M38 Voltage regulator

horse · Post by **horse** » Fri Feb 14, 2020 1:07 pm

Hi first post, Jeeps new to me so steady chaps. I have a electronic voltage regulator which produces 29.5v little more than standard 28.5V. at the batteries, I noticed when I had the top off the regulator there are two pots and was thinking they could be adjusted.
If they could, do you know which is the pot to adjust for the correct voltage?

Thanks.

Jonathan[/img]

Post by **wesk** » Fri Feb 14, 2020 9:03 pm

The old mechanical regulators are adjustable. The newer transistorized regulators may be but we have no technical support pubs to tell us where and how. The Army bought them as throw away units. There are no support manuals for them and no internal wiring diagrams.

To give you an idea of how complicated a few simple adjustments on these solid state regulators can be here's a link to the 100 amp regulators service manual.

https://www.liberatedmanuals.com/TB-9-2920-225-34-1.pdf

It is not uncommon for a test and adjustment procedure to require the temporary removal or bypassing of some of it's electrical components in the regulator when making adjustments.

The bulk of the motor pool technicians I have spoken to over the years indicated they were usually just tossed in the trash and a new one installed.

Post by **wesk** » Sat Feb 15, 2020 12:40 am

I opened an album for you to help with your photo posting.

Here's your album:
http://www.willysmjeeps.com/v2/modules. ... _album.php

Here's the rules:
http://www.willysmjeeps.com/v2/modules. ... opic&t=498

Here's the tutorial for loading your album:
http://www.willysmjeeps.com/v2/modules. ... _album.php

Here's the tutorial for posting your photos:
http://www.willysmjeeps.com/v2/modules. ... pic&t=8670

horse · Post by **horse** » Sat Feb 15, 2020 12:42 pm

Thank you very much Wes for the reply and opening the album. Looks like I should just get a new unit.

Post by **wesk** » Sun Feb 16, 2020 12:08 am

Anytime a solid state regulator fails you should always bench check the generator and inspect all associated wiring before installing a new replacement regulator.

In case where the amp meter has been replaced with a voltmeter to read the solid state regulator output check disposition of wires carefully.

horse · Post by **horse** » Sun Feb 16, 2020 1:50 am

Thanks for the tip, more research.
I do not have any problems like boiling batteries or blown head lights but the volt meter does read an extra volt when under load and the gauge is over to the right all the time.
I am being fussy?

horse · Post by **horse** » Sun Feb 16, 2020 7:12 am

Hi back again, I found a section on testing the generator in place.
He suggested using the volt meter to the battery positive 24V cable that connects to the starter switch and nearby ground.

At idle I got 60 volts not the 23 to 25v suggested.
So would that mean the generator is faulty or am I taking an incorrect reading?

As a side issue I have been getting the vent lines working to give the distributor a nice air flow which is now working but noticed the breathers on the axel are not taken up and above the water line, so did the military allow water into the axels when fording?

Any help would be appreciated.

Thanks Jonathan

Post by **wesk** » Sun Feb 16, 2020 8:39 am

I found a section on testing the generator in place.
He suggested using the volt meter to the battery positive 24V cable that connects to the starter switch and nearby ground.

To properly test a generator in place it must be connected to a working regulator. There is a "Full Field Test" we perform to determine the generator's capability but that is a momentary procedure performed quickly by momentarily applying full battery voltage to the field terminal on the generator. This will yield the generator's maximum output voltage which in this case is 28.5 to 32.0 V's.

When you install a new voltage regulator you should properly bench test the generator to insure it will not damage the new regulator. To do this properly the generator must removed and placed on a test bench per TM 9-1825B Dated Nov 1952 starting on page 101 Section VI, Group 4 thru page 108 of the testing values charts. These same principals apply when you install a new generator, the original regulator must be bench checked per TM 9-1825B dated Nov 1952 page 168 Section V Group 4 page 204 of the test value charts.

These chapters of TM 9-1825B that must be used are our "Downloads Section" and are free in PDF format.

At idle I got 60 volts not the 23 to 25v suggested.
So would that mean the generator is faulty or am I taking an incorrect reading?

Extremely high and uncommon. I would first check the settings on your voltmeter IE AC vs DC and then the range in use. + or red cable to the large battery terminal on the starter and - or black cable to nearest good, unpainted, non-rusted ground.

noticed the breathers on the axel are not taken up and above the water line, so did the military allow water into the axels when fording?

Surprisingly the Willys folks on the prototype M38's had the axle vents plumbed to the air cleaner. They for some reason abandoned that idea on the production M38.

horse · Post by **horse** » Sun Feb 16, 2020 11:06 am

Again thank you,I will check all that you said and download the section required. If it comes to it I will remove both and try and find a place in the U.K who knows about these things, but not given up yet. Jonathan

horse · Post by **horse** » Sat Feb 22, 2020 8:46 am

The weekend, so out with the Volt meter again.

This time I get 27.9V at 46 degrees F, 8 Degrees C, off the generator measured from the cable that connects to the starter switch and a ground.

This seems okay by one standard answer but a little low on the answer from Wesk, max output 28.5 to 32V.

I am unable to do a full field test.

What do you think?

Thanks.

Jonathan

mdainsd · Post by **mdainsd** » Sat Feb 22, 2020 9:20 am

Is that number with the engine at idle? If so re-measure with the engine speed up off of idle (around 1200 rpm). Generator systems are not as effective at low speeds.

What I like to do is turn the headlights on, and select high beams. Bring the revs up off of idle and check that voltage again. Whats that number?

horse · Post by **horse** » Sat Feb 22, 2020 10:23 am

Hi.

That is the engine at revs not idle.

I will try tomorrow with the head lights on.

Thanks for the input.

Post by **wesk** » Sat Feb 22, 2020 11:42 am

27.5 to 32.0V is an absolute high RANGE not a specific single number limit. Your 27.9 falls in that range so that is good. The 1200 RPM range mentioned above is the "Come In " range for DC Generators. You actually want your RPM up at 2000 RPM similar to 30 MPH in 3rd gear.

Why can't you do a "Full Field" check of the generator. You clearly have the needed tools. The simplist way is to disconnect the long cable assembly between the voltage regulator and the generator. Then I usually attach a jumper wire to the generator case and then to a good ground. Then you connect another jumper lead to pin "B" on the generators plug adapter. Then hook up your test meter tp the A terminal and a good ground. Then start the engine. Now while watching your volt meter touch the jumper wire from Pin "B" on the generator to the battery cable on the starter. Note your peak voltage and quickly remove the jumper wire from pin "B".

horse · Post by **horse** » Sat Feb 22, 2020 12:50 pm

Thank you, I will try and follow that excellent explanation.

horse · Post by **horse** » Sun Feb 23, 2020 5:37 am

Okay done the easy test again with the lights on, reading just the same but as Wesk said in range.

So to the stupid questions.

I put an extra ground from the generator to an earth.

Put another jumper lead to pin B on the generators plug, ready for the test.

Meter onto terminal A , which one is that on the picture?

Thanks

P.S.If I am not completely happy I will not carry out the test.