1998 Chevy Truck temp gauge shows over heat

Tiny
VERNON JAY
  • MEMBER
  • 1998 CHEVROLET TRUCK
  • V8
  • 4WD
  • AUTOMATIC
  • 200,000 MILES
The gauge shows overheat once normal tempature
is reached.
I have checked with michanical gauge and temp.
is normal.
I have changed the sensor more than once.
I have also changed the gauge cluster.
There seems to be no short or voltage drop.
I have tested the gauge at 1300 ohoms ( cold )
and at 50 ohms ( hot ) everything seems to be fine.
When back testing the circut there is no resistance
When the truck is cold the gauge works fine untill
the temp. Starts to rise.
I am out of ideas!
Do you
have the same problem?
Yes
No
Friday, February 5th, 2010 AT 11:40 AM

11 Replies

Tiny
FACTORYJACK
  • EXPERT
What happens if you create another ground at the sender. And which sensor have you changed. The two wire sensor for the ECM, or the one wire sender near the #1 spark plug?
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Saturday, February 6th, 2010 AT 2:15 AM
Tiny
VERNON JAY
  • MEMBER
It's the sending unit for the gauge by the #1 plug
I have changed.
I also have tried grounding at the connector for the
sending unit and the gauge tops out on the hot side,
(which I think is normal).
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Sunday, February 7th, 2010 AT 4:02 PM
Tiny
FACTORYJACK
  • EXPERT
Open circuit would give you full cold, and shorting the sender wire should send the gauge full hot. See if you can get a resistance value of the sender when at operating temperature, and we can translate that to a temperature, to see what the sender is actually reading. Compare that to a thermometer reading if possible, for cross checking. If the senders resistance value corresponds to the correct temperature, then the problem would have to be in the wire or the cluster. It is a simple circuit, I don't see what else would influence it. It would be interesting to know as well, what temp the ECT sensor for ECM is registering on the data stream. If you have a scan tool, you may be able to view it.
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Monday, February 8th, 2010 AT 12:31 AM
Tiny
VERNON JAY
  • MEMBER
Yes, open circuit does give me full cold and closed
full hot.
When motor reached 165 degrees the gauge
showed full hot and the sending unit showed
89 ohms.
What confusses me is I have tried changing the
cluster and also the gauge.
I am going to try and test the ECT sensor temp.
And get back to you.
Please tell me if you think of any other test or ideas.
Thank you!
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Monday, February 8th, 2010 AT 6:29 PM
Tiny
FACTORYJACK
  • EXPERT
Here is what I have for some more info, The temp vs. Resistance table didn't copy/paste too well, hope you can decipher it. One thing stands out, you say the engine reached 165, but the gauge showed full hot, and the sender read 89 ohms. On this chart, 89 ohms would put you at about 250 F, which is overheat. Just out of curiosity, where are you taking your raw temp at?

Temperature Versus Resistance
C
F
OHMS

Temperature vs Resistance Values (Approximate)

150
302
47

140
284
60

130
266
77

120
248
100

110
230
132

100
212
177

90
194
241

80
176
332

70
158
467

60
140
667

50
122
973

45
113
1188

40
104
1459

35
95
1802

30
86
2238

25
77
2796

20
68
3520

15
59
4450

10
50
5670

5
41
7280

0
32
9420

-5
23
12300

-10
14
16180

-15
5
21450

-20
-4
28680

-30
-22
52700

-40
-40
100700

Engine Coolant Temperature Gauge Inaccurate or Inoperative
Step
Action
Value(s)
Yes
No

1
Did you perform the Instrument Cluster System Check?
--
Go to Step 2
Go to Instrument Cluster System Check

2
Disconnect the connector at the Engine Coolant Temperature Sender.
Connect one red lead of the J 33431-C to connector cavity A of the Engine Coolant Temperature Sender connector.
Connect the other lead of the J 33431-C to ground.
Turn the ignition switch to the RUN position.
Adjust the resistance dial to 1305 ohms and then to 49.5 ohms.
Does the temperature gauge match the values given?
1305 Cold, approx. 100 F (Gas)/160 F (Diesel). 49.5 Hot, approx. 260 F.
Go to Step 3
Go to Step 4

3
Replace the Engine Coolant Temperature Sensor. Refer to Engine Coolant Temperature Sensor Replacement in Engine Cooling.

Is the repair complete?
--
Go to Instrument Cluster System Check
--

4
Remove the I/P Cluster.
Connect a B+, 10 amp fused jumper wire to cavity 22 of the I/P Cluster assembly.
Connect one red lead of the J 33431-C to cavity 13 of the I/P Cluster assembly.
Connect the other lead of the J 33431-C to ground.
Adjust the resistance dial to 1305 ohms and then to 49.5 ohms.
Does the temperature gauge match the values given?
1305 Cold, approx. 100 F (Gas)/160 F (Diesel). 49.5 Hot, approx. 260 F.
Go to Step 5
Go to Step 6

5
Locate and repair the cause of resistance in CKT 35 (DK GRN) between the I/P Cluster and the Engine Coolant Temperature Sender. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?
--
Go to Instrument Cluster System Check
--

6
Replace the I/P Cluster. Refer to Instrument Cluster Replacement.

Is the repair complete?
--
Go to Instrument Cluster System Check
--
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Monday, February 8th, 2010 AT 11:04 PM
Tiny
VERNON JAY
  • MEMBER
Sorry, I haven't gotten back to you for a while.
I made a mistake it was 389 ohms.
I do not have a J 33431-C. It's only
a dealer item. I was wondering if you could tell me
where cavity 22 and cavity 13 are.
They aren't numbered on this cluster.
Also, would you have any ideas where a short
might be?
Thank you.
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Friday, February 19th, 2010 AT 3:15 PM
Tiny
FACTORYJACK
  • EXPERT
Pins 18 and 22 are meant to be power and ground to the cluster for use of the tool. Actually, there is an error somewhere cause the connecto view for the cluster lists pin 18 as water in fuel indicator for diesel. The cavity you are concerned with for the sender is 13 dark green. There are two dark green wires next to each other, pins 13 and 14. The easiest way to describe the location in the connector, is they are two pins in from one of the ends, and the two pins are 15(tan), and 16(purple). There are numbers on the body, they are just about microscopic.

16(ppl)15(tan)14(dk grn)13(dk grn)
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Saturday, February 20th, 2010 AT 12:27 AM
Tiny
VERNON JAY
  • MEMBER
I guess I don't know what you mean by this pin 18,
you never said anything about it before.
Is this the connector on the back of the cluster?
Is there any thing else I can test?
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Saturday, February 20th, 2010 AT 2:05 PM
Tiny
FACTORYJACK
  • EXPERT
Sorry, I don't know where pin 18 came from either. Yes this is the connector at the back of the cluster. The only other thing I can think of is running a wire from the sender, to the cluster outside the harness and tapping it to pin 13. Maybe the wire has a fault in it. The temp gauge is the only one with issues, is that correct?
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Wednesday, February 24th, 2010 AT 12:41 AM
Tiny
VERNON JAY
  • MEMBER
Hello, sorry it's been a little while.
Now the gas guage doesn't work.
What should I do next?
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Friday, February 26th, 2010 AT 9:39 AM
Tiny
VERNON JAY
  • MEMBER
Hello, sorry it's been a little while.
Now the gas guage doesn't work.
What should I do next?
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Saturday, February 27th, 2010 AT 10:00 AM

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