Water temp gauge/sending unit

[Flip]

I'm pretty sure that either my water temp gauge and/or the sending unit went out. How do I test either one? If I do need a new sending unit, how do I tell which one I need. I'm sure there is some kind Ohm's test I need to do, but I don't know what I'm looking for.

[Mighty Thor]

Quick simple test to see if the guage is functuning at all is to pull the wire off the sending unit and ground it, if the guage hand sweeps it is probably not the problem. if it doesn't move you might check the other wires to the guage before you start trying to work on the sender. Doing this once or twice should not harm the guage in any way.

[Flip]

Cool, I'll try that. Anyone else?

[Oldsquirt]

SInce the sending unit is usually easier to access than the back of the gauge, I would start like Thor said. With key on, jump the terminal of sender to ground. If gauge reads full hot, sender is bad .
If no reading in above test, crawl under dash with voltmeter and jumper wire. With key on, verify good power and ground at the gauge. If OK, jumper the "S" (sender) terminal of gauge to ground. If gauges reads max, wire from gauge to sender is "open". If guage still doesn't read, double check power and ground and if they really are OK, gauge is bad.
Problems in order of likelyhood(in my opinion):
1. Bad connection(ground, power or sender wire)
2. Bad sender
3. Open in wiring to sending unit on engine
4. Bad gauge
If you need a new sending unit, it needs to match the temp range of the gauge. Try to get one that is the same brand as the gauge itself.

[Flip]

Ok, so I tested both gauges (water and oil). Both gauges max out when grounded. The water temp sender is definately bad. The oil sender kinda bouncing around at times. I'm pretty its not an accurate reading of whats acutally going on in the motor. Pretty sure the sender is just going bad.
So I guess the question is now, what kind and where do I get new sending units? I've got original '77 Hardin Marine gauges. I'm just wondering if there's an ohm's rating that I'm gonna have to match up with or can I just run down to the auto parts store and get senders with the same fittings and temp/pressure ratings?

[Oldsquirt]

You might find some numbers on the sending units that would help in matching to a generic sender.
CPPerformance.com sells the Hardin line. Not sure if Rex does.