System charges at high RPM only

[Mid70's Chrysler Fanatic]

My '74 T&C won't start without a jump. The battery is less than a year old. I noticed that the system doesn't charge unless the engine is running at high r.p.m.'s. At lower speeds the ammeter reads low and I can't even get the turn signals to work; the w/s wipers are really slo-o-o-w, etc. Put the pedal to the metal and the ammeter suddenly jumps to to a slight high side and the signals, wipers etc. work normally. Driving slower than about 70 mph is not an option.

Something obviously is needing to hit a threshold of some sort before the charging system kicks in. Any ideas of what to check first?

 

[Big_John]

That sounds like a voltage regulator.

First, take a look at all connections and grounds for the regulator. Check the battery voltage with everything off. Check the battery cables for clean, solid connections at both ends of the cables, not just the battery. I'd do a load test on the battery too. If you can't do that test yourself, take the battery to your local Autozone type place and they will test it for free.

If all that looks good, there's a simple check that is very similar to the procedure in the FSM. There are 3 connections at the alternator. Two of them are spade connectors. You'll need to make a jumper wire from one of the connectors to a good ground. There will be a blue and a green wire. Pull the green wire off and connect your jumper to the field connection on the alternator and a good ground. Start the car and immediately turn on the headlights. This simulates the load shown in the FSM. Check the voltage at the battery. It should be over 14 volts. Don't run the car very long like this as it may overcharge the battery.

If the voltage is higher in that test, that points to a problem with the regulator. If it isn't, that points to an issue with the alternator.

 

[Davea Lux]

You have an electronic voltage regulator, also check to be sure the mounting bolts for it are tight and that the unit has a good ground. You might also have a burned out diode in the alternator, the test john suggested will confirm if the alternator is capable of charging.

Dave

 

[RJRENTON]

I concur with the two previous gentlemen. If the regulator is OK, then there is an OPEN diode. The alternator is a three phase full wave bridge design and an open diode (either a positive or negative diode) in any phase will result in half of the published amp capacity produced and lower under high loads.
Bob Renton

 

[Mid70's Chrysler Fanatic]

Finally got to work on the car last week. This relentless winter we've been having...aargh! Anyway:

It was the alternator. I replaced it with a lifetime-warranty rebuilt unit and it worked like a charm!

Incidentally, I decided to replace the V-belts, and the parts clerk brought out their in-house brand "MasterPro." They looked really chintzy, almost worse than the existing ones! They're described on oreillyauto.com as:
"Designed for the budget conscience [sic] customer who wants quality, but does not require the superior performance of the premium belt. Not intended for severe driving, high-performance or heavy duty applications."

I bought Gates instead.

 

[Davea Lux]

Finally got to work on the car last week. This relentless winter we've been having...aargh! Anyway:

It was the alternator. I replaced it with a lifetime-warranty rebuilt unit and it worked like a charm!

Incidentally, I decided to replace the V-belts, and the parts clerk brought out their in-house brand "MasterPro." They looked really chintzy, almost worse than the existing ones! They're described on oreillyauto.com as:
"Designed for the budget conscience [sic] customer who wants quality, but does not require the superior performance of the premium belt. Not intended for severe driving, high-performance or heavy duty applications."

I bought Gates instead.

The Master Pro and Master Crap belts sold at Car Quest and some Auto Zone stores are made in China or Taiwan and they are not worth putting on.

Dave

 

[Mid70's Chrysler Fanatic]

The Master Pro and Master Crap belts sold at Car Quest and some Auto Zone stores are made in China or Taiwan and they are not worth putting on.

Dave

Master Crap: I love it!