Voltage gauge waay high, way low
03-22-2008, 09:38 AM
#16
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Tim - there is no diode - I think he means the exciter resistor. Its possible you have an issue (again) with the alternator/regulator but I really don't know what would be killing them - shouldn't be anything in the pod. The primary suspects are excess loading related - which would be battery and aftermarket equipment (audio or other high current) usually. Certainly could be other high current consumers on the car - like ABS pump etc...
But there aren't so many of these...
Monitor the voltage for a while and report what you see.
But there aren't so many of these...
Monitor the voltage for a while and report what you see.
Seriously!!!!
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Alan's suggestion of checking the voltage at the panel is a good one, but the 84 panel is the last of the early style with the ceramic fuses and the main power leads coming in the front between plugs N and O rather than studs on the top edge like the later style. You can check it at any of the three big red terminals you'll find there.
The idea is to divide and conquer; If you don't get stable voltage there, you work back through the jump post, alternator main output terminal, and battery to see at what point it begins misbehaving. If you DO get stable voltage there, then you move forward through the panel, harnesses, etc to narrow it down.
Did you replace the alternator before all this or did the symptoms appear after changing it out? Either way, check for a slipping belt. That could cause low voltage under load and much higher voltage when the belt heats up and begins to grip, and it charges like crazy to make up for the time it wasn't charging.
If the problem seems to be in the CE panel/harness, try just unplugging and reconnecting each harness connector at the base of the panel. If this changes things, then it's most likely corroded contacts(and not just the ones that lead to the gauge) and you can choose whether to leave it at that(band-aid) or go for a more thorough approach.
The idea is to divide and conquer; If you don't get stable voltage there, you work back through the jump post, alternator main output terminal, and battery to see at what point it begins misbehaving. If you DO get stable voltage there, then you move forward through the panel, harnesses, etc to narrow it down.
Did you replace the alternator before all this or did the symptoms appear after changing it out? Either way, check for a slipping belt. That could cause low voltage under load and much higher voltage when the belt heats up and begins to grip, and it charges like crazy to make up for the time it wasn't charging.
If the problem seems to be in the CE panel/harness, try just unplugging and reconnecting each harness connector at the base of the panel. If this changes things, then it's most likely corroded contacts(and not just the ones that lead to the gauge) and you can choose whether to leave it at that(band-aid) or go for a more thorough approach.
03-23-2008, 02:21 PM
#17
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Easter Sunday - homework report...
Connected a DMM to CE panel - thanks to Dave for specifying the voltage supply location.
Voltages to report:
No key = 12.8V
Car at idle = 13.8 to 14.2V
The gauge, if anything, shows a lower voltage than actual.
So I decide to take the car for a touch-free car wash so as to introduce a little moisture to the matter. (And we all know how important that can be...
) Car shows 14V throughout the car wash. Then I exit the car wash and turn the ignition off. I restart the car and the voltage on the DMM soars to 18.8V. The gauge is way past 16V. During my five minute drive home, the voltage fluctuates between its normal 14V and the elevated 18V.
So the source of this issue seems to be BEFORE to CE panel.
If the gurus have a next assignment, I'm ready for it. Can a slipping belt cause this? I installed the new Bosch alternator myself and tightened the belt by feel. I don't know what the tightening specs are.
Thanks, peace, and Happy Easter guys.
Tim
Connected a DMM to CE panel - thanks to Dave for specifying the voltage supply location.
Voltages to report:
No key = 12.8V
Car at idle = 13.8 to 14.2V
The gauge, if anything, shows a lower voltage than actual.
So I decide to take the car for a touch-free car wash so as to introduce a little moisture to the matter. (And we all know how important that can be...
) Car shows 14V throughout the car wash. Then I exit the car wash and turn the ignition off. I restart the car and the voltage on the DMM soars to 18.8V. The gauge is way past 16V. During my five minute drive home, the voltage fluctuates between its normal 14V and the elevated 18V.So the source of this issue seems to be BEFORE to CE panel.
If the gurus have a next assignment, I'm ready for it. Can a slipping belt cause this? I installed the new Bosch alternator myself and tightened the belt by feel. I don't know what the tightening specs are.
Thanks, peace, and Happy Easter guys.
Tim
03-23-2008, 05:21 PM
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Well - then the regulator on the alternator doesn't work properly.
Don't know why its happened on another alternator - but that can be the only cause of these voltage levels.
Alan
Don't know why its happened on another alternator - but that can be the only cause of these voltage levels.
Alan
03-23-2008, 11:13 PM
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If Alan says we need another new alternator - his wish is my command.
Once the new/old alternator comes back I will have it tested.
Roger
Once the new/old alternator comes back I will have it tested.
Roger
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Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission?
George Layton March 2014928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
03-23-2008, 11:32 PM
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Even with my shallow know-how with these electronics, I can see that today's discovery does narrow the search; the only place to get 18+V is at the alternator. So it does seem like something's afoot there.
Thanks, Alan.
Tim
03-24-2008, 12:12 AM
#22
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How about a loose belt? Can that cause spikes like this?
Even with my shallow know-how with these electronics, I can see that today's discovery does narrow the search; the only place to get 18+V is at the alternator. So it does seem like something's afoot there.
Thanks, Alan.
Tim
Even with my shallow know-how with these electronics, I can see that today's discovery does narrow the search; the only place to get 18+V is at the alternator. So it does seem like something's afoot there.
Thanks, Alan.
Tim
Good luck
03-24-2008, 12:14 AM
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A loose belt will cause low voltage drop out - usually at idle but can't cause stable high voltage conditions. The alternator should never generate a stable voltage above its regulator set point (say about 14v). Instantaneous spikes can be created by heavy duty switching but will only last for an instant - too short for a meter to detect or display. You can find them with an oscilloscope and they may cause noise - esp. audio noise - but you have something quite different...
Alan
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03-24-2008, 03:21 AM
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When I swapped my alternator out I had to max the belt tensioner out and the belt didn't feel really tight enough. If my belt has started to slip it would fit my alternator issues as well. I'm going to buy a shorter belt tomorrow and check the exciter wire just to make sure. Does anyone know the stock size alternator belt for my 81 OB? Jason
03-24-2008, 12:59 PM
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A loose belt will cause low voltage drop out - usually at idle but can't cause stable high voltage conditions. The alternator should never generate a stable voltage above its regulator set point (say about 14v). Instantaneous spikes can be created by heavy duty switching but will only last for an instant - too short for a meter to detect or display.
Alan
Alan
The ever-unbelievable Roger Tyson has already called and offered to send a new alternator. But some history is important to note here. He sent me a remanufactured Bosch alternator just a couple months ago. This SHOULD be a fine alternator. So I want to get it tested before he sends me another.
Jack time!
Thanks to all!
03-24-2008, 04:17 PM
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Tim since it doesn't happen all the time - they need to load test it while running for a while. I'd guess its heat related. Heat is self generated under load as well as environmental (exhaust proximity). Do you have the shroud and air tube installed ?
My alternator is much happier at idle in the winter vs the heat of the summer.
Alan
My alternator is much happier at idle in the winter vs the heat of the summer.
Alan
03-24-2008, 06:52 PM
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Tim since it doesn't happen all the time - they need to load test it while running for a while. I'd guess its heat related. Heat is self generated under load as well as environmental (exhaust proximity). Do you have the shroud and air tube installed ?
My alternator is much happier at idle in the winter vs the heat of the summer.
Alan
My alternator is much happier at idle in the winter vs the heat of the summer.
Alan
I concur that the true test of this alternator is after heat or moisture build-up. But I'm not sure how to get the unit tested in-store under such conditions. I do indeed have the shroud and intake tube hooked up. The tubing is damp from dripped power steering fluid from the reservoir above (think I've fixed that issue) but otherwise seems in decent shape.
Roger and I are going to catch up by phone soon. Neither of us is an electrical genius, but we'll try to figure out our options.
And Alan, I don't blame your alternator for not liking your summer heat.
Peace,
Tim
03-25-2008, 03:58 AM
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Tim, as Alan says your alternator is charging higher than it ought to, which points to a regulator problem. I'm suspicious about the problem occurring after you get the car wet. Do you have the drip shield over your jump post & 14-pin connector? Is it possible the car wash splashed some water into the Alt. cooling duct and messed with the regulator?
03-25-2008, 10:46 AM
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I've asked about replacing the wiring harness, but a couple of folks have indicated this is not the likely culprit.
Thanks, folks.
Tim