Assuming Jerry has solved the first part of his problem -- finding the little bugger (i.e., coolant temp sensor), let's try to solve the second part -- getting the temp gauge working.
Instead of just addressing his problem, I want to take a more general approach on the assumption other people will run into similar problems.
This will require several posts, as I take time to gather information and write the posts while taking breaks from other stuff. (Such as getting things ready for winter ... which will be here in a few weeks or a month.)
Most of our older motorhomes have four gauges: Fuel Level, Oil Pressure, (Engine) Coolant Temperature, and Electrical Ammeter. The first three operate in a similar fashion and the same basic problems and troubleshooting techniques apply to all three. The fourth (Ammeter) is an evolutionary holdover who's operation is an anomaly compared to the first three.
I will be focusing on the first three because of their similarities and ignoring the fourth because of it's significant difference. (Also, I consider ammeters to be functionally outdated, an opinion I will explain if anyone's interested.)
The following is oversimplified diagrams of basic gauge circuits. The top circuit demonstrates the basic wiring of gauges in older vehicles, such as most of our motorhomes. The bottom circuit reflects that found in newer computerized vehicles.
Note: Leeann may recognized this diagram. It's part of an article I started writing several years ago but haven't (yet) gotten around to finishing. (Back then, Leeann helped me with proofreading and editing what I've gotten done so far.) Also, this diagram is copyrighted by me and may not be used without my permission.
Missing from this diagram is a voltage regulator between the +12v and the gauge. Most gauges operate on nominal 5 volt current, which is an apparent technological holdover from the days of 6 volt automobile electrical systems.
In most older vehicles, a single voltage regulator (which is often referred to by other names) serves the fuel, oil, and temp gauges. This regulator is usually mounted on and wired into the instrument cluster.
Aftermarket gauges have a voltage reducer/regulator built into the gauge itself so it can be simply wired to +12VDC supply.
The gauges themselves are actually just voltmeters, displaying voltage levels ranging from zero to five volts.
The sensors, in turn, are variable resistors which vary the amount of current (voltage) in the circuit based on whatever they're monitoring.
When the fuel level/oil pressure/coolant temperature is low, the sensor increases resistance in the circuit. This causes the voltage to drop to near zero and the gauge needle to remain towards the left side. As these conditions rise, the sensor resistance drops, allowing more voltage into the circuit and causing the needle to move to the right.
At this point, you're probably thinking, "Jeez, this is simple." Well, you're right. It generally take me less than an hour to find and fix gauge problems and sometimes a little as five or ten minutes.
I have a good idea as what Jerry's problem is but I prefer the "teach a man to fish" approach.
BTW - there's been a revision to that saying ... "Give a man a fish and he'll eat for a day. Teach a man to fish and he'll sit in a boat all day drinking beer."
I'll stop here temporarily while I go do other things. When I take another break from those other things, I'll describe what can go wrong with gauge systems and how to find the problems.
(Yes, I will get back to starter vs deep cycle batteries eventually. However, Jerry's problem presented an opportunity to take a break from that and look at something that's simpler.)
