1999 Accord V6 overheating mystery

My 1999 Accord V6 with 180k miles has started overheating. The fans will not run unless the A/C is on. I replaced the coolant temp switch A, because the fan came on when I jumpered the pins of the fan connector. The fan will not come on, even when the temp gauge goes all the way to the top. Jumper the pins or turn on the climate control, the fans kick on, and the temp drops. Tested the old temp switch in boiling water and it tested fine. Tried spreading the pins on the temp sensor in case the connection was just bad and that did nothing.

Otherwise, the system seems to work fine. Top coolant hose gets hot first, then bottom gets hot after 10 minutes. The heat works, and I didn’t see any signs of exterior leaks. What else might keep the cooling fans from coming on?

The problem could be with the radiator fan relay or with the radiator fan control module.

Try swapping the condenser fan relay with the radiator fan relay to see if the fan runs. If not, then problem could be with the radiator fan control module.


Since the fan comes on when you disconnect the connector and the contacts in the harness side are jumpered, that’s a pretty good clue there’s something wrong with the switch still. Just b/c you have a new one doesn’t guarantee it is working. hmmm … so you put the switch in a pot of hot water and it closes? That’s a pretty good test of the switch. How hot does the water have to get, on my Corolla this switch closes at about 190 deg F. Same for you? What is the resistance you measure when the switch closes in your test set-up with the hot water in the pot? It should be very low, less than 0.1 ohm.

It seems that if the switch closes at 190 deg F and you measure the switch resistance to be 0.1 ohms or less, then the fan pretty much has to turn on. The only reason it wouldn’t – since you know it turns on when the switch is jumpered – is that the switch contacts aren’t making a good electrical contact with the harness pins. Wouldn’t be unheard of for a connector like this to have corroded pins, might see if some contact cleaner would help. Look carefully with a magnifying glass and good lighting at the contacts on both sides too.

Sometimes you can’t see the contacts well enough to tell if they are corroded or not. But if I still thought they might be, I’d probably cut the connector off and just use some solder-less spade connectors temporarily to see if then the fan will come on when the coolant gets hot. If so you know you need to replace the OEM connector.

Tester, I saw that the two relays were the same, so I switched them and still had the same issue.

GeorgeSanJose, the switched closed at 202F and opened again at around 192F. I was reading 0.6 ohms, which I thought was good considering that I’m getting 0.4 when I touch the probes together. I’ll try cleaning the contacts and see if that does anything.

The temps for opening and closing seem about right. But 0.6 ohms when closed is too much for a new switch. You may not be making good contact with the switch though. Might want to repeat that experiment.

I agree with @GeorgeSanJose. There should not be an ohms reading. Especially when you touch the probes. Both of my meters read zero with probes touched. I’ve tested many switches, and the good ones show zero ohms, even when my meter can read milli-ohms.

Just for an experiment, when the temp gauge goes hot, pull over and shut down the engine to see if the fan comes on. It should come on within 30 seconds of shut down, key in off position or removed from the ignition.

Don’t know about Accords, but for my Corolla, the key must be in the “on” position for the fan to spin. The radiator fan won’t spin with the key removed or in the “off” position. Varies from car to car. If I remember correctly, on old VW Rabbit, if the coolant was hot enough to set the temp switch, the radiator fan would spin even with the key removed.

One thing OP could do as a diagnostic in any event, if OP thinks the engine is hot enough that the fan should be spinning, remove the connector and measure the resistance between the pins of the fan’s temp switch.

Accords have a 10 minute timer after the engine is shut down that provides power to another temp sensor with a higher set point than the one used for when the engine is on. If the coolant around the second sensor goes above the set point, which it often does about a minute or two after shutdown, then the fans run.

The reason I suggest this test is to confirm if the engine is really overheating or if it just has a sensor issue.

Update: Sunday I decided to clean the contacts on the connector as @GeorgeSanJose recommended. I did that as best I could considering the connector has only about 2" of wire on it. When I started up the car, I noticed a small amount of coolant leaking, which hadn’t happened before. I thought I just needed to re-torque the temp sensor, but that was fine. As the engine warmed, it started leaking more and it was apparent that it was coming from the coolant return hose. The car is now 15 years old and I can’t recall if the coolant hoses were ever replaced, so I figured I’d switch them out. When I got them off, I noticed a lot of corrosion on the thermostat cover and when I cleaned it off, it was pitted and it looks like it’s very close to going all the way through. So I’ve got that part on order and will try putting it all back together when that comes in. It would be great if I was just getting air in there that was causing all this.

The temp switch is in a difficult location for me to get to while the battery is in and the engine is running. Originally, I fashioned a connector with extension wires but I never got continuity, so I just assumed that I wasn’t able to get them connected properly. This time, I’ll get them on before I put the battery back in and hope for a better test.

It always bugs me when a post leaves you hanging and you never find out what the final problem was, so here’s another update. I still don’t know what the final problem is. But I’m getting closer.

It seems as if the cooling system is all falling apart at once and has multiple issues. Coolant hoses and the thermostat cover were replaced since my last post. I was still having the temp gauge go into the red zone, but I’m still leaking coolant. More and more each day. I did get a laser thermometer and nothing on the cooling system indicates that there is actually an overheat problem. 175 - 177 degrees is what I’m seeing. I decided to get the leak looked at first by a professional.

I learned that I was leaking coolant from the water pump, and the water pump is on the timing belt, so that was a nice little chunk of change to have all that repaired.

The next day, the temp gauge goes into the red zone again on the way to work. After sitting for 9 hours on a high 50’s day, the temp gauge pops up to 1/4 way within 5 seconds of starting the car. I stopped by the mechanic on the way home. He checks and says that I’m definitely not overheating, so he thinks it’s the temp sending unit or the associated wiring. I’ll take it back and have them put in a new sensor and see how that goes this weekend.

Thanks for posting the updates. You’re right that it’s always better to hear back about how things proceed and what ends up happening, so it is appreciated. I’m wondering, btw, if it isn’t time to check for exhaust gases in the coolant. This can leave you with hot pockets (thus having the sensor and independent thermometer reading different things), and create higher than normal pressures in the cooling system and set things to leaking. I’m also wondering whether you’ve tossed a new radiator cap on it at any point.

+1 on appreciation for updates. Memories…I had a '77 Fiat with a cooling fan that ran after taking the key out. One day, I was walking away from the car and a woman nearby yelled, “Sir, sir, you’re engine’s still running!” My first thought was that that fan motor had almost as much power as the 1.3L engine.