Low charge when hot.

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LivewireBlanco

LivewireBlanco

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So I went on a cruise last night and it was a lot of stop and go traffic. Car was running about 200, no biggie there but I was noticing coming to the stops that it was not charging or barely charging. The voltmeter usually is about 14 but this was about 13 or just under, sometimes 12 and I would put it in neutral to help it a little and get it above 12.5.

What would cause this? It has only happened when I'm driving it for a while in stop and go traffic. Long highway drives are fine and general running around is fine also.

Car is a 71 Dart with 318.
 
If this is a stockish 50+ year old system it could very well be considered "normal." it also might be that you have a small alternator, or even a diode has died. Without a good test at full output load, no way to tell. That is all documented in the service manuals. You need a carbon pile load tester and multimeter
 
So it's fairly normal to have this issue when it's hot and not when it's being driven and cool? Everything works fine and charges great until extreme stop and go diving. The alternator is about a year old and is 70 amp I believe. The voltage regulator is original.
 
FIRST thing I'd do is check the belt for slippage

........and not enough information. Could be as simple as the idle speed sags a bit when hot. Many many causes here. And VR regulation changes with temp.

...........right out of section 8 of the 72 service manual..........

voltage.jpg
 
67Dart273 said:
FIRST thing I'd do is check the belt for slippage
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Think it might be slipping when it gets hot? I'll see if I can get more tension on it. Good call. I have a multimeter but no carbon pile load tester.
 
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It would seem its not stock.
In daytime, without the lights or anything else on, battery voltage should have not dropped down to 12.0 V.
The fact it came up to 12.5 V with a small change in rpm suggests the alternator is partially working.
 
Mattax said:
It would seem its not stock.
In daytime, without the lights or anything else on, battery voltage should have not dropped down to 12.0 V.
The fact it came up to 12.5 V with a small change in rpm suggests the alternator is partially working.
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The only thing not stock is it has a relay that accounts for the crappy bulkhead connector and a bypass wire from alternator to the starter relay to help charging. Everything works great when it's nice and cool, only when it's hot does it barely charge.
 
LivewireBlanco said:
The only thing not stock is it has a relay that accounts for the crappy bulkhead connector and a bypass wire from alternator to the starter relay to help charging. Everything works great when it's nice and cool, only when it's hot does it barely charge.
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LivewireBlanco said:
charges great
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We commonly say charges when discussing alternator producing power.
If that's what you are infering from the voltage readings, that's fine.
Just be aware that the battery may not be charging even when voltage is above 12.8
Without an ammeter, we generally assume it is, and that's fine.
When troubleshooting a problem like this, there is a possibility may not be charging the battery regardless of voltage on a dash meter.
 
When its hot, could check for the voltage at the field terminal with blue wire.
Could also full field the alternator to see if that brings the voltage up.
 
You never answered if you have electric cooling fans.

Did the car ever run "correctly" if so what had been changed since then?

Like Mattox said, without knowing what the current is doing you don't know what is really going on. At idle you might me pulling from the battery because the alternator is not keeping up.



LivewireBlanco said:
The alternator is about a year old and is 70 amp I believe
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Post a photo of your alternator rear side view.
Did it do this before the alternator replacement? Reason I ask, many times the reman alternators have a larger diameter pulley and turn too slowly at idle.



LivewireBlanco said:
The voltage regulator is original
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How original is original?
Post a photo of it as well.
 
LivewireBlanco said:
So it's fairly normal to have this issue when it's hot and not when it's being driven and cool? Everything works fine and charges great until extreme stop and go diving. The alternator is about a year old and is 70 amp I believe. The voltage regulator is original.
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It's entirely possible it is normal, since a lot of times when engines heat up to about the max temp they run, they idle down a hair, causing the alternator to not put out its normal crappy output. Idle "AIN'T" the time Chrysler's stock charging system shines.....and I don't really know when is......lol........but that could be the issue. Where is your hot idle speed?
 
Also, some of the early alternators have that mafungo huge pulley and that slows alternator speed down even more at idle.
 
Dana67Dart said:
You never answered if you have electric cooling fans.

Did the car ever run "correctly" if so what had been changed since then?

Like Mattox said, without knowing what the current is doing you don't know what is really going on. At idle you might me pulling from the battery because the alternator is not keeping up.





Post a photo of your alternator rear side view.
Did it do this before the alternator replacement? Reason I ask, many times the reman alternators have a larger diameter pulley and turn too slowly at idle.





How original is original?
Post a photo of it as well.
Click to expand...

I'll post a photo when I get off work. I do not run electric fans because I know my stock style alternator couldn't handle it. The alternator is a auto parts store remanufactured stock style for 71 Dodge. It charges at 14 volts and I've confirmed it at the battery when the car is first started and idling. After driving for a long time in stop and go it only barely charges and sometimes doesn't charge but is discharging at idle. I bumped up the idle rpm to help compensate for this and putting in neutral usually brings it up to charge at about 12.8 volts. You can visibly see the volt meter jump from 12 to 12.8 which I know is low but at least not going to die.
 
RustyRatRod said:
It's entirely possible it is normal, since a lot of times when engines heat up to about the max temp they run, they idle down a hair, causing the alternator to not put out its normal crappy output. Idle "AIN'T" the time Chrysler's stock charging system shines.....and I don't really know when is......lol........but that could be the issue. Where is your hot idle speed?
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The pulley looks normal size to me but I could be mistaken. It's not abnormally large if that helps. Also, the rpms aren't dropping when it's hot they actually are going up a bit and the problem is even there when it's in park and idling higher than when in drive, just not as bad. For example normal charging is about 14 volts in park but when it's hot is only about 12.8 at the same rpm. Something is getting hot and reducing the charge or limiting the charge. I have never changed the voltage regulator and looks like it's from 1971.
 
Here's the thing. Depends. If you don't have EFI or some other critical electronics, your car should run off the battery for HOURS. When I first trunk mounted the battery in my 70RR, I did not realize it had been sulfated (discharged and sat at a dealer for a month). Coming back to San Diego from Vegas one weekend, "we had fumes." I knew it always held the needle a bit past zero. So we just unplugged the VR and drove it.........for HOURS. No radio, no blower/ heater, just the engine ignition and occasional brake lights.

Now of course this does not mean you don't have something wrong.
 
LivewireBlanco said:
The pulley looks normal size to me but I could be mistaken. It's not abnormally large if that helps. Also, the rpms aren't dropping when it's hot they actually are going up a bit and the problem is even there when it's in park and idling higher than when in drive, just not as bad. For example normal charging is about 14 volts in park but when it's hot is only about 12.8 at the same rpm. Something is getting hot and reducing the charge or limiting the charge. I have never changed the voltage regulator and looks like it's from 1971.
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Then I would check every connection to make sure everything is clean and tight. Beyond that, if you don't see anything obvious, the alternator or regulator could be suspect. Have you run a good load test on it? Like with an old style carbon pile tester?
 
RustyRatRod said:
Then I would check every connection to make sure everything is clean and tight. Beyond that, if you don't see anything obvious, the alternator or regulator could be suspect. Have you run a good load test on it? Like with an old style carbon pile tester?
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No, I don't have one.
RustyRatRod said:
Then I would check every connection to make sure everything is clean and tight. Beyond that, if you don't see anything obvious, the alternator or regulator could be suspect. Have you run a good load test on it? Like with an old style carbon pile tester?
Click to expand...
No, I don't have a load tester. Best I can do is get the battery or alternator checked at an auto parts store. At idle I know the alternator would fail when the car is hot but pass when it's cool. I guess I could get another alternator since it's under warranty but I don't know if that would fix it.
 
Ok, in doing some more digging isn't one function of the voltage regulator based on temperature? Doesn't it dial back charging voltage at higher temps and increase it at lower temps? Could it be that my regulator is getting too hot or dialing back too much when it sitting in stop and go traffic? This does only happen when the car is about 210ish and not when lower than that.
 
LivewireBlanco said:
No, I don't have a load tester. Best I can do is get the battery or alternator checked at an auto parts store. At idle I know the alternator would fail when the car is hot but pass when it's cool. I guess I could get another alternator since it's under warranty but I don't know if that would fix it.
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Mattax said:
When its hot, could check for the voltage at the field terminal with blue wire.
Could also full field the alternator to see if that brings the voltage up.
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How would I check the voltage at the field terminal? Just put my multimeter positive on the blue wire and negative on the engine block?
 
LivewireBlanco said:
How would I check the voltage at the field terminal? Just put my multimeter positive on the blue wire and negative on the engine block?
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Yes.
The backside of the connector is open. If the meter has thin probes the tip can be slipped in the back to touch the metal terminal.

For the full field test, remove the connector with the green wire and install a jumper from the terminal on the alt to a good ground.
 
Temporary jumper for full fielding as shown.
Not sure where things are tapped or connected for your relay so not showing that.
upload_2020-10-5_13-21-13.png
 
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