldn't think a car should behave this way. My 944 never did, but it had much less accessories. Now certainly, all the above was unloaded - no fans on, no radio, no lights. Just power to fuel pump, engine computer, ignition. You know - bare minimum. I'm running out of ideas
12-02-2014, 06:37 PM
#2
Team Owner
Thats strange what your reporting, have you done any cleaning of the CE panel and the Pod connections?
spray Deoxit everything electrical, do the Ce panel front and back and the pod connections
replace the battery ground strap and clean both battery terminals.
Clean the engine ground strap and inspect and clean the 14 pin connector under the hood.
Check the starter connections make sure the wires are tight and clean and not grounding on the engine
spray Deoxit everything electrical, do the Ce panel front and back and the pod connections
replace the battery ground strap and clean both battery terminals.
Clean the engine ground strap and inspect and clean the 14 pin connector under the hood.
Check the starter connections make sure the wires are tight and clean and not grounding on the engine
12-02-2014, 08:01 PM
#3
Intermediate
Thread Starter
Thank you for the suggestions Mr Merlin!
I refurbished the pod about a month ago, polished all connections with dremel or eraser, soldered riveted connections, deoxit all of the connections. Also cleaned and polished and deoxit engine ground strap as part of all the stuff I did above. It also had minimal resistance.
I have not touched the CE panel yet. looks a bit hairy but I'm not scared. I worked on flight simulators for quite a while so I T-shot interface panels with many thousands of connections! In the CE I guess I just might touch on a blade connector that isn't making a solid connection. In servicing the CE I take it just to remove all the plug in connectors along the bottom, pull it out and remove relays and go to town?
I also thought to Ohm out the battery ground terminal to the alternator body. That ought to prove out half the circuit.
I suppose I ought to mention ALL of the ground points have been cleaned and deoxit -ed in the last couple months. And not sure if related, but with all power off I checked the battery drain. Somewhere I'm getting a .2 Amp load which makes the battery go flat if left for two weeks. This one is a little lower on the totem pole as I figured 200mV wouldn't cause the low idle voltage issue. If you feel there's something here I'll move it up.
Again, thank you for the suggestions
I refurbished the pod about a month ago, polished all connections with dremel or eraser, soldered riveted connections, deoxit all of the connections. Also cleaned and polished and deoxit engine ground strap as part of all the stuff I did above. It also had minimal resistance.
I have not touched the CE panel yet. looks a bit hairy but I'm not scared. I worked on flight simulators for quite a while so I T-shot interface panels with many thousands of connections! In the CE I guess I just might touch on a blade connector that isn't making a solid connection. In servicing the CE I take it just to remove all the plug in connectors along the bottom, pull it out and remove relays and go to town?
I also thought to Ohm out the battery ground terminal to the alternator body. That ought to prove out half the circuit.
I suppose I ought to mention ALL of the ground points have been cleaned and deoxit -ed in the last couple months. And not sure if related, but with all power off I checked the battery drain. Somewhere I'm getting a .2 Amp load which makes the battery go flat if left for two weeks. This one is a little lower on the totem pole as I figured 200mV wouldn't cause the low idle voltage issue. If you feel there's something here I'll move it up.
Again, thank you for the suggestions
12-02-2014, 09:08 PM
#4
Team Owner
you might chat up SeanR he is in your neck of the woods and a great resource.
12-03-2014, 12:50 AM
#5
Intermediate
Thread Starter
Deal. Was hoping to have breakfast with the DFW crew again this weekend, but have to head out of town. If cleaning up the CE panel doesn't clear it up, I'll look up SeanR. Thanks again Merlin
12-03-2014, 01:48 AM
#6
Nordschleife Master
It could be an open in the alternator. Bad brush. Open winding. Open in the regulator. Something like that.
I'd remove it and take it to a FLAPS and have it tested. Or take it to an alternator shop.
I'd remove it and take it to a FLAPS and have it tested. Or take it to an alternator shop.
12-03-2014, 08:41 AM
#7
Team Owner
once the alternator comes on line it is self sustaining,
However this doesnt explain that two different alternators ( Bosch and PR ) did the same thing
However this doesnt explain that two different alternators ( Bosch and PR ) did the same thing
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12-03-2014, 09:03 AM
#8
Rennlist Member
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"The 4 rib belt is quite snug. "
If you use the Porsche belt tension tool to set the alternator belt, it is "very tight". The old "push 1/2 inch with your thumb in the middle of the belt" doesn't apply to the 928...
If you use the Porsche belt tension tool to set the alternator belt, it is "very tight". The old "push 1/2 inch with your thumb in the middle of the belt" doesn't apply to the 928...
12-03-2014, 10:16 AM
#9
Intermediate
Thread Starter
Thanks for the suggestion.
Additionally, I have a second 928 ('84) that's been a parts car. Not so much so that it's disabled yet. This gives me the opportunity to test both my alternators in another system. While I'm at cleaning up the CE panel I can take both alternators to get them tested as well. I do take the free alternator tests with a grain of salt. All I think they test for is output voltage at unspecified RPM. This may not mean much if they don't do a load test. I would think this would provide more clues to what's good/bad. Another wrench in the works is that this car hit a deer a few weeks ago and is going to the shop. I have a feeling this will take a little time to get to the bottom of.
Last edited by rotenDrachen; 12-03-2014 at 11:03 AM.
12-03-2014, 10:45 AM
#10
Electron Wrangler
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Sounds like the belt is too loose, your resistance readings must be wrong - I suspect the measurement device - many are not accurate for small values. If you had 1 ohm in the battery to alternator connection when the alternator charges the battery at say a 10A rate you'd be dropping 10V! (so obviously that never happens).
If any appreciable amount of that resistance was in the battery to starter connection you'd never be able to turn the engine over since it can take >100A.
I think you need to reconsider how to do those measurements and basically forget the results for now.
It seem clear the alternator doesn't charge - except marginally at cold - take it out and take it to a good alternator shop for evaluation (don't just replace - test - you want to know its condition for diagnostic purposes). It's interesting you say the previous alternator did the same thing - but I wouldn't ignore the obvious diagnostic path.
Once you know you have a good alternator - check the exciter line while running at the 14 pin connector (at both engine cold & hot conditions). You want to see that is continually driven high and not drooping when hot.
Maybe get a better meter (or just change the battery first) and see if you can get more likely resistance readings.
Alan
If any appreciable amount of that resistance was in the battery to starter connection you'd never be able to turn the engine over since it can take >100A.
I think you need to reconsider how to do those measurements and basically forget the results for now.
It seem clear the alternator doesn't charge - except marginally at cold - take it out and take it to a good alternator shop for evaluation (don't just replace - test - you want to know its condition for diagnostic purposes). It's interesting you say the previous alternator did the same thing - but I wouldn't ignore the obvious diagnostic path.
Once you know you have a good alternator - check the exciter line while running at the 14 pin connector (at both engine cold & hot conditions). You want to see that is continually driven high and not drooping when hot.
Maybe get a better meter (or just change the battery first) and see if you can get more likely resistance readings.
Alan
12-03-2014, 12:15 PM
#11
Rennlist Member
Rennlist Site Sponsor
Yes, the belt is loose. A properly (according to Porsche) tensioned belt has virtually no "give" in the belt - it feels like a steel band.
I agree with Alan, and want to emphasize one thing - take the alternator to an auto electrical shop, not an auto repair shop or auto parts store.
A good electrical shop can and will test the alternator correctly. They also can and will properly rebuild it, including full load and voltage testing afterwards. They also can usually install any one of several available Bosch regulators, with differing temp compensation, etc.
That way, you get the correct alternator, complete with the original cooling duct (assuming that you have it now, of course), pulley, etc.
I agree with Alan, and want to emphasize one thing - take the alternator to an auto electrical shop, not an auto repair shop or auto parts store.
A good electrical shop can and will test the alternator correctly. They also can and will properly rebuild it, including full load and voltage testing afterwards. They also can usually install any one of several available Bosch regulators, with differing temp compensation, etc.
That way, you get the correct alternator, complete with the original cooling duct (assuming that you have it now, of course), pulley, etc.
12-03-2014, 01:11 PM
#12
Intermediate
Thread Starter
I contacted Metroplex Alternator and Starter in Arlington, the fella spent a full half hour on the phone going over my indications and suggested a few more troubleshooting steps. Namely checking the Amp draw from the alternator at idle using a clamp over ampmeter and running an additional heavy cable (jumber cable) from the B+ post to the battery pos and see if it is indeed a problem in the cars + leads. He was very interested to hear the brushes on the VR had not bedded in. I explained the forward slip ring was pretty deeply grooved and the brush was riding on a few knife edges. He was also interested in the RPM ratio for the alternator. It seems that the full Amp rating for an alternator is near max RPM and they will generally put out half rated at idle. So I'll check the Amp draw at idle to see if I have a lot of current draining somewhere. At any rate, I now have several more tests to perform and will post results / resolution as soon as I can.
I do indeed have a new cooling shroud and duct. These were missing when I got the car because I'm guessing the PR had been replaced by the Bosch and the ducting didn't fit so was discarded. If I understand correctly a 1983S US car came with the PR alternator. Also, if I understand correctly, a 1983 car had a 4 rib belt and a 1984 had a 5 rib. I had to change out the pulleys when I put the PR alternator from the 84 and put it on the 83, where it has been installed for this whole evolution.
Thanks again everyone
I do indeed have a new cooling shroud and duct. These were missing when I got the car because I'm guessing the PR had been replaced by the Bosch and the ducting didn't fit so was discarded. If I understand correctly a 1983S US car came with the PR alternator. Also, if I understand correctly, a 1983 car had a 4 rib belt and a 1984 had a 5 rib. I had to change out the pulleys when I put the PR alternator from the 84 and put it on the 83, where it has been installed for this whole evolution.
Thanks again everyone
Last edited by rotenDrachen; 12-03-2014 at 01:27 PM.
12-03-2014, 01:28 PM
#13
Intermediate
Thread Starter
Yes, the belt is loose. A properly (according to Porsche) tensioned belt has virtually no "give" in the belt - it feels like a steel band.
I agree with Alan, and want to emphasize one thing - take the alternator to an auto electrical shop, not an auto repair shop or auto parts store.
A good electrical shop can and will test the alternator correctly. They also can and will properly rebuild it, including full load and voltage testing afterwards. They also can usually install any one of several available Bosch regulators, with differing temp compensation, etc.
That way, you get the correct alternator, complete with the original cooling duct (assuming that you have it now, of course), pulley, etc.
I agree with Alan, and want to emphasize one thing - take the alternator to an auto electrical shop, not an auto repair shop or auto parts store.
A good electrical shop can and will test the alternator correctly. They also can and will properly rebuild it, including full load and voltage testing afterwards. They also can usually install any one of several available Bosch regulators, with differing temp compensation, etc.
That way, you get the correct alternator, complete with the original cooling duct (assuming that you have it now, of course), pulley, etc.
12-03-2014, 01:39 PM
#14
Intermediate
Thread Starter
Sounds like the belt is too loose, your resistance readings must be wrong - I suspect the measurement device - many are not accurate for small values. If you had 1 ohm in the battery to alternator connection when the alternator charges the battery at say a 10A rate you'd be dropping 10V! (so obviously that never happens).
If any appreciable amount of that resistance was in the battery to starter connection you'd never be able to turn the engine over since it can take >100A.
I think you need to reconsider how to do those measurements and basically forget the results for now.
It seem clear the alternator doesn't charge - except marginally at cold - take it out and take it to a good alternator shop for evaluation (don't just replace - test - you want to know its condition for diagnostic purposes). It's interesting you say the previous alternator did the same thing - but I wouldn't ignore the obvious diagnostic path.
Once you know you have a good alternator - check the exciter line while running at the 14 pin connector (at both engine cold & hot conditions). You want to see that is continually driven high and not drooping when hot.
Maybe get a better meter (or just change the battery first) and see if you can get more likely resistance readings.
Alan
If any appreciable amount of that resistance was in the battery to starter connection you'd never be able to turn the engine over since it can take >100A.
I think you need to reconsider how to do those measurements and basically forget the results for now.
It seem clear the alternator doesn't charge - except marginally at cold - take it out and take it to a good alternator shop for evaluation (don't just replace - test - you want to know its condition for diagnostic purposes). It's interesting you say the previous alternator did the same thing - but I wouldn't ignore the obvious diagnostic path.
Once you know you have a good alternator - check the exciter line while running at the 14 pin connector (at both engine cold & hot conditions). You want to see that is continually driven high and not drooping when hot.
Maybe get a better meter (or just change the battery first) and see if you can get more likely resistance readings.
Alan
Thanks again
12-03-2014, 01:42 PM
#15
Chronic Tool Dropper
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My 'rule of thumb' on belt tension is half a thickness deflection with thumb pressure mid-span. So the ribbed alternator belt maybe 1/8" deflection. It's pretty darn tight.
---
Clamp-on Ammeters for DC circuits can be a little spendy. The typical one you use for house or other AC circuits won't give a reliable reading on DC circuits like the battery and alternator. The ones for DC induce a high-frequency field through the cable, and look at distortion of that field to determine current flow. Bottom line: Make sure your clamp-on meter is designed for DC measurements.
In addition, the load in the car splits forward and back at the alternator, so you get to take several readings and put them together to get a total picture of current flow in the car. From the alternator, current flows forward in the car to the ABS connection if you have ABS, and to the jump post and on to the CE panel for most but not all loads. From that same alternator, current flows to the rear to charge the battery, and also to run the fuel pump, injectors, and on S4+ cars the cooling fans, all of which connect directly to the battery positive.
---
Clamp-on Ammeters for DC circuits can be a little spendy. The typical one you use for house or other AC circuits won't give a reliable reading on DC circuits like the battery and alternator. The ones for DC induce a high-frequency field through the cable, and look at distortion of that field to determine current flow. Bottom line: Make sure your clamp-on meter is designed for DC measurements.
In addition, the load in the car splits forward and back at the alternator, so you get to take several readings and put them together to get a total picture of current flow in the car. From the alternator, current flows forward in the car to the ABS connection if you have ABS, and to the jump post and on to the CE panel for most but not all loads. From that same alternator, current flows to the rear to charge the battery, and also to run the fuel pump, injectors, and on S4+ cars the cooling fans, all of which connect directly to the battery positive.