After putting a new double post send unit in my 51 flatty my temp gauged worked fine. but once I connected the passenger side single post to it the gauge no longer works. So I disconnected it from the double post unit leaving it (the double post unit) hooked up to the gauge, the gauge still won't work. My questions are...
--- Would a bad single post unit cause the double post unit to go bad?
---- Are the both really needed?
Thanks

 

No and yes.

Remember the single post unit completes the ground for the temp gauge circuit. So there has to be continuety from the gauge to the single post unit.

The double post unit is a simple overheat sensor - a switch - it's either on or off. Inside a contact switch is closed allowing power to flow out of it over to the single post sender.

If the engine boiled, the switch would open - breaking the electrical circuit, and not allowing the power to reach ground via the single post unit.

Without a ground the needle on the gauge, basically, turns off, and returns to it's default position on "H."

So, power runs from the ignition switch "ACC" post, to the gauge, out of the gauge to the double post sender; then [if the temp isn't boiling and the switch inside it is closed] the power then flows out of the double sender and over to the single sender. The single post sender varies resistance with changing temps (causing the different gauge needle positions), then the varied power flows to ground.

Get a meter and check for continuety between the two posts of the double post sender. If you have continuety there then the two post sender is ok.

Take the wire coming from the gauge that attaches to the double post sender and attach it to the single post sender with a jumper wire. Start up the truck. If the gauge works the single sender is good - if not, it is bad.

Just out of curiosity, why did you change it?

 

Not sure if you've seen this before, it explains the function of the system. It isn't like anything on modern trucks.

 

keep in mind that flatties are, for the sake of conservation, two four cyl. units. two independent water pumps. with only one send unit, you would not know if other half of engine was hot until too late. i use two totally independent send units and gauges. i KNOW how hot both banks of cyl,s are. just my 2cents.

 

Thanks for the input. I am finding it hard to locate a new single post unit that is not a repro. I have been told that the newer ones are not very consistent. I have 2 (one for a spare) new double post on their way now.

 

SO Julie, Can you re-Explain that for use with 12 Volts neg ground?

Just Kidding

I never knew all this info, great explanation and the diagrams by Ross help too.


Cheers

Josh

 

Sure, it's exactly the same! You just have to reduce the voltage from 12 to 6 volts before it goes into the gauge (smart Alec! )