1989 244 manual tranny. Been running a 100 amp alternator from a 740 for the last 10 years. I was changing my HID lights and had a bulb short out. I right after that I noticed the battery bulb was lit. I had no fuel or temp gauge and the tach was not working. I drove the car and noticed that the speedo was jumping up and down. My volt meter gauge showed that it was charging. The next day everything was normal. Today it was back to the battery light illuminated, the gauges and tach not working and speedo jumping again. But still the volt meter read a charge. Is this typical of a bad alternator or maybe bad diodes in it? Thanks for any help you can provide.

How about pulling the brushes from the alternator and checking them?

When they are worn, they can stick and not make proper contact, then a bump might free them and things are OK until they stick again.

How many miles on the alternator - I had two fail at almost exactly 215K.

Thanks guys. I will do a voltage drop on the neg and pos sides. The HID system has been in the car for the last 7 years with no problem. It was only when I swapped out one of the bulbs that all this started. As for the 100 amp alt., It has been in the car with no problems since 2005. The wiring was replaced when I installed the E-Codes and went to a higher wattage bulb until I got the HIDs. All wires are 12 gauge with relays for the high and low sides. The famous factory headlight relay is bypassed. The factory bight light relay is still in use. And its the speedo thats jumping up and down. The tach is dead. I checked the voltage to D+ and had voltage there so I know it is activating the field. The Batt terminal also had 12 volts (12.76) As for the volt meter I really do not remember where it is connected. I did that when I first got the car in 2000. Being over 70 now, my memory seems to be wondering. Mostly wondering where it is and why I can't find it. I exchanged the Bosh alt today for a new one so as soon as it stops snowing and the weather gets above freezing I will swap it out. The only good thing that came out of this is that I discovered that the bottom mounting bolt had sheared off at the block and I now have to drill it out and replace it with a higher grade bolt. I will let you know what I find in a day or two. Thanks again for the suggestions. P.S. Art I hope you are doing well, I haven't talked to you in several years.

Hi there,

I will agree with Art on the sudden loading theory.

I purchased the "Automotive Electric/ Electronic Systems" book that Bosch prints. Art recommended to me a few weeks back. I got it off Amazon for about $8 shipped! Looked really new and its well illustrated too!
It's a great little book, Art, thanks!

You did not mention if the instrument panel goes "nuts" if the headlights are on or not?

The I don't know much about HID headlights. I think they use a transformer don't they?
Any people that I have known, that used aftermarket head lights, have had issues with the transformers, sooner or later!

All of this might be a coincidence that started from worn brushes or stator rings in the 100 Amp alternator. 10 years is quite a bit of time if you have not serviced them.

I'm curious to know where the voltmeter is connected. Is it into the cars circuitry or is hooked directly to the battery's posts?

The instrument panel is having an issue to be acting that way.
It seems to me I would look art the fuse panel out feeds to the instrument panel or the headlights switch itself. Most of the power should goes through the "infamous" headlight relay connector, on a regular 240.

For some reason, the battery light is seeing a ground circuit through the alternator or a part of the diode circuit for several lights mounted on the instrument panel.
The tachometer bouncing sounds like a on-off grounding or instrument powered issue to be along with the gauges.

If you remove the cluster you will access to almost all of those goodies!

I hope you are able to keep it down to a low tech hunting endeavor!

That's about all I could handle if it were my car.
(:)

Phil

There's nothing typical about your symptoms but neither are HID lamps or 100A alts typical in a 240, so looking for a consensus derived internet solution is probably not expected to be fruitful.

The high-tech approach to solving this would employ a scope and experienced troubleshooter; the low-tech solution is inevitably part swapping, perhaps starting with the alternator and battery.

Meanwhile you could look for a lousy connection opened up by the sudden load using voltage drop (high-tech), or visual inspection (low-tech).
--
Art Benstein near Baltimore

Why don't you ever see the headline 'Psychic Wins Lottery'?