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IMPORTANT: Charging System Design Flaw? (long)

It may be that what I found is specific to my 914, but from what I can tell, this issue should apply to all 914's, unless they have a different engine relay board than mine.

In a separate thread, I report (very long) on recent things I've done while chasing down idle instabilities with my car - the main one being a large drop when I add electrical load (e.g. lights). While tracking this problem, I found what appears to be a signficant design flaw in the charging system. First, some background on the charging system.

See the following URL for an excellent description and troubleshooting guide for VW-based charging systems:

http://www.ratwell.com/technical/ChargingSystem.html

I used this article as my guide when tracking down the problem I describe.

A properly-operating charging system supplies sufficient current from the alternator under all running conditions (except starting) to power the car and to keep the battery charged. The voltage regulator works with the alternator to make this happen. The voltage regulator accomplishes this by measuring the voltage of the battery and controlling the voltage put out by the alternator. This means that when the car is running, the alternator voltage will always be somewhat higher than the fully-charged battery voltage, to keep it charged.

A fully charged auto battery produces about 12.6 to 12.8 V. Even a small decrease in battery voltage indicates that it is not in a fully-charged state. From the article referenced above, a battery at 12.35 V is only about 50% charged, at 12.0 V its at 25% charged, and at 11.8 V it's completely drained. When your charging system is working correctly, your system voltage will be at least at 12.8 V or greater.

For the voltage regulator to work properly, it has to have the correct ground reference. If you look at the following diagram of the 914's relay board:

http://www.pelicanparts.com/914/parts/Electrical/914_electric_73E.jpg

.... part number 4 is the voltage regulator. Note that the D- connection, the ground reference, is connected to the case of the voltage regulator, which is shown connected to ground.

BUT WHERE IS IT GETTING THE CASE GROUND?

The relay board is made of PLASTIC. The screws which hold the voltage regulator to the relay board do not connect to chassis ground. As a result, the current path to ground for D- has to go through either the alternator or through the voltage regulator itself. I've measured the resistance to ground of D- on my car, and I got a value of 4 ohms. When the car is running, this resistance causes the voltage measured at D- not to be zero volts (as it should be, as per the reference I provided above), but to float to -1.1V. This causes the output of the voltage regulator to never exceed about 12.0 to 12.2 V, preventing the battery from ever charging to more than about 50% of a full charge.

I did some further measurements of the resistance of D- to ground. If I pull the alternator plug from the relay board, the resistance increases to about 100 ohms. If you follow the current path to ground, you can see that it now goes through the voltage regulator, through the D+ connector, and though the alternator warning light circuit - which has a bulb in it that has about 100 ohms of resistance. If you open that circuit, you get no path to ground - the case of the voltage regulator isn't grounded.

I connected a short ground wire between the voltage regulator case and chassis ground. Now, I read 13.0 V or greater under all running conditions, DF is at about 5 to 6 V when idling, and my battery stays charged to 12.8 V. The charging system is now working as it should have.

EDITED: DON'T ADD A GROUND JUMPER TO THE VR CASE, IT'S UNSAFE! SEE MY POSTING BELOW.

The question here is - is this common to all 914's, or is my car somehow different? If your voltage regulator is as mine is, connected only to the plastic relay board plate, and with no case connection to ground, then I see no way that your system should be any different from mine, and you should be seeing a lower charging voltage than is needed to keep your battery fully charged.

I'd like to see some readings from other 914 owners cars. Here's what I'd like for you to measure, it only takes a few minutes with a simple digital voltmeter to take the readings.

1. With the ignition switched off, measure the resistance from the case of your voltage regulator to a chassis ground. If your charging system is set up correctly, you should read about 0.1 ohms. Anything more than 0.5 ohms is suspect.

2. Remove your relay board rain cover, and start the car and let it idle, headlights off, heater/blower fans off, stereo off. The alternator connector is the three-pin connector on the side of the relay board. Set your DMM to DC volts and measure the voltage between a chassis ground (there's a ground connection on the firewall beside the relay board that's nearby) and each of the pins (you can stick a probe down beside the wire into the connector and contact the pin easily). Here's what you should get:

D+ (pin that's nearest to the front of the car: greater than 12.8 V, no more than 14 V

D- (next pin down, brown wire): no more than 0.1V

DF (next pin down): about 5 V or so (depends on the electrical load of the engine and the charging state of the battery)

If you don't get the results above, then your car has the same problem as mine did. A simple ground jumper between a chassis ground and the case of the voltage regulator fixes the problem. Let me know what you find.

Last edited by pbanders; 01-08-2007 at 07:51 PM..
Old 01-07-2007, 09:08 AM
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Brad, this is pretty timely. I'm starting the rebuild of the 2.0. But I was going to send out the alternator to check for output. I works fine. But with all the lights on it runs at 11v. Which starts to drain the brain. So the next start is wimpy.
I'm going to have to get a bigger binder for my Brad Anders info. LOL.
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73 914S 2.0 AG
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Old 01-07-2007, 09:15 AM
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My 76 had a weak voltage output when running, I wonder if this could be the source there too......

We're all lucky you are so obsessed, Brad :-)
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Old 01-07-2007, 10:37 AM
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Everybody - with the help of people on the other boards, this problem has been solved. There is no design flaw with the 914 charging system, the D- ground is provided by the alternator case. The problem is that the wiring diagrams here on the PP site and in the Hayne's manual for the all but the 75-76 cars don't show that D- is connected to ground through the alternator case - but it is, as the Factory Workshop Manual shows. The problem with my car is that this ground is faulty, and by tying the VR case to ground, I'm bypassing the fault. In the stock configuration, despite what the wiring diagram for the relay board shows, there is no ground connection to the VR from the relay board.

DON'T ADD A GROUND JUMPER TO THE VR CASE! IT'S UNSAFE, SEE MY POSTING BELOW

Low charging voltage seems to be a common complaint, and it's likely that as in my case, a poor alternator ground may be the cause of the problem. The test is simple: just measure the voltage on D- with respect to chassis ground while the car is running. If you get a signficant negative voltage (mine was -1.1 V), then your alternator ground is faulty.

I didn't want to lead anyone astray by what I found - that's why I asked the community to check what I found and see if it made sense. We're lucky to have a large community of smart people who can figure out these issues quickly! It didn't make sense to me that ALL 914's would have this problem, I'm glad to see it's just a fault and not a design flaw.

Last edited by pbanders; 01-08-2007 at 07:49 PM..
Old 01-08-2007, 06:37 AM
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DON'T ADD A GROUND JUMPER TO THE VR CASE - IT'S UNSAFE.

If you have an alternator ground fault, and add a ground jumper to the VR case, ALL OF THE CURRENT GENERATED BY THE ALTERNATOR WILL FLOW THROUGH THE JUMPER, 20A or more. You could start a fire. DON'T ADD A GROUND JUMPER.

Instead, if you suspect you have an alternator ground fault, measure D- with the car running. If you get a value more negative than -0.5 V, then your ground is bad and must be repaired.
Old 01-08-2007, 07:48 PM
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Brad, I took the alternator out of the box. NO ground strap.

Looking at the parts diagram. It shows a ground strap for the alternator. But nothing is shown or called out in factory manual during testing or replacement. ???

So with that being said. Where would you mount/attach the ground wire ? Both of the attachment bolts are pivot points so they would be suspect. There are thru bolts on the alternator, but they're like 4mm.
Thoughts ?

I just got a couple new battery's for my good old Sony digital camera. So I should be able to take some JPGs.

Which one is D- ? Where would you measure it ?
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77 Carrera RS w/3.2 #59
73 914S 2.0 AG
73 914 1.7 Driver ( daily driver, under complete rustoration )
74 914 2.0, 71 914 Tub, 74 914 2.0 Tub + 73 914 donor
Old 01-10-2007, 06:10 AM
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Cary, I'm seeing the same thing. The part exists, it's shown on the factory 914 parts diagrams, but it apparently isn't on anyone's car nor is it shown in the factory workshop manual or aftermarket manuals. Several other owners have noted the poor nature of the alternator ground connection through the pivot bolt, and have added their own connection between the alternator case and chassis ground. How are they connecting it? It's still unclear to me what they're connecting to on the alternator case, unless it's the pivot bolt.

D- can be accessed from the alternator plug on the relay plate. The connections are actually printed on the plug, though they may be hard to read. D- is the one with the brown wire, it's the middle one of the three connections, the one that's closest to the rear fender. You can touch the back of the spade connector in the plug by sticking in a digital volt meter plug. You measure the DC voltage between D- and a chassis ground (there's a ground point right next to the relay board) while the car is running. D- should be very close to 0 V, but if you have a bad ground, you'll see a sigificant negative voltage, something like -0.5V or more.

Here's a diagram of the relay plate that shows the alternator and voltage regulator connections:

http://www.pelicanparts.com/914/parts/Electrical/914_electric_73E.jpg

Last edited by pbanders; 01-10-2007 at 10:09 AM..
Old 01-10-2007, 07:48 AM
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I'll tinker tonight.
When my engine blew, the alternator got coated in oil. What would you suggest to clean the outside of the alternator with ?
I used carb cleaner a little bit last night. But don't want to use anything that would harm the inerds.
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77 Carrera RS w/3.2 #59
73 914S 2.0 AG
73 914 1.7 Driver ( daily driver, under complete rustoration )
74 914 2.0, 71 914 Tub, 74 914 2.0 Tub + 73 914 donor
Old 01-10-2007, 08:16 AM
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