Water Temp Sending Unit

Boze · Post by **Boze** » Tue Jun 28, 2016 7:14 pm

Is there a simple way to check these for function? I installed a new gauge cluster with new gauges on my M38 and the temp gauge won't move. The old gauge didn't work so I assumed the sending unit did. Wrong? The harness is relatively new, too.

OKCM38CDN · Post by **OKCM38CDN** » Tue Jun 28, 2016 8:00 pm

Place the Sending unit in HOT water get it to temp; then using an Ohmmeter check the resistance of the sender and see if it changes as it cools off.

Need to do this fairly quickly before the sender cools off...

This is not a perfect test but should tell you the sender is not dead...

Another way is to use a heat gun and warm the sender up while connected to a meter...

Hawkshadow · Post by **Hawkshadow** » Tue Jun 28, 2016 8:38 pm

Check the pam on here for a step by step walk though to test the individual items.

http://willysmjeeps.com/downloads/instruments.pdf

I would suggest that if the gauge passes, and all of the wiring works properly and you have no resistance in the line to the sender… it's probably the sender.

4x4M38 · Post by **4x4M38** » Tue Jun 28, 2016 9:11 pm

The sender has to be grounded as well.

One thing when testing a gauge with a meter. Do not tie one of
the test leads to the Douglas case to get ground.

You must tie to one of the mounting screws. The metal Douglas
case is not a proper ground.

charlesfitton · Post by **charlesfitton** » Wed Jun 29, 2016 4:57 am

Thanks Hawkshadow...A useful instruction sheet tat I will read through later

Hawkshadow · Post by **Hawkshadow** » Wed Jun 29, 2016 8:30 am

No problem. It's a simple diagnostic process of elimination. I used it to fully go through my system. It lead to a new oil pressure gauge, fuel level sender, and coolant temp sender.

oilleaker1 · Post by **oilleaker1** » Sat Jul 02, 2016 5:48 am

The early temp gauges with the built in resister and angled black connector use a different sender than the later ones. I found a NOS sender through Brett Mullins Jeep Parts in Texas. It now works great. The "test" procedure showed the gauge to be fine, but the sender was the wrong rating. John

4x4M38 · Post by **4x4M38** » Sat Jul 02, 2016 7:28 am

Going to step out on a shaky limb and suggest the oil pressure sender
might have the same issue. Not sure about the fuel.

Post by **wesk** » Sat Jul 02, 2016 9:09 am

It's best to not offer a guess for troubleshooting advice. It usually leads to confusion later!

The senders were not changed because we dropped the external resistor on the gauges.

The only difference between an early oil press or fuel gauge with the external 95 ohm resistor and the later fuel or oil press gauge with the built in 95 ohm resistor is the location of the resistor. Therefore fuel and oil pressure use the same senders with either gauge. The problem with the water temp gauge was the new internal resistor gauge required a different value sender due to additional internal changes on the new water temp gauge.