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Jeremy, Certified Truck/Bus Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 3304
Experience:  ASE Med/HD Truck Technician, NATEF Diesel Engines and Electrical/Electronics Technician, EPA 608/609 HVAC Certified, Steering/Suspension and ABS/Brakes Technician
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I have a 2005 Isuzu NPR 6.0 EFI. I've owned the truck years

Customer Question

I have a 2005 Isuzu NPR 6.0 EFI. I've owned the truck for 3 years and within a month after I bought it the temp gauge pinned as high as it could go. The truck wasn't over heating, the gauge just spiked and has stayed there ever since. As soon as I put the key in the ignition the gauge pitches right up to the H. So far I've changed the coolant temp sensor check all fuses. Pulled the gauge cluster out to verify it was clean and all wires are solid and they are. I also looked over the wires coming off the sensor itself and all looks fine. Idk what to more I can check. 3 years of not knowing my engine temp is stressful considering my truck is my job. One last thing the truck has 185k on it but the problem started at 110k
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Jeremy replied 1 year ago.

My name is ***** ***** I will assist you.

When you check the voltage coming off of the 2 wires going to the gauge, what is the reading you are getting on them?

Have you traced the wires at all to make sure the is no rub through on the wires?

Have you tested the gauge using a dial knob switch and putting direct power to it rotating the switch to see the gauge go up and down?

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Customer: replied 1 year ago.
On the back of the gauge "cluster" its more like a automobile. No specific wires going directly too the gauge itself just one big circuit board with three several wired plugs that go into that. Im confident that the gauge is getting power but its giving me a temp reading of redhot. One other thing. When I changed the temp sensor the fan now runs continuous. Now I know this is a secondary problem caused by the cpu thinking the engines running way to hot but prior to the sensor being replaced the fan never came on. So idk if this might have been a combination with the temp sensor accompanying the problem with the gauge
Expert:  Jeremy replied 1 year ago.

Which sensor did you change out? There are 2 sensors for the coolant. On on the front and one on the back of the engine. I will uplad the pics of them both.

The one on the front, right head is the one for the fan. It is the one that sends the signal to the PCM to turn the fan on. The one on the back of the head is for the gauge only. There may be a counter with one working and the other not causing the signal to be off or an issue in each circuit.

I can load up more info on each if needed. I have full Isuzu access. Everywhere there is a box on the diagram, I can pull up the plug pin chart info if needed.

Check the main wires at these 2 sensors and see that you have the proper voltage of at least 5, 9, or 12 volts and ground return. Do no jump the wires for the PCM for it doesn;t get the full voltage back. It only gets a partial and you don't want to pop the PCM. Use a digital multi meter for the reading across the pins.

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