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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:

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:

.... 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.


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.


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 08:51 PM..
Old 01-07-2007, 10:08 AM
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