When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Just got back from a road trip in the ‘79. About 30 miles from home on the way back, the voltmeter started reading erratically, bouncing around 15+ volts. Put a voltmeter on it and it reads about 15 volts. Changed the voltage regulator for a good used one. Voltmeter has settled down, but it still reads a little high. Put the digital meter on it and it reads a steady 14.9 volts. Still seems high. Is there another component that could be causing an over voltage condition?
Check the water levels in the battery if you can.
Also do a load test of battery.
You don't want it to boil dry if it is overcharging
Service Manual says between 12.5 - 15.5 your good.
Page from 77 Service Manual
Just did a load test on my 5 year old battery a few weeks ago.
Car Running at normal operating temperature.
You can see No Load I'm at 14.41 charge rate.
And with Headlight On High Beam and fan running on High Speed my Loaded is 13.65 charge rate.
Last edited by bmotojoe; Jul 19, 2022 at 07:54 AM.
13.6 Is showing that your Alternator is on the edge of your needs.
Although that is really how a new car was in 79.
Your used regulator is kicking out at 14.9
Yes that's a touch on the high side. But still in spec. Just
No. nothing else, battery water levels if you actually have such an ancient battery have nothing whatsoever to do with Alternator output.
The voltage regulator does that. Nothing else.
It's called a "Voltage Regulator " for a reason.
Perhaps try another regulator from a reputable firm like Tuff Stuff.
And Interpon, At 15 plus. I would repair that.
Yeah 15 plus is not ideal... I believe I read anything over 14.4 can be detrimental to battery life and some modern 12v electronics one might run in or off the car but its been a while and I dont remember specifics. seems many here have no issues with 15volts though.
Check what the alternator is seeing by checking the voltage sensing wire and alternator case ground. 43 year old wiring is going to have so voltage drop.
Check what the alternator is seeing by checking the voltage sensing wire and alternator case ground. 43 year old wiring is going to have so voltage drop.
So what is the relationship between field voltage and alternator output. If my field voltage drops, would that increase alternator output voltage?
So what is the relationship between field voltage and alternator output. If my field voltage drops, would that increase alternator output voltage?
As the field voltage increases so does the alternator output. In a internal voltage regulator alternator there is no practical way to check field voltage as there no field wire outside of a internal voltage regulator type of alternator. Only the voltage sensing lead and excite circuit. How an alternator works is the voltage regulator switches the field current on and off very quickly. The field current from the spinning rotor induces current into the stator. Let's say the targeted voltage is 14.5, when the systems voltage drops below 14.5 it turns on the current to the field and when the voltage goes above 14.5 it turns off the current to the field. So basically what I'm saying is if there's any voltage drop to the voltage regulator current to the field will be turned off at a higher voltage and cause overcharging.
Skids,
Is this a good way to test the complete circuit charging path?
Get a benchmark.
1. With engine running N.O.T. ALL accessories OFF take a volt reading at the car battery, record.
2. Car still running, Take a reading at terminal 1 on alternator Sensing (Brown Wire/GEN Light) to alternator case ground, record.
3. Car still running, Take a reading at the BAT terminal to case ground, record.
These should be +-10% of each other.
Circuit:
Skids,
Is this a good way to test the complete circuit charging path?
Get a benchmark.
1. With engine running N.O.T. ALL accessories OFF take a volt reading at the car battery, record.
2. Car still running, Take a reading at terminal 1 on alternator Sensing (Brown Wire/GEN Light) to alternator case ground, record.
3. Car still running, Take a reading at the BAT terminal to case ground, record.
These should be +-10% of each other.
Circuit:
bmotojoe, a couple of things. The voltage sensing is terminal 2, red wire and 10% difference is a lot as that would work out to 1.45 volts. 0.3 of a volt is considered the maximum voltage drop in an automotive circuit. Otherwise that would work. Also what is N.O.T. ?
bmotojoe, a couple of things. The voltage sensing is terminal 2, red wire and 10% difference is a lot as that would work out to 1.45 volts. 0.3 of a volt is considered the maximum voltage drop in an automotive circuit. Otherwise that would work. Also what is N.O.T. ?
OK. Maybe I’m starting to get it. The voltage I need to test is the voltage sensing connection (voltage regulator terminal 2, red wire), not the actual field voltage. If this is low due to resistance in the circuit that will cause the alternator to overcharge?
If this is low due to resistance in the circuit that will cause the alternator to overcharge?
Correct. Don't forget the ground too. If the ground has resistance the voltage regulator isn't going to see the correct voltage ether. There are many ways to perform voltage drop tests. Besides the method Bmotojoe described YouTube also has videos.
What does bouncing around look like? And did replacing the voltage regulator fix it? What was voltage prior to any issues?
i just don’t see The 15 volts as an issue…
how is your tachometer?
How is the voltage actually being measured? decent meter? https://www.mercurynews.com/2007/03/...s-telling-you/
As someone whos played quite a bit with different charging voltages and technics with lead acid batteries in my solar RV system I can confidently say that 14.8+ does boil the acid in the batteries quite a bit more than 14.1v causing more off gasing and then requires more maintenance to keep the plates submerged and happy. If your just short tripping the car it wont be as big of an issue as if you tend to take long drives.
Just one more thing to check. The main ground for the Alternator is through the engine block. A number of years ago I was experiencing strange charging behaviour as well.
I found the ground cable from the block to the frame. (near the starter to the idler arm) Had grown green inside under the insulation. I replaced that ground cable with a nice heavy one I got at a farm implement. Problem solved.
Alternator advice
