Occasionally my Bronco II's coolant temperature gauge would read high. I've changed the thermostat multiple times over the years and found it hard to believe that all these thermostats could go bad. After doing some research into the issue I read that the instrument cluster voltage regulator (ICVR) is an electromechanical device that sometimes goes bad and causes the gauges to misread. I checked the regulator with an oscilloscope tied between the coolant temperature sensor and ground and found that it generated a regular square wave with KOEO. On a road test, however, or even when lightly tapping the instrument panel, the voltage pattern went nuts. You can see this in the attached image. The first half shows the uniform regulator pattern and the second half shows what happens when I begin tapping the instrument panel. The same crazy behavior repeats on the road.

A note of caution here. Subjecting the oil pressure and coolant temperature sensors to constant 12V destroys them so you have to be careful. The idea behind the ICVR is that it generates a square wave at around 40% duty cycle, i.e., it's only on 40% of the time, thereby approximating a constant 5.5V, or thereabouts.

I removed the ICVR from the instrument panel and replaced it with a variable buck regulator (an LM2596, that costs about $1 from AliExpress), set to output 5.4V. I wired this up to 9V battery terminals so I could hook it up in place of the ICVR which uses the same terminals. After soldering on 3 wires (common ground, + voltage in, + voltage out), I placed the circuit board inside a small dental floss container and hot-glued the wires (not the circuit) to the container so it wouldn't rattle around. I ended up having to do this twice because the first time I tried this the regulator kept cutting out. I had hot-glued the back of the circuit to the dental floss container and maybe that was what made it misbehave.

The ground terminal screws to the back of the instrument panel in place of the ICVR and the other two terminals snap on to their mating connectors (be careful not to switch them when you're soldering the wires).

This works like a charm. The gauges read about where they used to read, but the coolant temperature is now much more stable. A bonus is that the ammeter, which hasn't worked at all over the past 35 years, has started working.


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