vze2jshn

Yes different circuit, but bulb (3watt) and holder is the same as back lights as opposed to my other pod lights and '79 on 1.5 bulb with additional parallel resistor circuit. Since I did not know what I know now, thanks to rennlisters, when I went to LED on the bulb cancellation and initial exciter circuit did not occur.

RCinXS

Hmm.

I have no idea if this could be related, but here it is.

I have changed every bulb in my 84 to LEDs , except the overheat and no charge bulbs. I didn't change them because an EE said that they are part of a logic logic circuit that takes resistance into consideration and that LEDs would not work, as they would throw the values out if wack.

There are some odd things about changing to LEDs.

For instance, my rear tails lights have a very, very dim glow all the time, when the ignition is on.

This is due to the bulb check circuit that sends a very small volatge or current to "see" if the bulbs are good.

A regular bulb would not recieve enough power from the bulb check circuit to glow as the LEDs do.

A regular bad bulb, would cause an open circuit which would trigger the bad bulb light on the dash.

LEDs can operate dimmly, on a much smaller voltage / current then they are rated for.

Good news is that if a LED bulb blows, the warning light will still come on.

If the EE was right about the value balance issue for the overheat and no charge circuits, you might have to somehow fool the circuit into seeing more resistance than the LED provides.

Again, I don't know if this is part of the problem, but I had to use resistors here and there to solve other "logic" problems in the 928. When I converted to Xenon headlights, I had to add a resistor to fool the Hi / Low circuit, otherwise it would not work properly.

vze2jshn

Thanks, I'll keep an eye out but my '78 has no computer and I only changed back lights--hope the situation you describe does not apply to my OB.

tmpusfugit

The exciter circuit in the alternator depends upon a modest current flow through the warning light bulb (and the 68 ohm resistor in parallel with the bulb in later cars) Some or perhaps many of the alternators will "self-excite" at about 1800-2000 rpm, but not all of them will. VW has used this approach in their cars since the dawn of time thru at least 1975, maybe they still do.....in an early car replacing the bulb with an LED will not draw enough current to excite the alternator. In later cars with the resistor, it may work OK with an LED. The resistor was most likely added to allow charging, until it could be replaced, if the bulb did burn out.

Landseer

This has been posted elsewhere, but for completeness for the next guy searching...