The new temp gauge for my 79 f150 is reading a temp of 180 when you turn the key on and climbs to 250 degrees which is it's max reading.

Holding an infrared gun to the thermostat housing the engine (71 351c boss) reads at a normal operating temperature. I have as well replaced the thermostat, water pump, temp sender and obviously the gauge as well as flushed the system and filled with 50/50. I don't believe there are any hot spots in the engine as I've let it run with the radiator cap off to burp any air pockets, however it was suggested to me to do this again without the thermostat in the truck.

The temperature gauge is very similar to an Ultra-lite gauge (2 1/4) and bought new through LMC. As far as wiring goes, the gauge is grounded under the dash to the cross member at the column. I'm sure this is a stable ground since it is bolted down, soldered and the ground placement is connected to a steel ground cable back to the cab of the truck.

The Ignition post is connected directly from the ignition switch line, or RUN which is hot with the Key on. Someone suggest I run this line to the fuse panel as the gauge may be getting too much power. I haven't done that yet.

The sending line is run to the sender on it's own lead with good connection.

I'm really posting this to bounce this stuff off you guys and get any tips you might have. If there is more information you need from me, just reply here.

 

My guess would be that the temperature gauge is calibrated to a different resistance than your temperature sender puts out. That's how these things work; As the temperature increases, the resistance decreases, and the gauge is calibrated to this specific range of resistance.

It's strange though that one from LMC would be incorrect for your truck... Is it possible for you to post the exact model of the gauge you bought, and to find out what range resistance your temperature sender puts out?

I'm not sure if the temperature sender for a 351c out of a Cougar would be the same as the one from your truck (is that even the stock engine? I didn't think the 351c came in trucks) If it is though, this site lists the specs of the gauge:

Temp Sender Switch Temp Rating : 22.7-25.5 Ohms Resistance At 220 Deg F
Temperature Switch Thread Size : 3/8" x 18 NPTF

Replaces Ford Part #: D0WY-10884-A (SW-924)

So, your temp. gauge would need to read in that same range of resistance.

 

No Jeremy ... 351C is not stock to a truck. 351M or 400 .... yes but bell housing pattern is different than a 351C.

Cbad .... I know that Ford used a "instrument voltage regulator" for gages that supplies like 5 vDC .... it fit on back of the instrument cluster.

 

Like Jeremy said, the gauge and sender need to be a matched pair. Did you install a new sender with the new gauge?

As far as voltage regulation, aftermarket gauges generally run on full 12 volts. As long as you have a good switched source the power supply is not the issue.

 

The temp gauge is a short sweep electric 2 and 1/16 white from LMC (pg 54) and came with an 1/8 NPT electric sender so I imagine the sender is rated for the gauge. it is rated for 100-250 degrees.

I was using the temp sender that came with the gauge but somehow it burned out. I believe I had a bad ground in the truck at the time. I've since fixed any ground issues I might have had and got another sending unit for a the engine, specifically for the 71 351C.

And no, this engine is not stock. Someone intended this truck to be a hot rod at one time and put this rebuilt motor in before I bought it.

I cant tell you what rating the sending unit is, as I can't find the box or the reciept but as I look at it now it seems like they gave me something for a merc cougar. So im going to say somewhere in the 73 cougar area rated for a 351c.

As I said before this engine is a 351c Boss R series

 

if you disconnect the sender wire from the back of the gauge does it still peg out when you turn the key on?