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willysmjeeps.com :: View topic - Temperature Gauge Issues
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Temperature Gauge Issues

 
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SCJeep
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Joined: Jul 20, 2012
Posts: 11

PostPosted: Thu Sep 13, 2018 5:30 pm    Post subject: Temperature Gauge Issues Reply with quote

So I followed the troubleshooting guide and found that I had a bad gauge, the gauge would never read anything. I replaced the gauge and it operated properly all was good.

Suddenly after about 1-2 weeks the gauge goes to full scale (260 degrees) pegged. Went back to the troubleshooting guide, immediately check the sender, it ohms out properly.

Back to checking the gauge. It reads full scale(pegged) in the following scenarios:
-> Sender hooked up (24v applied)
-> Sender wire removed (24v applied)
-> Ground the sender terminal (24v applied)

Couple of odd things:
If i check continuity of the panel while removed from the dash it shows that it is grounded to the body.
If I remove the gauge from the panel and check it again it works. Hook up 24v, remove the sender wire, ground gauge body to Jeep body and now it reads 0.

I'm a bit perplexed at this point. Do I have a bad gauge, is something wrong with grounding on my gauge cluster.
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wesk
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Joined: Apr 04, 2005
Posts: 13735
Location: Wisconsin

PostPosted: Thu Sep 13, 2018 8:31 pm    Post subject: Reply with quote

Grounds are the king of all nemises in an old vehicle.

Keep in mind that the gauge on it's own requires a good ground for it's internal circuitry. This ground is empowered by the case to the mount bracket to the instrument cluster to the dash. To test gauges while they are installed in the cluster but the cluster is disconnected from the dash requires that you add an alligator clip test lead between the cluster and the dash so that needed ground is available.

The secondary ground is for the needle of the gauge thru the wire to the sender and then thru the sender to the chassis ground.

Either of these grounds become lost and there will be several different symptoms.

You're biggest enemies on an old jeep are:

1 - Rusty surfaces that won't ground together.
2 - Over painted surfaces that have paint covering the little areas the factory originally left bare so the gauge case grounded to it's bracket which in turn grounded to the cluster which in turn grounded to the dash.
3 - Rotted wires that allow the bare wire to short to the metal connectors which in turn occasionally or intermittently short to nearby bear metal.

Since the oil press & water temp senders are screwed into the block their ground is seldom an issue but the gas quantity sender is grounded to the tank but with all the rust or overpainting the tank cannot find a ground to the tub and in turn to the chassis.

Yes, this was long winded but until you tell us you are positive none of these grounding or shorting issues exist in your system we can't offer any intelligent comments concerning your gauge's behavior.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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SCJeep
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Joined: Jul 20, 2012
Posts: 11

PostPosted: Mon Sep 24, 2018 7:42 am    Post subject: Reply with quote

So to confirm I went and added a grounding strap to my instrument panel which I verified.

Following the troubleshooting guide I get the following result:
READS FULL SCALE [PEGGED]
-> Sender hooked up (24v applied)
-> Sender wire removed (24v applied)
-> Ground the sender terminal (24v applied)

So I pulled the gauge out and checked it on the bench:
READS FULL SCALE [PEGGED]
-> Sender wire removed, OPEN (24v applied)
-> Ground the sender terminal (24v applied)

This appears to indicate that the gauge is bad, however, this seems like an odd failure mode to me. Any thoughts?
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wesk
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Joined: Apr 04, 2005
Posts: 13735
Location: Wisconsin

PostPosted: Mon Sep 24, 2018 8:40 am    Post subject: Reply with quote

The troubleshooting service bulletin was published by the Army to get rid of all those not so sure phrases like "Appears to be" . "Seems to be" or "Could it be"!

It is a detailed, step by step precision piece of work that takes you to a very clear, concise and direct determination.

You have only given us a very small piece of the test shown in the troubleshooting guide. Yet you cannot possibly expect us to reach a conclusion without seeing each and every step used in the guide?

What was the verdict of your properly completed troubleshooting guide?
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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SCJeep
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Joined: Jul 20, 2012
Posts: 11

PostPosted: Mon Sep 24, 2018 9:54 am    Post subject: Reply with quote

5. Gages and Related Wiring
a. Check vehicle battery voltage. Charge or replace battery if voltage is not as prescribed
in pertinent technical manual. Result: Battery voltage good.
b. Check battery cables and terminals for good condition. Repair or replace battery cables
and terminals, as required. Be sure that they are properly secured. Battery cables good.
c. Disconnect wire from ignition switch to gage being tested at gage, and connect wire to
positive lead of voltmeter. Complete
d. With ignition switch ON, ground negative lead of voltmeter (B, fig 1) to vehicle
ground, voltmeter should indicate battery voltage. Note. If lower or zero reading is indicated,check
circuit for loose or broken connections. Repair as required. Complete, battery voltage 24.6V.
e. Remove voltmeters negative lead from vehicle ground and connect to case of gage (C,
fig. 1), voltmeter should indicate battery voltage. Note. This check is important. If case of gage is not
properly grounded, gage will not operate. Complete, battery voltage 24.6V. Case grounded properly.
f. Turn ignition switch OFF. Complete
g. Disconnect voltmeter positive lead from ignition switch-to-gage wire and connect wire
to gage. Complete.
h. Disconnect wire from gage to sending unit at gage. Complete
i. Connect suitable jumper wire to sending unit side of gage. Complete
j. Turn ignition switch ON. Complete.
k. With jumper wire grounded to vehicle (D, fig. 1), the gages should indicate the
following:
Temperature gage Maximum temperature-> Reads Maximum temperature
l. Remove jumper wire from ground (E, fig. 1), the gages should indicate the following:
Temperature gage Zero temperature-> Reads Maximum temperature
Note: If gage does not conform to reading outlined in k and l above, replace gage in accordance with pertinent
technical manual. If gage readings conform with this test, proceed with steps m through r below.
m. Turn ignition switch OFF.
n. Remove jumper wire from gage and connect wire from sending unit to gage.
o. Disconnect wire from sending unit to gage at sending unit.
p. Turn ignition ON.
q. Ground wire removed from sending unit (F, fig. 1), the gage should indicate the
following:
Temperature gage Maximum temperature -> Reads Maximum Temperature.
If gage does not conform to reading outlined above, repair or replace wire from gage to sending unit.
r. Turn ignition switch OFF. Connect wire from gage to sending unit.
Note: Electrical gages indicate an approximate reading, for accurate readings, it is recommended using master gages
connected to vehicle.

Steps of the guide indicate a bad gauge. I am asking if this is a normally seen failure mode.
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wesk
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Joined: Apr 04, 2005
Posts: 13735
Location: Wisconsin

PostPosted: Mon Sep 24, 2018 5:23 pm    Post subject: Reply with quote

There are 3 common failure modes:

1 - gage reads zero all the time
2 - gage does not read actual temp
3 - gage reads max scale all the time.

Over the last 40 years I have seen many always pegged gauges.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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