Alternator Questions

[MJP]

The Marchal alternator on my 74 isn't working. I get 12+ volts at the battery with the car running from idle up to around 2500 rpm. The voltage climbs to the expected 14+ when I get to around 3000 rpm. The dash light is on while the voltage is close to battery voltage, and goes out as it climbs. I swapped it with a Bosch alternator that I intend to install in a 68 that I'm restoring. That one works fine. Light on at start, blip the throttle and it's out, voltage is up and all's well.

I had the Marchal checked at a local shop that's well respected in this area. They replaced bearings and brushes, nothing else was wrong. I put it back in the car an no change. Back to the shop, they put it on the fixture and it pumps out 50 amps at a reasonable rpm. Back in the car and no change.

I didn't want to hijack "car-wont-charge-melted-alternator-ground-why". I've been following the discussion. I don't understand the purpose of the D+ other than its connection to the dash light. What should I get at that terminal on the alternator while it's spinning? Is there a problem with that post that the shop wouldn't spot under a normal test cycle? Any help would be appreciated.

[ossiblue]

Have the shop (or do it yourself) measure the output from terminal D+ at varying rpms of the Marchal. I remember reading a thread about faulty connections inside the alternator to D+ but I can't be sure if that was in reference to generators or alternators. Regardless, that would be my first step, possibly even test both alternators at the D+ and compare the outputs since you know the Bosch works well.

[grizzfan]

Every time I read a post about alternators I can't help but think about this decal that comes with our cars:



I'm having issues similar too and replaced the regulator with a new Bosch but I don't know what brand the alternator is. Guess I had better find out!

Tom

[theiceman]

doesnt the 74 have an extrnal regulator ? maybe that is the culprit ?

[Targalid]

Check the voltage regulator, next to the CDI box. It is energized through the noise suppressor (right next to the voltage regulator) from the 14 pin connector blue wire. Have an assistant sit in the car to watch the alternator voltage output (voltage tester plugged in at cigarette lighter or directly from battery) while you jiggle wires around the 14 pin connector and voltage regulator. When I did this the charge would change from off to 14.4 volts as those wires were jiggled. I ultimately rewired a new 14 pin plug (mine turned out to be cracked giving poor connections) and all was well with the charging system.