Getting ready to put a new radiator in my 78 nova. While the coolant is drained I am going to install a temp gauge. So I started looking around the front of the engine For a place to install the new sender for the gauge. I noticed that there are 2 hot light senders wired in such a way that either one can turn on the hot light. One sender is in the left front corner of the head, the other one is in the water neck below the thermostat. What gives? Anyone know anything about this? Are both sending units the same? Any help would be apprenticed. Jay

Don't know why you would have two. Is the one on the head actually connected to anything?

Most people use the one on the neck if it's there (mine is just a blank boss). The one on the head will give faulty readings.

Do you have an electric fan? If so maybe one is connected to the fan and the other to the dummy light? Maybe gauge and dummy light?

No electric fan. Just a 3 blade clutch fan from a 350 pickup truck. This is all factory. The wire from the harness runs to a single terminal sender on the water neck with 2 wires connected to the plug. The other wire that is connected to the first plug goes to the sender in the head. That way if either sender gets hot and grounds the hot light comes on. I even looked up the wiring diagram at Steve's Nova Site and it shows one temp sender on V8 models and 2 temp senders used on 6 cylinder cars. The senders look simular but I don't know if they are the same or not. Maybe one comes on at a higher temp than the other one? I wonder why would the factory do this? Jay

No Chevy Inline came with an electric fan. They were either the fixed four-blade fan, or the later three blade clutch fan on the trucks. Yeah, my '78 Camaro with the 2nd version integrated head has two temp sensors as well. One in the bottom portion of the water neck (right next to the vacuum valve for the EGR and EVAP), and one in the cylinder head too. That seems to be the way they are set up in the integrated cylinder heads from at least 1977 onward. If someone with the '75-'76 integrated head could look and confirm if they have it too? I'm not sure on the '79 non-integrated 250 head in the Camaros, Novas, Caprices, and Impalas; or the 292 truck engines, was also the same way?

Lifeguard's engine compartment is a much cleaner, nicer version of mine. The places that he has circled in the photo is where mine are also. Trust me GM would not have spent the money for 2 senders without there being a reason. I have heard that members that put a gauge in the head the gauge reads too high compared to the water neck. I am going to put my gauge in place of the sender in the water neck and leave the one in the head hooked up. Still wonder why it was done though. Jay

Langdon explained it to me: The one in the head reads high because the heat from the header transfers to that area very easily. Mine reads high when sitting still but as soon as air flows over the area it comes down to the actual reading. I suppose if I had a mechanical fan it wouldn't be so far off but when it's idling and the electric fan isn't engaged it reads 220 or so when I know it's really 195.

I just haven't felt the need to go get a new neck to relocate my sending unit. Rather put the money elsewhere.

I could see why being that close to the exhaust would make the gauge hooked up at the head read higher. Lucky for me I have a easy choice of where to put the sender for the gauge. Looking at the 2bbl intergrated heads that I have, same thing, sender on the water neck and one on the head, but in a different location. Still would like to know why two senders were installed. Jay

The Camaro and Nova.....kissing cousins. So they should be near identical under the hood. Then again the Chevy Impala and Caprice for 1978 would also have the same setup. Not sure how things changed for the final year of 1979 when the non-integrated head was reintroduced.

Jay,
I think they knew the cylinder head was prone to cracking if overheated so the two switches.
One for coolant temp in general, the other for the protection of the head if it had no coolant or no flow through it.

Just my thoughts. I've always seen or heard the integral heads were prone to cracking. But I have no personal experience.

Best of luck with it.

Thanks 292 wagon. That is what I think too. If the fan belt broke for example, the head might get hot enough to do damage before the switch in the water neck turned the hot light on. I know that the 2bbl heads crack easy, I don't know how easy the 1bbl heads crack. This is one of the reasons why I run a 180 thermostat. Jay