Joined: Oct 06, 2014 Posts: 252 Location: South Dakota - Aberdeen
Posted: Mon May 02, 2016 1:07 pm Post subject: volt versus amp gauge
Simple question I hope! This gauge came with some others that I bought to replace my missing gauges. Is it an amp gauge or a volt gauge? How can you tell?
The wiring harness I bought came with a 4 wire spider and the cluster this gauge came off had a 5 wire spider.
Easy way to tell is a voltmeter reads voltage so is only hooked up to the harness with one wire, usually through that extra wire on the five wire spider.
An ammeter measures load, so has to be in the circuit, so has two wires to the harness, 8 and 9.
Without a complete circuit not much happens on either an amp meter or a volt meter. The volt meter's case is grounded to complete the volt meters circuit. The extra stud is often used as a ground when too much paint exists on both the gauge housing and the instrument cluster. Most of the M170's and very late M38A1's came with the voltmeter. Earlier jeeps when upgraded to the GI 60 Amp alternator kit or the transistorized 25A volt reg used the voltmeter. Since you only want the volt meter to draw juice when the jeep is powered up they are connected to an extra spider harness leg. When the volt meter is installed the old amp meter wires # 8 & 9 are not used. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Feb 20, 2007 Posts: 606 Location: Illinois
Posted: Tue May 03, 2016 6:30 am Post subject:
I replaced the amp gauge in my M38 with a volt gauge. The old amp gauge was faulty - the needle was always pointing to a slight charge condition, which is good normally, but not when the charging system was not working! Which is what I found out the hard way. The volt meter, for me, gives a better visual impression of charging system condition. The military switched to volt meters, I believe, for later models of the M38A1 and other vehicles of the era for era for simplicity. The electronic voltage regulator does not have the circuit for the amp meter. As mentioned you only need the 5 wire spider harness to convert or, if you have a direct reading oil pressure gauge, the 4 wire harness works. Options! _________________ 1951 M38
Joined: Oct 06, 2014 Posts: 252 Location: South Dakota - Aberdeen
Posted: Tue May 03, 2016 10:23 am Post subject:
I am converting back to 24 volts and am using all 24 volt components. i.e. generator, distributor, regulator, etc. These components are not original to the jeep but were later added or changed out in the jeeps life.
I guess since I have this volt gauge, I would like to use it. If I have vintage wiring swap out the 4 wire spider for the 5, will this allow me to use the volt gauge? _________________ Aaron
1950 M38 - #MC11328, 24volt, 1948 CJ2A (Lefty)
Just about all the military surplus dealers carry some of it if not all. You can save a few bucks when you are not in a hurry by shopping the MV shows and flea markets and EBAY.
Joined: Feb 20, 2007 Posts: 606 Location: Illinois
Posted: Wed May 04, 2016 7:46 am Post subject:
Wiring the gauge panel is pretty simple. Using the multi-color wiring diagram in the "Downloads" section of this website, you can see the very light blue lines represent a 4 wire spider harness. The amp meter is connected to #8 and #9 wires while the other 3 gauges are connected to the common spider, #27, the 4th wire is for incoming power from the harness. To use the voltmeter instead of the amp you need a 5 wire spider - 4 wires for the gauges plus the one lead-in wire. Or, as I indicated, I have a direct read oil pressure gauge in my M38A1 - no wire needed for that gauge so a 4 wire spider works there. For terminals you can also check Ebay. The metal shells are pretty durable but the plastic barrels and rubber grommets are the parts that you have to find in new condition as NOS or take-outs tend to be deteriorated.
Wiring diagram: http://willysmjeeps.com/downloads/M38wirecolored.jpg _________________ 1951 M38
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