M38: Testing the charging system

1952 - 1968, M38A1, M170, questions, discussions, regarding anything related to the M38a1 series.
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thomasarnold
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M38: Testing the charging system

Post by thomasarnold » Tue Mar 27, 2007 7:00 am

Hello everybody.

Since I got the engine running on my M38 I wanted to check out the charging system (generator/regulator) before going any further in the restoration. Actually, I got tired sand blasting the tub and wanted to take a break...

Here's the setup: Both generator and regulator are Auto-lite. The regulator is the older 'bread box' version, not the flat electronic one. Battery voltage is 24.8 volts.

I followed Wes' instructions and built me a test harness using an old generator to regulator cable in order to be able to full field the generator and test it's output. The test results are:

- With the master switch ON and the engine not running I am getting 0 volts measuring from wire A+C on the cable against ground. So far so good.

- With the engine running I am getting more than battery voltage. Voltage increases when increasing engine rpm. According to Wes, this step of the test should give me at least battery voltage...

- Now the odd result of the test: With the engine running I connected a jumper wire from the battery (+) on the starter to the B wire (for less than 3 seconds) and I am getting a whopping 37 volts.

I repeated this test after opening the regulator and cleaning/filing the contacts but the results are the same. I am now wondering if:

(A) the generator is bad

(B) the regulator is bad

(C) the generator and the regulator are both bad

(D) the test results are normal or

(E) it was too early in the morning and I didn't have enough coffee yet.

Please let me know your opinion about this...

Thanks!
Thomas G. Arnold
Hampton, Georgia

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Post by Cuz » Tue Mar 27, 2007 7:30 am

When you full field test a generator you should release the hot wire as soon as the generator reaches it's rated output. They can and often will go higher but then you risk damaging the units. Generators can and will runaway all the way to melt down if you let them. Obviously your full fielded generator output is more than adequate. If there has been no damage to the system and the measured normal outputs per TM 9-1825B tests values are correct then you are good to go.
Cuz (AKA Wes K)
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Post by thomasarnold » Tue Mar 27, 2007 10:48 am

Well, the 37 volts did not gradually build up, my voltmeter showed this value the second I touched the B wire with the hot jumper wire coming from the (+) on the starter. I disconnected the jumper wire immediately.

I am just concerned that the charging system on my jeep is not working since my voltmeter only reads battery voltage when I connect one test probe to the (+) on the starter and the other probe to ground, regardless of engine running or not. Shouldn't the voltmeter read at least 3 volts higher when the engine is idling fast?
Thomas G. Arnold
Hampton, Georgia

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Post by Cuz » Tue Mar 27, 2007 9:00 pm

Your full fielded generator reading indicates there is nothing wrong with the generator. So your voltage regulator needs to be tested and adjusted. For this you should be on pages 181 thru 187 in TM 9-1825B dated 1952. Use the values in tables VIII and IX. You should have a volt meter reading with the system operating at 70 degrees of 28.30 +- 0.40 volts.
Cuz (AKA Wes K)
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Post by maeserik » Tue Mar 27, 2007 11:25 pm

28 volt is far enough for loading and good for the batteries.

more than 29 volt can make them boil and shortens their life!

you can adjust it on the middle contact in the regulator , the more tension by the spring the higher volt on the batteries

erik
1942 GPW 71336 - 1951 M38 MC11891

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Post by thomasarnold » Wed Mar 28, 2007 3:33 pm

Thanks. Let me go ahead and test the voltage regulator. All the info you provided here would explain why my jeep voltmeter is constantly in the yellow. I'm just glad it's not the generator.

Maeserik, you say
'you can adjust it on the middle contact in the regulator'
Wouldn't this change the setting of the current regulator instead of the voltage regulator. I'm looking at Figure 119 of TM9-1825B and it shows circuit breaker, current regulator and voltage regulator from left to right starting at the connector side... :?:
Thomas G. Arnold
Hampton, Georgia

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Post by maeserik » Thu Mar 29, 2007 12:01 am

you are right it is the voltage regulator on the right, the one with the small wirings, the middle is the current regulator

under the spring there is a nut you can turn to in- decrease the tension on the contacts more tension is higher voltage

Image

sorry wes i used one of your pics i thinck
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Post by Felix » Thu Mar 29, 2007 11:10 am

One suggestion:
If you are going to adjust the spring tension, which is an acceptable way for regulating charge, do it with the engine off and the battery ground disconnected.
Some persons do it with the engine running and battery connected so they can quickly verify the effect of the adjustment and re-adjust if necessary, but believe me, horrible things happens if you touch something that you should not have in the voltage regulator.
It may take a little longer but you’ll save yourself a headache.
I am stating the above based on my personal experience.
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Post by thomasarnold » Thu Mar 29, 2007 1:23 pm

Felix, I appreciate your advice and I will make sure to follow it! Do I have to full field the generator after every step during the voltage adjustment process in order to see if the 37 volts decrease or is there a different, maybe easier way? Thankful for every suggestion! :)
Thomas G. Arnold
Hampton, Georgia

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Post by Cuz » Thu Mar 29, 2007 8:12 pm

Just forget full fielding. It is a one time check to ensure that the generator is capable of making full rated output. The final number realized during the check is a product of RPM and electrical efficiency. We already know the generator is good to go. This has nothing to do with the proper setup of the voltage regulator assembly IAW TM 9-1825B. Just follow the detailed adjustment steps in the TM for the regulator.
Cuz (AKA Wes K)
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Post by thomasarnold » Fri Mar 30, 2007 7:30 am

Thanks for all the tips. I will be visiting the 7th Army Training Command in Grafenwoehr Germany for the next three weeks. The results of the regulator adjustments will be posted as soon as I'll be back and get a chance to mess with it...

Regards,
Thomas G. Arnold
Hampton, Georgia

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Post by thomasarnold » Tue Apr 24, 2007 11:01 am

Just wanted to thank everybody for the information you provided me. I finished adjusting the voltage regulator and have now a steady 27.90 volt output. Ammeter needle is now on the charging side.

Regards,
Thomas G. Arnold
Hampton, Georgia

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Post by maeserik » Tue Apr 24, 2007 12:30 pm

That is the meaning of this forum !

I also have received many good info in the past from other jeeplovers.

Erik
1942 GPW 71336 - 1951 M38 MC11891

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Post by Cuz » Wed Jun 06, 2007 9:08 am

That is the meaning of all the helpful M series Willys Jeep forums.

The G503.

the http://www.willysmjeeps.com/v2/modules.php?name=Forums

the http://autos.groups.yahoo.com/group/M38-M38A1-M606/


and many others.

The files source for the bulk of the troubleshooting data presented here is
http://www.willysmjeeps.com/v2/modules.php?name=Forums
and
http://autos.groups.yahoo.com/group/M38-M38A1-M606/
Cuz (AKA Wes K)
45 MB, 51 M38, 54 M37, 6? M101A1, 60 CJ5, 76 DJ5D & 47 T3-C

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Post by petesilfven » Wed Jun 06, 2007 3:03 pm

The voltage adjustment should be done with a fully charged battery because the voltage regulator doesn't operate until the battery is charged enough to bring the voltage up to the point at which the regulator is set. It helps to have a voltmeter that can be read while driving so one can monitor voltage and make fine adjustments.

When a regulator is adjusted on the bench, a resistor is used to simulate a fully charged battery. This provides a stable reference for adjustment.
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