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Cat Man
Cat Man, Caterpillar Engine Expert
Category: Heavy Equipment
Satisfied Customers: 8912
Experience:  23 Years Caterpillar Engine experience.
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1998 GMC: Liter..CAT engine..light stays..times the engine runs

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I have a 1998 GMC Topkick With a 7.2 Liter CAT engine. The engine heater light stays on entirely to long after initial start. At times the engine runs poorly during this time. While checking the amp draw of the heater on one occassion it was drawing approx. 120 amps. Which was more than the alternater could supply, thus pulling battery voltage down considerably. No one knows how much amperage the heater grid should draw. or why it stays on so long can you supply some testing specs?
Hi. Thanks for using JA! I will try and help you out.
The heater should come on every time the engine is started. The engine ECM checks the temperature of the engine coolant sensor and the intake air temperature sensor. The ECM adds these readings together. When the sum is greater than 119 degrees F, the engine ECM should turn off the heater. Most of the time I have seen the intake heater saying on, the problem is most often a faulty relay solenoid. It is located on top of the engine near the heater.
The next most common item item that causes it to stay on is a wiring problem in the harness that controls the heater replay. Basically, the connector for the relay always has a 12 volts supply. The engine ECM simply applies battery ground to turn the heater on. IF the ground wire from the ECM has rubbed and found it's own ground, it would keep the heater on. I would find the 2 wire plug to the relay and see if the ground wire is grounded.
The last possibility is that one of your sensors, either the coolant or intake air temp, are reading way to low causing the heater to stay on longer than it should. To verify, you would need to have someone connect a service tool like Cat ET to monitor the readings of these sensors. If they are reading wrong, they will see it.
Please let me know if you need more help or have more questions.
Cat Man and 2 other Heavy Equipment Specialists are ready to help you
Hey. I just wanted to se how things were going. Please let me know if you need more help.
Customer: replied 5 years ago.

I was wondering if you had specs for normal initial amp draw for the heater say @ room temp and do you have a voltage vs. temperature chart for both the Air Intake sensor and the Coolant Temperature so the temp of the sensors can be checked with a voltmeter as I dont have a scanner. Thank you!


Michael Staples

I do not have a voltage vs temperature chart for the sensors. Sorry. I wish Cat made one but they do not for truck engines.


They also do not give a spec for how many amps the heater draws. But there is a way to find out by using Ohms law. To find the amps, all we would need to do is measure the Ohms of the heater apply 12 volts divided by how many Oh,s we got when we measured it.


I do not have a good heater that I can measure the Ohms for you or I would.


However, the main problem that heaters ever give is on open circuit and fail.