m37 voltmeter

[nutsfor59]

hi,my trusted friend says that the voltmeter gauge is supposed to read in the yellow with the engine off/not running but my gauge goes all the way left or bottoms out when i turn the switch off and goes to yellow when i turn the switch on,what is the correct way ??? looking for a couple opinions,thanks

[nutsfor59]

sorry all i guess i asked this once before but my friend has never steered me wrong before,i have now a completely NOS system,regulator/rectifier and generator and still no reading in the green on the voltmeter,getting 24.9 or so at the batteries while running,guessing my generator even though it's NOS,anyone have ideas now ??? thanks

[cuz]

Your friend must have his volt meter connected to a terminal that is always hot. That's a no-no. But that would make it read low volts in the yellow when his engine is not running. It should be connected to circuit that is only hot when the master switch is turned on. Then when you turn the master switch off the gage drops to zero where it belongs.

still no reading in the green on the voltmeter,getting 24.9 or so at the batteries while running

Does this figure 24.9 come from interpolating the yellow green dash volt gage or using a good quality hand held VOM?

[nutsfor59]

hi,thanks for the input,my reading was with a hand held voltmeter across the batteries,whats on your mind about this ?? thats also while running

[Glenn]

I believe you should be reading 28 volts when running, that should put you in the green. My son and I adjusted the regulator on mine, going by an electrical manual I have. Do you have the old style regulator?

[cuz]

Nuts for,

I wanted to be sure you had an accurate reading. I don't know where you are in the troubleshooting guide pages I sent you so I can't offer any help. I would suggest that you carefully go thru the guide one step at a time in sequence and record your results for each test.




Hello Glenn,

He has the 100 amp alternator system.

[nutsfor59]

hi all,thanks for the responses i have a NOS transistorized regulator i just got from midwest military,it does have the adjusting plug on the top and yes i tried moving that and got no response on the hand held voltmeter that i had across the batteries,still more than willing to listen to all ideas and thoughts,my whole charging system is nos and i'm really bummed now that this crap is still giving me grief !!!!

[Joe]

Adjusting the regulator shouldn't produce an instant change on your voltmeter! Your voltmeter reads the battery voltage and it (the battery) takes time to charge and discharge even with different regulator settings. IF you had your regulator and generator on a test stand with no battery then you could get instant results but not in the vehicle with the battery attached. In the test stand, the battery is replaced with a huge resistor bank and it responds instantly to changes in the regulator settings.

Your regulator settings primarily controls the peak (fully charged) battery voltage. Internally you can also set the maximum charge (current) rate, the (low) voltage at which it starts to charge the batteries, etc.

Also a full 28 volts on the batteries is much too high! 28 volts in the nominal vehicle voltage (just like certain boards are called 2" x 4" but none of them actually are!) I don't have the manual in front of me but IIRC the peak battery voltage is about 26.5 volts in the summer and 26.2 volts in the winter. Yes, it's supposed to be readjusted seasonally. If you can't find the proper setting let me know what model regulator you have and I'll look it up.

FWIW this is covered in detail in TM9-1825B, 'Electrical Equipment (Autolite)'. It's a great book to have for understanding the electrical devices in the M-37s!

Cuz is right, your dashboard voltmeter should drop to the LH end of the scale when the vehicle is turned off. Not just to the 0 volts but completely as far to the left as the needle will go since the meter shouldn't be getting any power at all with the vehicle turned off. As for it not reading higher when you adjusted the regulator, how long did you wait to see results? You shouldn't see any change instantly but if you let the engine run you should see the meter slowly change as the battery charges or discharges according to how you adjusted the regulator. How long it will take to see a change and how long it takes for it finally stabilize is difficult to say. It depends on your battery capacity and condition, the battery temperatures and a lot of other factors but typically it takes 12+ hours to fully charge auto batteries. Not that I suggest leaving it running that long! I would hook up a good 24V battery charger (or two 12v battery chargers) and fully charge the batteries and then see what the dashboard voltmeter show and also check it with an external voltmeter. You may well find that one of your batteries has a shorted cell and that your batteries will never come up to the rated voltage! Once you know that you're dealing with good batteries and good gauges then tackle the charging circuit but I suspect that it's probably working (but may need to be re-adjusted) since you've already replaced everything.

[nutsfor59]

hi,thanks to all of you for easy to understand explanations of this,i'll let you know results asap,i plan on working on the truck this weekend for several hours,thanks !!

[nutsfor59]

hi again,forgot to ask is there a place i can download this TM-1825B MANUAL ?? THANKS

[cuz]

TM 9-1825B deals with repair and overhaul on the bench of the applicable Auto-Lite electrical accessories. It will not help you much with on the vehicle troubleshooting and adjustments. Also it only covers early 25 and 50 amp 28 volt systems. I do not know of any web sites offering a free download of the manual. It sells about $60 at Portrayal press. There are separate 1970's vintage manuals for that specific alternator, regulator and rectifier but again these are upper echelon depot type bench rebuilding manuals. The applicable M37 manuals give you adequate guidance on upkeep of the original 24 volt/ 25 amp electrical system and it's components. The guide I sent you will cover the 100 amp system just fine.

Let me clarify that max battery voltage mentioned above is with the battery fully charged and setting with no alternator charging it. With your 100 amp alternator system running the voltmeter should read at least 27.0 volts and preferred setting is 27.5 volts and the max setting is 28.0 volts.

100 Amp alternator system installation and troubleshooting guide

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[hbb]

If i have missed it in the replies I apologize but if not the battery's should be load tested to make sure they have the same voltage drop when loaded or you will have the potential for charging voltage inconstancy's. the basics are the best place to start Your charging system can only be as good as the storage container.

hb