Voltage gauge reading low

[Michael]

I may take a Pic. and tape it over my gauge. Never had over 13.

What is weird is I have had three brand new alternators over the years (DMCH, SpecialT and aftermarket and not one of them showed the voltage at idle this locally rebuilt DMCH unit has.

I would suspect the old pigtail was suspect but I had not yet replaced it when I took that pic.

I don't know what the odds are of me getting 3 crappy nators in a row are but I'm leaving well enough alone. I am going by the parts store today and get some asbestos wrap for the wiring near the header.

[Bitsyncmaster]

What is weird is I have had three brand new alternators over the years (DMCH, SpecialT and aftermarket and not one of them showed the voltage at idle this locally rebuilt DMCH unit has.

I would suspect the old pigtail was suspect but I had not yet replaced it when I took that pic.

I don't know what the odds are of me getting 3 crappy nators in a row are but I'm leaving well enough alone. I am going by the parts store today and get some asbestos wrap for the wiring near the header.

Alternators should be putting out 14.5 to 14.7 volts. In cold temps the voltage should be higher until your engine warms it up. If you ground your fuel pump or inertia switch you should read about 0.30 volts higher on the gauge because it reduces the ground drop on the gauges. My DMCH alternator shows about 14.8 volts on the gauge.

[Michael]

Alternators should be putting out 14.5 to 14.7 volts. In cold temps the voltage should be higher until your engine warms it up. If you ground your fuel pump or inertia switch you should read about 0.30 volts higher on the gauge because it reduces the ground drop on the gauges. My DMCH alternator shows about 14.8 volts on the gauge.

Even ice cold and with a new batt/alternator, my gauge has never read as high as it does now. At best it would read 13 dead cold then gradually drop to around 12.5 to 12.8 and only hit 13 even when the revs are up. Whatever I have done, I like it. I have a theory that most people who sell alternators are either sourcing them cheap or having them built with cheap components.

If anyone is suffering similar problems you may want to try and have your present alternator rebuilt using quality higher end supplies. I told these guys to do whatever they needed to make it as good as they could and I was ok with any additional expense. I don't know if they actually had "a cheap parts pile and a expensive parts stash" but I am pretty happy with this thing.

[Michael]

Here is a screen grab from a cold start just a few hundred miles ago. Never got above 13v even ice cold and nothing on.(it only went lower when warm).

Screenshot_20191222-160919_YouTube.jpg

And compared with yesterday's cold start and rebuilt alternator.

20191221_124404.jpg

[Bitsyncmaster]

Here is a screen grab from a cold start just a few hundred miles ago. Never got above 13v even ice cold and nothing on.(it only went lower when warm).

Screenshot_20191222-160919_YouTube.jpg

And compared with yesterday's cold start and rebuilt alternator.

20191221_124404.jpg

I will run a test (I may have done this before) of what the linearity of the gauge is. I know I run at almost that 3/4 mark which I assumed was 15 volts.

[Michael]

I will run a test (I may have done this before) of what the linearity of the gauge is. I know I run at almost that 3/4 mark which I assumed was 15 volts.

If the markings are true, the upper mark should be 15.5v

[Bitsyncmaster]

If the markings are true, the upper mark should be 15.5v

This is an NOS gauge.

8 volt mark = 9.17 volts at 37ma.
1/4 mark = 11.15 volts at 47 ma.
13 volt mark = 13.30 volts at 55ma.
3/4 mark = 15.8 volts at 62 ma.
18 volt mark = 22.4 volts at 86 ma.

So the readings around the 13 volt mark are not to bad but elsewhere it's junk. I know the gauge in my car must have different calibration since I see it run pretty close to the 3/4 mark and my voltage is about 14.7 volts when running.

Note this gauge has a light blue calibration resistor on it.

edit:
I just thought of something. I was testing with the gauge laying flat on my table. Now testing with it vertical like in the car.

[Bitsyncmaster]

Test #2 with gauge vertical and light tapping on gauge after voltage change.

8 volt mark = 9.02 volts
1/4 mark = 10.82 volts
13 volt mark = 12.90 volts
3/4 mark = 15.62 volts
19 volt mark = 21.9 volts

So either the gauge is sensitive to being vertical or my test is not repeatable.

[Michael]

Wow, seems as long as you are within the acceptable range of a battery's voltage, the gauge is actually pretty accurate. (well yours is anyway)

[Michael]

Cleaned the main ground on the frame in the engine cradle. It didn't look bad and it barely gets wet but I decided to check it anyway. It had a thin dark gray layer of corrosion/sediment on the frame and fitting. I cleaned it with sandpaper and got another .5-.7v on my gauge.

Going to clean my fan grounds next time she goes in the air. It's been a minute since I have cleaned any of them.