1990 Honda Accord radiator fan is not running, overheating in traffic

This summer has been a hot one and I noticed that my 1990 Honda Accord began to overheat while crawling along in traffic. A quick triage revealed that the radiator fan was no longer running at all. In the end, the fix was to bypass a broken wire in the main wiring harness. More details below.

This article on troubleshooting Honda overheating problems is a great place to start, since it provides a fault isolation workflow.

Another great place to start is the Honda factory manual.  The cooling fan troubleshooting starts in two separate places, page 15-47 in the air conditioning section and 16-103 in the electrical section.

I was able to quickly determine that the fans were not at fault by connecting battery voltage to them. However, jumpering the coolant temperature switch pigtail at the thermostat body with the ignition on did not cause the fan to run.

I swapped the cooling fan relay (in the underhood relay box) with the identical power window relay which I knew worked. The fan still did not run. I could not hear the relay click when plugging it in.

I checked the voltage at the relay coil terminals with the key on. There was less than 1 volt. I compared to ground and both had battery voltage.  That seems strange but the fan timer module is connected to both the source and drain side of the relay coil, so this state at least makes a degree of sense if the fan timer module is driving both high.

However, the blue wire at the coolant temperature switch pigtail at the thermostat had no continuity to either relay coil pin in the relay socket. (Don’t confuse this pigtail with the pigtail at the water outlet on the front of the cylinder head.)

Using the schematic in the Honda manual, I checked that the blue wire on the fan timer module (under the dash) was connected to the blue wire of the relay coil in the C254 wire harness connector, using a long wire from the connector so that I could reach it with my meter from the engine bay.  It showed continuity.





After checking and re-checking, I determined that there was a wire break somewhere between the relay box and the pigtail, since there was no continuity and no voltage.

I used a 22-16AWG weatherproof butt connector to create a three-way splice under the relay box.  I severed the blue wire on C254 and spliced a new wire onto it which I then spliced into the blue wire of the coolant temperature sensor pigtail at the thermostat.

After splicing, I was able to activate the radiator fan by jumpering the pigtail, as expected.  Everything works now!

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