With everything switched off and the battery disconnected, I measure about 5 ohms to ground from bus 15. Is that about right ? What leaks ?

 

I'm pretty sure that resistance checks like that aren't very meaningful. Too many active, non-linear devices in the path that change resistance with current flow.

If you want to check drain rate, put an ammeter (usually a multimeter in "current" mode) in series with your ground strap. Make sure that you've got things arranged such that your interier lights won't go on (on my '90 S4, there's a convenient two-pin connector right next to the hatch release mechanism that I can disconnect to prevent the open hatch from turning on the interior lights). If you have selectable ranges on your meter, you might want to select a higher range to start with, as there is an initial high-current transient when you first complete the circuit as capacitors, etc. charge up.

On my S4, I see a 37 mA "everything off" drain. I think that's pretty typical.

 

Bus 15 is only powered when the key is "on", so it's not draining your battery (if that's what you're worried about) since it's not connected to the battery if the car is off. There are a lot of things on it that can't be "switched off" - like the fuel pump relay, FI brain, instruments, etc, etc, etc. But they are all off if the key is off.

 

Thanks-- I was actually checking the alternator D+ wiring for a short to ground when I found that Bus 15 had a ground path. I have a high voltage/red light at higher speed alternator problem that was not fixed with a new regulator.

I guess it is anything electronic that leaks ? My 79 has a few relays that are on Bus 15.

 

The resistance to ground isn't "leakage" on bus 15 - it is the total of all of the devices that get turned on when the ignition switch is on. A resistance measurement on bus 15 is meaningless under normal conditions.

The charge light is hooked to bus 15 on one side, and the alternator field on the other. When you turn the ignition switch on with the engine not running, power flows from bus 15 thru the charge light and the parallel resistor to the alternator field terminal, and thru the field coils to ground, so the light glows.

When the alternator is turned fast enough to start generating power, the field coils are powered thru the diode trio, so that the field is self-powered. There is now voltage on both sides of the charge light, so it doesn't glow.

Since you have a charge light when the alternator is turning, you still have power only on one side of the charge light. One possibility is an open diode.

My suggestion is that you find a good local auto electrical shop. Not a general garage, not an auto parts place, but a shop that repairs alternators, generators and motors. They can repair or rebuild your alternator.

 

Thanks Wally. I did just that and was at an alternator shop this morning. We put it on his test rig and he immediately said the regulator was bad. So maybe my new regulator is bad. We ordered a new one and will try again tomorrow. At least now I know that it is in the alternator. I went thru all the D+ wiring in the car thinking that a short to ground would not only light the red light but also would fake out the regulator to put out a high armature current because the D+ signal was grounded. I was a little disappointed in the test rig the shop used. Looked like just some analog gages. He must have known how to interpret the readings to tell that a diode is not bad. Maybe one of the gages is AC, which would give a higher reading than a rectified AC with all good diodes. I still think that a diode is the problem, but we will see if yet another regulator cures things. I checked all 9 diodes with a meter and they seemed OK (correct forward voltage drop, nothing in reverse), but they were installed in the circuit and I am not sure if that could fool me.