Low voltage at idle loaded

[pjm8047]

My 71 road runner at idle with loads on voltage low blinkers run slow wipers slow and headlights dim but when revving or accelerating voltage goes up and everything runs much faster. What could be ? I still have factory alternator and voltage regulator single pulley .

 

[Pops1967GTX]

Mopar alternators are weak at idle, going to need more information. Get test light out And Mult- meter.. Need to
Measure your voltage with the car off, then start it and re-check the reading.... charging voltage. should be 13.8 to 14.2.
You could have voltage drop at idle. I would start off checking grounds and ground, check wires, the really big problem is usually the bulkhead connector

Mopar Headlight Dimming - MyMopar.com - Mopar Headlights

 

[pjm8047]

the gauge on the dash is on the plus side but when loaded it goes towards the negative side with the more loads I put on it but when rpms go up the voltage goes up.

 

[Pops1967GTX]

the gauge on the dash is on the plus side but when loaded it goes towards the negative side with the more loads I put on it but when rpms go up the voltage goes up.

That is correct...the dash gauge reading are not the best judgement of the system performance... Put a voltage meter on the battery for a better reading.

 

[pjm8047]

I will but wouldn't slow wipers at idle with headlights on indicate low voltage ? The more loads I put on the slower they go. Rpm's up = higher voltage faster wipers

 

[Pops1967GTX]

You need do the system tested to be sure that the alt and reg are doing their job right. It could appears to me that the alternator is not putting out enough amps to keep all these items running... but more info is needed...

 

[pjm8047]

I'm thinking a powermaster 75 amp alternator would help.

 

[jimt1971rr]

That is probably your best bet. My 71 does exactly the same thing, as did both of my 60s Barracudas. The stock alternators are just weak, so you either need to up the amps or the rpms.

 

[pjm8047]

Would this work? I only have one belt. https://m.advanceautoparts.com/p/dr...NBgZRq8K9CXKtaTcs3V4DI50J3aj7SSP1LCRCuVDMRC3f

 

[jimt1971rr]

It should work but you might want to see if they offer it with a single pulley.

 

[pjm8047]

So I ordered a new powermaster alternator. But I was thinking could a bad battery cause low amperage output even when the vehicle off? For instance my wiper blades run just as slow at idle as the vehicle is off but do speed up with acceleration . Vehicle does start no problem. The battery is about seven years old I remove it every winter and throw it on the battery charger.

 

[pjm8047]

So I tested my alternator here are test results
No loads idle 14.57volts 2.7amps and loaded 11.94 volts 7.3 amps. Bad alternator?

 

[black64]

So I tested my alternator here are test results
No loads idle 14.57volts 2.7amps and loaded 11.94 volts 7.3 amps. Bad alternator?

Make sure your connections at the battery are clean. Also it wouldn't hurt to have the battery load tested to check it out. The alternator has an output curve that increases with rpm. If the load is more than the output the battery voltage will be lower a fully charged battery with engine and loads off should be in the 12.5 volt range. If your battery passes load test and battery connection are clean then either a new smaller diameter alternator pulley to increase alternator rpm at idle or a new alternator with more output throughout the rpm range will improve required amps needed at idle so you don't discharge. There was some idle discharge allowed in design of the cars manufactured back then.

 

[pjm8047]

Battery test results
12.25 volts
Measured 553cca
Rated 800cca
Not a load
Test
Starter test
Voltage 10.34volts
Amps 240.9amps
Voltage went back to 12volts after starting.

 

[black64]

Battery test results
12.25 volts
Measured 553cca
Rated 800cca
Not a load
Test
Starter test
Voltage 10.34volts
Amps 240.9amps
Voltage went back to 12volts after starting.

The battery didn't fall below 10 volts so it should be fine.

 

[mopar73dge]

Use a 1973 Alternator, get a new battery for reliability and clean all your battery connections. My 318 with an alternator that was for a up to 1972 car with steel strips in between the two halfs was not the correct one, The alternator I have now is correct complete cover to cover no metal layers type, works better, much better no dimming at all.

 

[72SebringPlus]

Use a 1973 Alternator, get a new battery for reliability and clean all your battery connections. My 318 with an alternator that was for a up to 1972 car with steel strips in between the two halfs was not the correct one, The alternator I have now is correct complete cover to cover no metal layers type, works better, much better no dimming at all.

Interesting solution. I've had this same issue with our 72 440 RR so thought I would look into it. I just checked Rock Auto to see if there's any difference between the part numbers of a reman alternator for 1972 and 1973 and there's none.