HELP with 1941 temp guauge

[kkarrer]

Hello,
     I'm wiring my '41 coupe and need to know three things regarding the temp gauge,etc.:
1.) There are two studs on the back of the gauge.  One is a little larger than the other.  I would think that the larger on should have the black wire that goes to the sending unit and the other is the feed. Is that correct?  The wiring diagram shows it that way, but wiring diagrams are not always correct when it comes to right and left justification. My wiring harness is new.
2.) Does the polarity matter?  I've changed this car to 12v neg. ground.  (planning to put AC in this car...I did this with my '50 and didn't need to switch the wires) and just to be safe I installed a  Runtz resistor to protect the gauge.
3.) How do you test the sending unit? Are they available is this one is bad? It looks the same as the one that was on my '50, but that was an ohv, 331 engine.

Unlike when I did this on my '50, this '41 was not running when I got it.  I don't know for sure if the gauge is good or if the sender is good or not. I did clean the sender and it's installed without thread sealing so that it is in positive contact with the head.  I do know that when I switch the wires and apply power that the needle "pegs" to the right and when they are in what I think is the correct position the needle does not move when power is applied or when the car is running.  I have an extra gauge and can remove the sender and plug the head and conduct testing with a remote power source. 
As always,
Thanks very much,
Ken Karrer 1941 6227D coupe
PS my extra gauge cluster is complete with good glass, 4 gauges, and a chrome surround that had NO pitting. 

[Bill Ingler #7799]

Hi Ken: The black wire goes on the larger stud of the temp gage and that same black wire goes to the temp sender on the engine. Since you have converted to 12 volt negative then you need to find a 12 volt temp sender for the engine. The 41-6227 coupe is one of the best looking Cad coupes of the 40s. Have fun     Bill

[kkarrer]

Bill,
     Thanks for the quick reply.  That's helpful and it's how I have it hooked up right now, but I have a Runtz resistor between the power source wire and the gauge.  I would think that should "fool the sender" into thinking that we're still working on 6v, but I'm getting nothing at the gauge.  Need a way to test the gauge and the sender I guess to get it down to which component is not working.  I'm hoping to get the interior in this thing by the end of next week and then I have stuff like this and some lifter noise to sort out and we should be good to go.
Thanks again,
Ken

[pauldridge]

Ken,

It drives me crazy to see someone opt out of their original gauge and install a NAPA set under the dash.  Unfortunately, I've had pretty much the same results (or lack of) with my own '40 temp gauge, and finally had to reluctantly resort to installing a small underdash gauge.  I haven't given up, but so far no luck with mine (12V system also).  I think AllCads sells a 12V sender, but it's pricey and I just can't bring myself to springing for it with no assurance of success.

If you come up with a solution, I'd sure like to hear it!
Phil

[kkarrer]

Phil,
    Don't worry, I won't go the under dash gauge route.  If I get something from NAPA it will only be a sending unit.  I have two gauges and I'll get this to work.  The only question is should I staywith the Runtz resistor and try to fine another 6v sending unit or remove the Runtz and go with a 12v unit.  From what I gather, it doesn't matter to the gauge (6 or 12v or pos or neg ground).  I hope someone will correct me if I'm wrong.  This is a frame off restoration.  I'll take my time to get it right.
Thanks,
Ken Karrer 1941 6227D coupe

[Paul Phillips]

Ken
I have not experimented with my 41 gauges, so not sure if they are polarity-sensitive, but electrical intuition would say 'yes'.  With that being the case, you would want battery + connected to the large terminal and the sending unit connected to the smaller terminal, then to ground.  The temp sender resistance drops as the temp increases, increasing current flow thru the gauge, which moves the needle to the right.  There is a bit more on this in a previous post http://forums.cadillaclasalleclub.org/index.php?topic=117815.0

I would suggest you consider a gauge voltage regulator instead of a dropping resistor (or a '12 volt sender' which essentially just includes an internal dropping resistor)  to  feed +6V to the temp and gas gauges.  That way you have a stable voltage feed regardless of battery voltage.  A web search should yield you several options.

FYI, gas sender appears to be 0-30 ohm rated.

Paul