1952 M-38 Temp. Sending unit

jerry72 · Post by **jerry72** » Sun Dec 28, 2014 7:28 am

Hi guys,

Had to replace my temp. sending unit ac-7389566 with NOS ac-1512245.
Gage= ac-1512422.

Gage reads 0 with new sending unit with engine hot.

Checked sending unit resistance according to troubleshooting instructions.
Reads 163 ohms when cold and drops when hot.
According to trouble shooting guide sending unit should read 2000 ohms @ 120F and 300 ohms @ 260F
Opposite of my new unit readings.

Could it be the new unit is not compatible with my gage?

Thanks for the help.

Jerry

4x4M38 · Post by **4x4M38** » Sun Dec 28, 2014 10:35 am

Jerry,
Do you have a good ground between the sender and block?
Maybe check continuity there.

4x4M38 · Post by **4x4M38** » Sun Dec 28, 2014 10:46 am

Also, should not matter, but what gauge do you have?
The one with an external resistor in the L-shaped fitting or no external resistor? 1512422 should be without I believe.
Brian

jerry72 · Post by **jerry72** » Sun Dec 28, 2014 11:43 am

Thank you for your response Brian.

Sending unit screws in the block so grounding should be fine. When I measure the ohms on the sending unit I ground one lead to the engine block. That's when the new unit reads 163 ohms when cold and drops as the engine warms.
If I read the troubleshooting guide correctly, the sending unit should have a high ohm reading (2000) when cold and drop as the engine warms.

My gage does not have the L shaped resister.

skyjeep50 · Post by **skyjeep50** » Sun Dec 28, 2014 1:50 pm

Your new sending unit does not conform to the readings suggested by the troubleshooting guide. Either its not the right unit for your application or it isn't operating correctly.

Post by **wesk** » Sun Dec 28, 2014 2:39 pm

You have a late gauge & a late sender. If your readings do not agree with the TS guide your sender is bad, NOS or not makes no difference. Always keep one thing in mind when buying NOS surplus. The military surplused out both good units and units that did not meet specifications. You can never be sure which you get til you try to use it.

BCA · Post by **BCA** » Mon Dec 29, 2014 12:16 pm

My experience is that NOS, new-the-box" AC brand temperature sending units are often bad. I initially thought it might have been incompatibility with SW temp gauges but finally concluded they were defective or aged poorly. I always bench test sending units and gauges (a pot of boiling water guarantees a good temperature reference: testing any kind of sending unit is too frustrating when installed in a vehicle. .... Brian

jerry72 · Post by **jerry72** » Mon Dec 29, 2014 2:15 pm

Thanks for the feedback guys.

I'll try my luck with a repo unit and see how that goes.

Jerry

Post by **wesk** » Mon Dec 29, 2014 3:38 pm

Jerry, Make sure your gauges are grounded to the chassis. It is the most overlooked ground on the jeep. Those 3 sender served gauges require two grounds to work. One thru the sender to supply the signal and one thru the gauge case to display the signal. Originally the factory ensured there was adequate bare metal contact points between the gauge, cluster panel, dash and chassis for this. With all the corrosion and layers of paint since these grounds are proving very unreliable. I always add a bonding strap between the gauge cluster panel and the firewall. I also make sure that the gauge and it's "U" bracket have bare metal at their contact surfaces with the gauge cluster and I make sure the gauge cluster panel has bare metal where the gauge and their "U" brackets contact it.