Overheating or Undervolting?

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Jul 19, 2018
Kodiak, AK
This weekend’s efforts to improve the FJ ended in a bit of a stalemate. Dual battery charging system from Blue Sea is mocked up and ready for install, new radiator fan dropped in, and some other small gremlins wrangled. But the heat remains.

My 12/78 FJ40 has run a little hot (200 at the thermostat) since I purchased it about a year ago. After dropping a new 185 degree thermostat in, it’s kept that a clean 185 degrees ever since, when measured at the top of the thermostat housing.

However, the aftermarket IT gauge puts it at 210+ while running, with big jumps anywhere from 230-260 every time the fan or headlights come on). Gauge voltage from the CS130 alternator fluctuates accordingly, normally reading 14.7V but dipping as low as 12 when any increase draw in draw activates. When I turn the rig off and it’s pulling right from the battery, the temperature reads a cool 185, then creeps up slowly as you’d expect without water circulation.

Thinking it was a dirty/corroded ground, I jumped a line from the battery negative to the ground of the gauge with no change. I then thought the wire gage might be insufficient, and jumped a line from the battery direct to the positive gauge terminal, with no change. I suspected a loose alternator belt, but tightening it did nothing to improve the symptoms either.

I burped the cooling system with no improvement, then cracked open the coolant drain plug at the back of the 6th cylinder to make sure flow was getting back there and it immediately dripped out a steady flow of coolant, so I’m not thinking there’s any major obstruction. That said, while burping, the funnel at the radiator fill cap was somewhat rusty and murky after circulating coolant for 20 minutes...probably due for a flush regardless.

I’m thinking this is an instrumentation error and not the engine actually running hot, but want to be sure.

Is there something I’m missing to diagnose or fix this latest problem?
It is a lot to actually figure out your system without deep details. One thing is get a IR temperature gun and compare your instruments vs. reality.
Yeah, I suppose there’s enough changes to all these rigs to make electrical troubleshooting difficult without being there.

I guess my question is, how much voltage variation is “normal” when relatively modest draws activate?

My alternator is putting out 14.7V and capable of putting out 100+ amps, but comes down to just over 12V at the gauge when a humble 9A radiator fan kicks on. Is there a way to diagnose a bad voltage regulator? The rig’s wiring is a Kwik Wire kit without any abnormal electrical power draws, aside from the little radiator fan.

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