Gauge wiring.

[Gardy90]

Hello all. Have a few questions on the wiring. I’m am getting ready to pull my front harness and rebuild it, I’m doing so I’ve been going through and labeling everything since most of the metal tags are either gone or not readable. However the wiring to the voltmeter does not align with either schematic I have for the truck. On the early schematic it shows wires 8 and 9 going from the regulator to the voltmeter. On the late model schematic it shows a single wire coming from the spider harness. My truck seems to be different however, I have one wire going to the voltmeter going into the main harness. I have yet to find where this goes by tracing with a DMM. Does anyone have any insight on this?

 

[John Mc]

The earlier M37 trucks had an ammeter (not a voltmeter) which was fed from wires 8 & 9 from the regulator. Later trucks (M37B1) had a voltmeter, which was fed from wire 27 coming from the ignition switch - this fed the spider which then fed each of the instruments (except for the speedometer). If you actually have a voltmeter in there, there should be only one wire, since it's grounded through the case. It appears you have two wires. Maybe someone added a separate ground? Not sure what else that wire would be

 

[NDT]

Sometimes the ammeter was replaced with a voltmeter since the ammeter was phased out early on and the voltmeter remained throughout the truck’s service life. If this was done, one of the ammeter wires was used to feed the voltmeter and the other cut off and taped up. If your mystery wire ends up at the voltage regulator cannon plug, that is what happened.

 

[Gardy90]

The earlier M37 trucks had an ammeter (not a voltmeter) which was fed from wires 8 & 9 from the regulator. Later trucks (M37B1) had a voltmeter, which was fed from wire 27 coming from the ignition switch - this fed the spider which then fed each of the instruments (except for the speedometer). If you actually have a voltmeter in there, there should be only one wire, since it's grounded through the case. It appears you have two wires. Maybe someone added a separate ground? Not sure what else that wire would be

There is only one wire. Here is a better picture. That may explain it then, since this was converted to a B1 at some point. (Found bondo over the mount holes for the side mounted spare)
So say it was you rebuilding this harness 100%, go with a 5 wire or keep the single wire from the regulator.

 

[Gardy90]