I am currently seeing a red engine light on the highway with the engine warm and ac blower on. When I turn the blower off the light goes out. Water temp gauge if around the left marker (the mark left of the N in normal). Sender measured 30ish ohms after drive. Thermostat housing was 215 with temp gun. Oil gauge reads in the middle and doesn’t flicker. When the light comes on it goes out instantly when I turn the HVAC to off. Light comes on in vent and max. No other positions tested yet.

Does anyone have a diagram that might show how the blower might affect the engine warning light? Are there any tests for the light module to diagnose it?

1989 F250 460 C6
No ECU codes
Runs and drives good
The light worries me.

Thanks

John

 

215 T-Stat temp has me thinking the truck has a cooling system problem.

 

215 isn't scary high. Just looked at my 96 F-150 it has all the gauges and a red battery and brake light and a yellow check engine light. I remember those red engine lights. Sometimes they were for multiple things. Are you running the a/c also? I think I would check the charging sys. first. If your lucky enough to have a owners manual it will tell you exactly what it's for. You didn't say if you have a volt gauge, if not I bet that is what it is.

 

Possible causes of the warning light

• Low, leaking or old coolant
• A defective thermostat that is stuck closed
• A bad water pump
• Radiator is clogged with debris
• Radiator hoses have collapsed
• Radiator cap is worn out or defective
• Blocked coolant passages inside the engine
• Cooling fan has failed
• Low oil level
• Blown head gasket
• Bad temperature sensor, producing a false warning

 

Thanks for the replies. The warning light is for high temp and or low oil pressure. Since mine has the switch drive gauge for oil my guess is that it would only come on with oil pressure below 4-8 psi.

I have done a little work to check the temp sender. Looks like it should be 74 ohms cold and 9.8 ohms hot. I found a post that ball parked cold at 130 and hot at 250. When I substituted a 10 ohm resistor for the sending unit the gauge showed midway between C and the N in normal. At 32 ohms it showed a needles width left of the C line. This is where it runs at idle when warm. Temp gun shows 210 at water neck. 205 at radiator inlet. 190 at radiator outlet. I think my gauge is bad. I also think it is overheating when the warning light comes on.

The radiator stays full. When cold it will be between the overflow hose and the top of the core.

When warm (200) I revved the engine and the level dropped down to near the top of the core. I never did see strong flow or motion/turbulence. I kind of expected to see flow.

I think my next step is to pull the thermostat. If not that then how do I check the radiator and the water pump?

Thanks

John

 

No joy on the thermostat. Removing made no change. Eight miles at 70 mph with the ac one brought the light and a creeping temp.

I did see the coolant flow through the radiator as I was topping off after pulling the thermostat. Looks like good flow.

I think I have no leaks, proper level, good coolant.

I think I have eliminated the thermostat.

Waterflow looks good.

Hoses are tough to check. Not sure how.

Water pump issue as highway speeds is possible.

Radiator may be bad/insufficient. Not sure how to check.

Would a head gasket cause this if the coolant stays in the system? I don’t think I have lost any.

Thanks

John

 

Replace the radiator. Head gasket no.

 

Did you install a 195 degree thermostat?