Posted: Sun Jun 07, 2015 7:18 pm Post subject: M38A1 charging problem
I am working on a 1953 M38a1 which hadn't run for a while and the owner was not mechanically inclined so depended on local mechanics. It has a voltmeter. At low idle the voltage range is fine but when rev's go up it goes above the red zone and the fan belts squeal. The batteries are in good shape.. I changed the voltage regulator: same problem: very high output as rev's increase. Is it possible for a faulty generator to overcharge even if the regulator is otherwise OK. I was curious how the high voltage was getting but my digital voltmeter went very erratic with readings all over the map. The only history I have on this jeep is that at one time the batteries had been hooked up backwards for a while and it appeared to function with this reversed polarity. The battery situation was corrected a while ago is it possible that the generator suffered some damage from that incidence of of reversed polarity? What are the symptoms of a charging system when the generator is not properly polarize do?
Assuming stock 24 volt 25 amp DC system, Starting with the easy items:
1-Squealing fan belt usually means loose fan belt. Check tension first. Intermittent slippage of the pulley with loose belts will result in erradic readings. If belt squeeling steel occurs at higher RPM's check gen front bearing and water pump bushing for free and smooth ops.
2-Disconnect the gen to reg cable and full field test the gen. If she puts out 28 to 30 volts the gen is ok.
When the need for polarizing is ignored the system will simply not charge. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Still a mystery. Tried some other used breadbox voltage regulators and replaced the generator with a rebuilt one. My other used voltage regulators all had problems.
So I installed a new solid state regulator along with the rebuilt generator: the dash voltmeter is stable and in the green. But my digital voltmeters (which work fine to test the running voltage in my M38) bounce all over the place when the M38A1 is running (the dash voltmeter and an analog voltage gauge are stable at about 27 volts.).
My mystery now is why my digital voltmeters bounce all over the place while the jeep is running. The digits change very quickly with no sense of the voltage reading.
Posted: Tue Jun 09, 2015 3:52 pm Post subject: solved
Original problem: over-charging a lot as M38A1 engine rev'd up. Tried to monitor the voltage with a digital multimeter but it had extremely erratic readings.
Replaced voltage regulator with two other used ones: same problem. Thought generator might be the problem so installed a rebuilt one. Same problems. Went through all wiring to be sure there were no shorts etc: all OK. Took a chance and installed a NOS solid state regulator (I had been hesitant since I didn't want to fry it due to some other electrical issue.). Charging problem seemed solved, with the voltage gauge on dash reading perfectly in the green. But I still wanted to monitor the actual battery voltage and the digital multimeter was still giving wild readings.
Here's what I forgot to mention: the spark plug cables are not original shielded waterproof cables but regular cables adapted to the waterproof distributor cap and going to commercial spark plugs. A quick Google of "erratic digital multimeter readings" led to an old tractor forum: Digital multimeters do not like the radio interference from ignitions systems: it drives them crazy. Now a stock M38 or M38A1 has well-shielded spark plug cables so that is why my digital multimeter worked fine on my stock M38 but not at all on the M38A1 with commercial spark wires. Rely on an old-fashioned analog voltmeter when working around unshielded ignition wires.
That's my story and I'm sticking to it!
A lot depends on the quality & / or the sensitivity of the Digital VOM. I use a couple of Flukes and have never had an issue with spark interference.
I often see civvy adaptions on spark plugs and wires that result in a rather high resistance due to excessive gaps in the military distributor cap towers that results in a lot of unwanted signal interference. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
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