Charging / exciter circuit question

[LateFan]

Hi - new guy here. I've searched and read several threads on charging here to learn about these cars. My son and I are planning on picking up a nice '86 ES coupe this weekend that won't start.

Seller info (we haven't driven it yet):

175k, two-owner
Auto (hope to swap)
Ran well but wouldn't start.
Installed new battery - now dead.
Jumps and runs well - won't restart.
Starter and circuit are fine because it jumps.
They get a clacking starter when attempting a re-start without a jump.

I've read here to check the positive cable in the trunk, the fused link in the cable, the engine ground strap, and the exciter circuit wire from the dash to the alternator.

If we ground that exciter wire (disconnected from the alt?) and turn on the key, the dash charging light should light, correct?

The power to the alt and the charging circuit passes through this dash light bulb, correct?

If it doesn't light, how will we know if the circuit is not live or if the bulb itself is burned out without going into the dash cluster?

If it does light, we're looking at the V regulator or the whole alternator?

We want to verify what we think is wrong before we offer (it's a deal already), but we don't want to work on it in their driveway, and we'd rather not solve it right there and drive away with an easy fix.

Related question - if this car is not charging and we jump it to drive it, will it last 45 miles on the highway? Or will the battery die and the ignition start cutting out?

I'm ordering a Bentley manual right now.
Thanks for your help -

[bmwstudent]

The battery light should light up anytime the ignition is on but the car is not running(alt not charging). Do not ground that wire it has battery voltage on it. If it doesn't light up when the key is in the on position(engine not running), then it's probably burnt out. The first thing you need to do is start the car with a properly charged battery then check to see if the alt is charging. My guess is it is not. It's possible to replace just the brushes in the alternator, they unbolt and come right out the back, if that's what's wrong with it. I do not believe you will make it 45 miles unless you stop to charge the batt several times, unless you start with one that is fully charged. Good luck!

[LateFan]

Oh, I thought I read a thread where Jim Levie said to disconnect that wire from the alt and ground it to see if the light comes on, to see if that wire was hot.

I got this wrong.

Is that fantastic troubleshooting manual available to buy new in color? In there I see that there is an early model / late model change in a resister wire that bypasses that bulb if it's burned out - so I need to find out when this car was built.

[LateFan]

found it

Here it is....

"Inspect the exciter circuit on the alternator. D+ (small blue wire) should have battery voltage when the engine is off and ignition in run. Also confirm that with the key on run and the motor off that your alternator (small battery symbol) light is on. It should go off after the car is started."

and this...

"A shorted primary power connection could cause failure of the alternator. But a bad alternator/battery warning light bulb will prevent the alternator from charging as that bulb is the source of exciter power. A simple check of the exciter circuit is to ground the exciter (small) wire at the alternator. Then turn the key to ignition on. If the bulb lights the exciter circuit is good and the problem is in the alternator. If the bulb doesn't light there is a problem with the bulb, cluster, or wiring."

[jlevie]

You won't hurt anything by disconnecting the exciter wire from the alternator and grounding it to check the bulb. The bulb (and/or late cluster resistor) limit the current flow.

If the problem is just a burned out bulb, you can connect a similar bulb from a source of power to the exciter terminal as a temporary fix. The voltage regulator can be changed without removing the alternator and it would be a safe bet to say that the brushes are worn and that a replacement regulator is needed in any case.

[LateFan]

Thank you.

Where can I look up the size / resistance of that bulb?

Any suggested location to connect to hot in the engine compartment? Just learning this car.

[jlevie]

The easy way to jury rig this is to pick up power from the coil positive. Any .5-1watt bulb will be okay.