Help! Temp Gauge Stopped Working

[Gab73z]

Hey Guys,

My heater core blew and, as a temporary fix, I decided to bypass it entirely. It was an easy fix, just some new hose and clamps. After it was done, however, I took it for a spin to test it out and now the temp gauge doesn't work.

I have no idea how the bypass could have caused this. I'm kind of grasping at straws but the only thing I could think of is either I got air in the cooling system and it messed with the sensor, or the rad full of coolant that emptied into my passenger side footwell shorted something.

Any ideas?

A little back story - I bought my 73 240 last year. It was in decent shape, definitely neglected for years but entirely complete and solid. Since then I've been doing whatever I can to put it on the road for the summer. I bought it because I love the car, not because I have any mechanical knowledge. Before the Z, the most complicated thing I did was install new brake pads. But, with the help of some good friends, I'm learning. So treat my like I'm ignorant.

[FastWoman]

I think you're a victim of coincidence. The bypass shouldn't have caused this problem. However, you'll get better coolant flow to the radiator if you cap the heater hose outlets, rather than connecting a bypass hose between them.

The temp gauge system isn't very complex. Find the water temp sender (front top of engine), and see if you might have accidentally pulled the wire off while working on your car. The connection might also be dirty or corroded. The wire might even have corroded through where it enters the connector. (I'm guessing you get a bit more snow and ice up there in Canada than we get in Southern Virginia -- and probably lots of corrosive road salt to go with it.) FAIW, my old '75 quit on me one day at a mere 10 years of age when a fusible link wire corroded through. That was a Central Texas car -- no snow/salt. I'm still surprised at that one!

[stevef1972z]

The temp sender should be on the Thermostat Housing, under the upper hose, that wire just hangs there, and the sender tip gets nasty quick, clean and check everything visually and retry.

If not, you may have developed a short. When the core decided to retire, did it leak hard inside the car? If so, you may have gotten a few conections wet and need to clean them as well. My guage acted bizzare until I found a short that was causing the drop in and out of the guage, it was under and behind the heater controls and dash area, but it impacted the dash lights as well.

Good luck and keep us updated.