I'm thinking about changing out the stock instrument cluster with a custom digital setup on the track car. What would be the easiest way to keep the alternator charging without the light/resistor in the cluster? Would an old style electronic voltage regulator (like a Chrysler) work on my Bosch alternator?

 

John,

Just run a wire from keyed power through a 68 ohm 2W resistor to the exciter terminal, in place of the wire that's there.

(also a good diagnostic test / quick & dirty fix)

There was a long detailed thread on this, not too long ago.

 

The resistor alone probably won't do it. The stock set up is for the alternator to be excited by the power flowing thru a combination of the resistor AND the light.

 

So when the light burns out, it won't charge???



I was under the impression that the resistor was there to prevent that kind of failure.

There is about 0.1 amp current flow through one of those 1.2W instrument lamps. The 68 ohm resistor allows an additional 0.18 amps of current flow. If 0.18 amps alone isn't enough to excite it, try a 47 ohm resistor instead (0.26 amps). This is a standard value and will be easy to find.


Just make sure the wattage rating of the resistor is sufficient to prevent it from overheating / burning your fingers / starting a fire. A lower ohms value resistor may require a higher wattage rating - 5W is a common value.

 

So how does this explain why all of my idiot lights remain on until I rev my engine up to 1,800 rpm for the first time each time the car is started?

The last wrench I asked about this said it was normal for some cars. I doubt it.

<img src="graemlins/offtopic.gif" border="0" alt="[offtopic]" />

 

Hey Dave,

It *doesn't*

But if I had to venture a guess, I'd check the voltage at your battery after you start the car, with the idiot lights on, before you rev it. It should be over 12 volts, probably 13-14 volts.

If it is low and doesn't go up until you rev it, then your alternator isn't getting excited normally & the low initial voltage may explain the behavior of the lights. I don't even think these cars will run at battery voltage below 10V, due to the needs of the ignition system.

If all is well with the charging circuit, I'd check the voltage at the idiot light control box under the same conditions. You may have a low voltage condition at that point due to an electrical fault.

 

Hey Dave,

Sounds like you have some wreching to do. (I don't feel left out now. <img src="graemlins/bigok.gif" border="0" alt="[thumbsup]" /> ) I think you have some connections on your car that need cleaning, thats probably it.

John D

 

Maybe check alternator belt tension as well?

I had a low voltage issue w my '82 that was 'cured' (after removing and testing the alternator....good practice I guess) by cleaning the battery connections and the ground at the wing bolt ..I also cleaned the alternator connections so that could have helped as well.

Jim

 

John,
w a 47 ohm resistor you'll dissipate about 3.5 watts at 13 volts (battery voltage) but you shouldn't dissipate it for long because the excitor coil 'comes up' to voltage and no current will flow then. So you can probably get along w less if you have to, although higher wattage is safer if you ever get into a problem where the alternator circuit is failing and you end up w current flow all the time.

Jim

 

Dave,
With the Auto show around the corner, you could probably get one of the hot girls to excite your alternator! Are you going to the show?

Ell
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

 

You know, my '85 S2 does what he is talking about- all the idiot lights on [in German!] until I rev it past 1800 rpm.

AND...my first car [also with a Bosch alternator], a 1977 VW Rabbit did this...

AS DID my second car [also with a Bosch alternator...], a 1985 VW Scirocco.

Several mechanics told me this was normal- I doubt it too, but nobody seems to know how to remedy this. All the contacts are clean...

N!

 

I would first try checking and cleaning all of the electrical connections involved. If that didn't work, I would try increasing the excitation current thru substituting a lower resistance resistor.

 

Thanks for the replies...the idiot light thing has been doing this since I bought the car, but it's never really been a problem. I was just curious.

Ell:

Not sure if I'm going to the auto show yet....maybe we ought to discuss it at the next 928 beer night? It should be next week!