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willysmjeeps.com :: View topic - Temperature Gauge and Sensor matching riddle
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Temperature Gauge and Sensor matching riddle

 
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arisLgr
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Joined: Dec 25, 2016
Posts: 20
Location: Athens, Greece

PostPosted: Sun Jan 07, 2018 8:58 am    Post subject: Temperature Gauge and Sensor matching riddle Reply with quote

Hi all and Happy, Healthy and M-Vehicle-Problem-Free-New Year!!!!!

Way back I posted an article re my M38 Temp Gauge. At the time I had burned it out, but once I pried it open I found an open coil wire and fixed it. I did the same with two used gauges that I found on line so now I have TRHREE temp gauges that based on Bob Westerman's guide from our downloads, seem to work OK.

I also have TWO Sending Units that seem to work OK, HOWEVER neither oneís Ohm range fits the range mentioned in Bob's info (properly tested in hot water etc)

Iím younger than my M38 and can work the mechanics, but my brain cannot figure out if what I have can be matched. What with the part and ordinance numbers, what with the gauges having inner or outside resistors, Iím totally lost in the manuals.

Wes, could you be once more kind enough to help me out? I read your preference for mechanical gauges, but all these years Iíve tried to make the M38 as original as possible, actually most of the other gauges are SWís!

THANKS!

Here are my gauges and sensors

Gauge No1 AC 1512422 7389567 (Inside, this has TWO terminals for the coils)
Gauge No 2 SW 505-A 7389567 (Inside, this has THREE terminals for the coils)
Gauge No 3 AC 6401256 MS-24543-2
Sensor No 1 MS 245537-1
Sensor No 2 AC 7728851
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Aris
Willys M38
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1973 Mercedes 350SE
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4x4M38
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PostPosted: Sun Jan 07, 2018 9:03 am    Post subject: Reply with quote

Aris,
What were your test results on the senders?
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Brian
1950 M38
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4x4M38
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Joined: May 30, 2014
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PostPosted: Sun Jan 07, 2018 9:34 am    Post subject: Reply with quote

Also, can you double check the number on gauge
#1? Maybe it's supposed to be AC 1512244?

Sender 7728851 - we have documented replacement
senders from a vendor with this number stamped on it.
This sender is to be used with the early gauge only,
which uses an external 95 ohm resistor in a black
connector on the back.

Gauge markerd AC 1512244 is this early gauge.
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Brian
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arisLgr
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Joined: Dec 25, 2016
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Location: Athens, Greece

PostPosted: Sun Jan 07, 2018 12:43 pm    Post subject: Reply with quote

4x4M38 wrote:
Aris,
What were your test results on the senders?


Thanks for the reply, I will retest the senders tomorrow and post the readings.

Should I take one reading at room temperature, or chart them at 120,180,220,260F like in the troubleshooting guide?
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Aris
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arisLgr
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Joined: Dec 25, 2016
Posts: 20
Location: Athens, Greece

PostPosted: Sun Jan 07, 2018 12:54 pm    Post subject: Reply with quote

4x4M38 wrote:
Also, can you double check the number on gauge
#1? Maybe it's supposed to be AC 1512244?

Sender 7728851 - we have documented replacement
senders from a vendor with this number stamped on it.
This sender is to be used with the early gauge only,
which uses an external 95 ohm resistor in a black
connector on the back.

Gauge markerd AC 1512244 is this early gauge.


Again Thanks for the reply !

No 1 gauge I confirm the number is as I wrote it, I can post a picture of it.

As for Sender 7728851 , i just bought it awhile ago from a vendor, it is brand new and I am thrilled you mention it needs a resistor! I will try it , but which Gauge of my three, should it be matched with?


Two more questions from my brother in law, who as an Electronics Engineer is helping me:

a) The Temp gauges, do they need to be GROUNDED on the instrument panel? Because I tested them on the bench, pried open, without their cases on them !

b) Are the Temp gauges operating as Amp meters, or Volt meters?
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Aris
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4x4M38
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PostPosted: Sun Jan 07, 2018 8:05 pm    Post subject: Reply with quote

I would post results based on the testing
procedures.

Yes the gauges need to be grounded.

If the gauge numbers you posted are correct none will work
correctly with the resistor previously mentioned.
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wesk
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PostPosted: Sun Jan 07, 2018 11:04 pm    Post subject: Reply with quote

Aris' equipment:

Quote:
Here are my gauges and sensors

Gauge No1 AC 1512422 7389567 (Inside, this has TWO terminals for the coils)
Gauge No 2 SW 505-A 7389567 (Inside, this has THREE terminals for the coils)
Gauge No 3 AC 6401256 MS-24543-2
Sensor No 1 MS 245537-1
Sensor No 2 AC 7728851


From a post I made Nov 2017:

Quote:
The two water temp gauges are:
7728855 (AC# 1512244) (SW# 505A) - external resistor
7389567 (AC# 1512422)- Internal resistor

These are on page 151 of the M38 1955 ORD 9

The only sender in the 55 ORD 9 is:
7389566 (AC# 1512423) - By the listing works on both gauges.

This is on page 163 of the M38 1955 ORD 9. However the two senders shown under that ORD# are AC# 1512423 and AC# 1512245.

If you open the 1951 ORD 9 you will find the AC# 1512245 listed as ORD# 7728851 lending credence to an error in the 1955 ORD 9.


This is from A July 2016 post I made:

Quote:
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.


There are plenty of additional posts on this topic that you should research as well:

http://www.willysmjeeps.com/v2/modules.php?name=Forums&file=viewtopic&t=11514&highlight=gauges

http://www.willysmjeeps.com/v2/modules.php?name=Forums&file=viewtopic&t=11373&postdays=0&postorder=asc&highlight=gauges&start=15

http://www.willysmjeeps.com/v2/modules.php?name=Forums&file=viewtopic&t=10726&highlight=gauges

http://www.willysmjeeps.com/v2/modules.php?name=Forums&file=viewtopic&t=10696&highlight=gauges
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arisLgr
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Joined: Dec 25, 2016
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Location: Athens, Greece

PostPosted: Tue Jan 09, 2018 9:45 am    Post subject: Reply with quote

OK guys, I am at it, will post some interesting findings. As for my M38, I've got a set gauge with sensor, working OK on the bench!
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Aris
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1968 Corvette Stingray
1973 Mercedes 350SE
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arisLgr
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Joined: Dec 25, 2016
Posts: 20
Location: Athens, Greece

PostPosted: Mon Jan 15, 2018 4:59 am    Post subject: Reply with quote

OK, first I hope the pics show properly..... ANyway, I spent some time with the electronics wizard of the family, here he is:



Now on the issue, here are my two sensors:
AC 7728851 just bought new, reads 150ohms cold
MS 245537-1 NOS on my car, reads 5000 ohms cold

FINDING ON THE SENSORS,
-There is a HUGHE Ohm resistance between the two sensors, meaning that 7728851 in no way can work with ANY of the three gauges, because the resistance of 150ohms is way too low. I donít know if it is dud sensor, I just bought it on ebay is it looks unused,brand new.

And the three gauges:



AC 1512422 w/bullet connectors, bought used, had broken wire that I repaired, and tests OK with Bobís values
S.W. 505-A, w/bullet connectors, was on my car, the one I may have burned, because it does not test OK with Bobís values
AC 6401256 with Rubber Connectors

One observation on all three gauges, is that contrary to the books, none appear to have INSIDE any RESISTORS, at least obvious ones.

FINAL RESULTS !
A. Sensor 7728851 is no good for any of my gauges, period
a. Sensor AC 1512422 however, matched perfectly the sensor MS 245537-1 I should also poin out that mention that to read properly, instead of a 90 Resistor, it only needs a 8,5 resistor.
Here it is bench tested in water with celcious thermometer




Thatís it folks! I also worked out with my brother-in-law a workaround, how to make broken sensors work with modern electronic THERMISTORS, but this will follow in good time
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Aris
Willys M38
1936 Cord Phaeton
1954 Mercury Monterey
1968 Corvette Stingray
1973 Mercedes 350SE
1974 Montesa Cota 247T
1982 Kubota B7100
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4x4M38
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Location: Texas Hill Country

PostPosted: Mon Jan 15, 2018 7:56 am    Post subject: Reply with quote

Aris,
That's some great research. Glad you got a working combo.

Sender 7728851 is the one that requires an external 95 ohm resistor
installed on the back of the proper early gauge.

Take care,
_________________
Brian
1950 M38
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arisLgr
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Location: Athens, Greece

PostPosted: Mon Jan 15, 2018 8:38 am    Post subject: Reply with quote

Brian,

as far as the expert is concerned, Sender 7728851 wont work no matter what resistor goes with it, he tried everything.
my Of course let me add that he only had my three gauges to match it with, so maybe the gauges are not as they should be.

Like you wrote, I'm elated with the result and when ready will post more findings! Wintertime is warm-indoor-workshop time!
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Aris
Willys M38
1936 Cord Phaeton
1954 Mercury Monterey
1968 Corvette Stingray
1973 Mercedes 350SE
1974 Montesa Cota 247T
1982 Kubota B7100
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