M38 Temp Gauge

Boze · Post by **Boze** » Fri May 15, 2020 2:58 pm

Bought a new temp sending unit from a MV dealer at the convention last year and finally got around to installing it. It seems to read double the actual temperature. After running it for about 5 minutes it was reading about 235 but the top hose wasn't that hot nor was the radiator - just warm. Is there a ying and yang to senders and gauges? There is plenty of clean coolant, no leaks and water pump and thermostat work fine. Flow has always been good. Just wondering if the sender and gauge need to be the right ones before I start tearing down the cooling system.

Thanks for input.

Boze

kenperkins · Post by **kenperkins** » Fri May 15, 2020 3:16 pm

does it match your sender ?

Boze · Post by **Boze** » Fri May 15, 2020 4:39 pm

Don’t know, but it should. Both were sold to me by MV dealers who said they were for M38s and have Douglass connectors. Why wouldn’t they? How many senders can there be?

OKCM38CDN · Post by **OKCM38CDN** » Fri May 15, 2020 7:54 pm

Early gauges required resistors to be in line with the gauges... could you have an early gauge?

Post by **wesk** » Sat May 16, 2020 12:45 am

Let's skip the guessing and get right down to the meat. What are the part numbers of the your temp Gauge and Temp Sender? Is your M38 completely stock and still 24 Volt? Have you checked the resistance range of your sender and compared that to TB 9-2300-228-20 Troubleshooting Military Vehicle Instruments?

We can see you did not research the part numbers before you bought the new items so now is a good time to explain how to answer your question:

Both were sold to me by MV dealers who said they were for M38s and have Douglass connectors. Why wouldn’t they? How many senders can there be?

To make the answer as simple as possible I'll stick with manufacturer's part numbers:

Gauges:
AC# 1512244 Gage, Water temperature, with external resistor Used thru MC17855

SW# 505A Gage, Water temperature, with external resistor Used thru MC17855

AC# 1512422 Gage, Water temperature, with internal resistor Used after MC17855

Senders:
AC# 1512245 Sender, Water Temp (1951 ORD 9)

AC# 1512423 Sender, Water temp (1956 ORD 9)

Is there a ying and yang to senders and gauges?

Can you tell us what this phrase actually means?

Just wondering if the sender and gauge need to be the right ones before I start tearing down the cooling system.

Yes they usually do.

One very simple check you can make is checking your water temp with another gauging system. An infrared heat sensor or a simple mechanical temp gauge available at any auto store. I keep one hanging on the wall in my shop with an assortment of adapter fittings.

Finally, if gauge/sender mismatch is not the issue then the usual culprit is wiring and specifically bad grounds. The military temp, oil press and fuel quantity gauges reguire a separate ground for the gauges internal needle drive mechanism wiring as well as the needed ground at the sender. The restos we do all have way to much paint on them causing many grounding problems.

Boze · Post by **Boze** » Sun May 17, 2020 8:21 am

Thanks, Wes. I've got some work to do. My rig is 24volt. I have an early M38 but replaced the entire instrument cluster because the original was shot. I think I don't have a separate ground so that may be it. I'll start by checking the coolant temp with a thermometer and make sure I have good throughput of the coolant. Then I'll add the ground wire. Then I'll check the sender and the gauge for manufacturer and number.

Where would I find a correct resistor if that is what I need? An MV dealer or an auto parts store?

Thanks again. A letter on this site tends to be very fruitful.

Boze

Post by **wesk** » Sun May 17, 2020 7:38 pm

I wouldn't suggest a vendor for something you may not even need yet.

The important thing I had hoped for was that you would show an interest in properly researching your needed data in the applicable manuals. In your case TM 9-8012, ORD 9 SNL G-740 and TB 9-2300-228-20. All 3 are free PDF downloads here on our web site.

bluewatersailor · Post by **bluewatersailor** » Tue May 19, 2020 2:51 am

Wes, I may be having just another senior moment, but I am unable to find TB 9-2300-228-20 in this site's download section. Suggestions please.
Bluewatersailor

Post by **wesk** » Tue May 19, 2020 7:44 am

I suppose you could say it is a wee bit buried.

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