Car Overheating -- troubleshooting cooling fan

OK, so I’m trying to track down this cooling fan problem. When I turn the a/c on, only the passenger side fan comes on.(People assure me the driver’s fan is supposed to come on to. Can somebody confirm this is true? I have a 96’ Honda accord LX)

Here’s what I’ve done so far:

  1. Direct power to cooling fan(driver’s side) works
  2. Swapped relays(both turn on a/c clutch)
  3. Tested relay wires with test light(Bright as battery, 12v I assume)
  4. Tested thermostat sensor A/B. The one behind the engine is so dim I thought it wasn’t getting any power and the thermostat in front of the engine(top radiator hose. Green bulbous connector) was dim yet slightly brighter.
  5. Tested the wire that runs from atop the compressor to the fan with test light. Looks like it’s receiving no volts.(but I’ll double-check)

Where do I go from here? Pictured is my multimeter that I’m not 100% confident I’m using right.(I tested the battery and got 12 volts)

*Also, there was some green corrosion on both the brass pieces of the thermostats. Does that mean the wire before the connector is the culprit?

Both fans should come on with the a/c running. The temp sensors are by-passed when the a/c is on, so the circuit problem is somewhere else. Start at the fan relays and check that the circuit to the driver’s side fan is working. Check to see if the a/c circuit is activating the relay. Let us know.

The circuit? Are you talking about the fuses? I checked all those and they are all good.

Since there is low voltage on the connector wires to the thermostat I’m probably looking for some corrosion somewhere, right? Popular mechanics said that’s the most likely cause.

Possible corrosion. But, by checking the circuit, I mean check the impedance between the relay and the cooling fan connector. This checks the wiring and the connection ends for possible open circuit issues. If the ohm reading is less than infinity, there is a circuit problem.

ok, I now what know the ohms sign is, I have that on my multimeter-----------Ω – ohm’s symbol

Ok, I tested the connector by placing the positive inside the connector going to fan and grounding it. That’s just a voltage check, right?

Which connectors do I need to put which side in to test the circuit? (Also should I do this with the car on accessories II?)

Ok, good news I’m not an idiot. My voltage check wasn’t reading right because the main ground off the battery cable to below the engine was corroded as all get out and the wire insulation was exposed and fraying!. I did the laborious task of getting it off, cleaning off the corrosion with a wire brush, snipping of the extra wire, wrapping it in half of a lb of electrical tape, bolting it back on to the ground point and now I’m picking up DC voltage on the thermostat sensors, instead of just the light. Infinitely more helpful!

Well, the lights speak to the voltage. Thermostat A(behind engine) had a whooping .4 volts at first and at second test it came all the way up to .8 something(Woot.) Thermostat B(in front of engine) was testing at 10.58 volts…not too bad but still low.

The positive battery terminal is in okay condition. I’ll probably clean off the wires that bolt to the clamp on the battery. I thought about replacing it, that one looks tough to replace and I don’t really feel like paying the dealer $70 for new battery cables. I probably won’t replace it unless I can get it used.

Updated: I tested the connector that go’s to the driver’s side fan. Seems now that I have a proper ground from the battery it’s reading at a steady .4 volts, the same as the thermostat A, which I think if I’m not mistaken controls both fans. I shorted this days earlier and the passenger side fan came on, but the driver’s side didn’t.

The wires to the relays both read 12 volts, but the connector to the fan is getting .4 volts. Am I missing something?


This is why you need to check the circuit. There is a problem between the relay and the fan connector if the voltage drop is that much. Did you try running the 12V power through the relay connector with a jumper wire? You may need to run a new wire from the relay to the fan if you cannot isolate the circuit issue. This is assuming the relay is not the culprit.

Most likely the problem is with the fan itself. I had this same problem a few years ago with our 97 Accord and it was the fan motor. The passenger side fan is the radiator fan and is supposed to come on both by coolant temp and AC on. The drivers side fan is the condenser fan and come on only with AC.

The radiator fan relay is in the underhood relay panel. On the drivers side of the radiator, on the frame near the top are two relays, the relay towards the rear of the car is the compressor relay, the one towards the front is the condenser fan relay. Remove the plug.

Look for a blue/yellow wire and white wire. Short the two pins together and the fan should run, even with the ignition switch in the off position. If it does not, the fan is bad. Be careful, there is a yellow/white wire, do not mix this one up with the white wire.


I shorted the relay wire to the the driver’s side fan and it came on! Thanks so much! That’s so weird that it’s the relay, when both were able to turn on the AC clutch…real head scratcher

A brand new relay is only $31 at autozone. As for which pin is which. looking at the picture in your post, A is the upper right, C is upper left, B is bottom right and D is bottom left. That’s as the picture is oriented in the post. In other words, as viewed



If you are going to test it out of the car, you need to apply + to D and - to B, then check for continuity A to C.

If the relay checks good, then turn on the ignition and the ac and check for 12v at the yellow/white wire, pin D on the connector.

ok, it’s not the relay, it couldn’t be the relay because both switch on the ac clutch.

In better news: I repaired the small leak in my AC system. I had an o-ring that was about flat as a pancake on the high pressure side(Which makes sense because high pressure does tend to flatten things!). Since I had it off I went ahead and replaced the o-ring on the low pressure side. I also added some pag oil to live up the old stuff. It’s funny my gas mileage has improved ever since we re-filled the refrigerant in my AC 2 weeks ago. I wonder if that AC clutch wasn’t dragging on the belt a little?

More good news: The cooling fan is coming on when the car gets warm, so that’s good. Any ideas of what else it could be? Thermostat A is working because I had thermostat B(in front of engine) unplugged and the cooling fan came on. Does thermostat sensor B control the condenser fan?

There are two relays in the location to the right of the top tank of the radiator, one for the compressor clutch and the other for the condenser fan. So it can be the relay, but if it tested OK, then check for 12 volts on the yellow/white wire, pin D on the connector. It comes from the radiator fan control module located under the dash.

There are no sensors for the condenser fan, only for the radiator fan. However, there are three fuses that go to the radiator fan module. Fuse number 34 on the underhood fuse relay block, not likely the problem, fuse number 4 on the under dash fuse/relay panel, also not likely, and fuse number 8 on the under dash fuse/relay panel, likely if it is a fuse problem.

ECT switch B turns on the radiator fan for 15 minutes if the coolant temp goes above 223F when the engine is off. ECT switch A turns on the radiator fan when the coolant is over 199F and the engine is running. The radiator fan module is located behind the glove box.

Let’s be clear what is now happening.

The car has two cooling fans to the radiator, a primary and secondary.

The primary fan comes on when the engine gets warm as it should.

Only the secondary fan comes on with the A/C on. The primary is not.

Relay to the primary checks out

Circuit from the relay to fan checks out.

Check now to see if the primary fan relay is getting signal to turn the primary fan on when the A/C is on. It should, but I fear it is not. Now, you need a wiring diagram to determine how the relay is wired between the temp sensors and the A/C circuit and probe to find out the failure in the system.

The cooling fan or the primary fan is coming on with the A/C and when the AC is not. The cooling fan is the passenger side one, right?

The driver’s side fan, what I think is the condenser fan isn’t coming on with the AC on.

Both thermostat switches are opening at the correct temperatures.