Zero1Niner

I have a 1999 Accord EX Coupe V6 with cooling fan issues.

Fans come on when AC is on, but nothing when AC is not on.

Even when AC is on, the car overheats.

I followed this helpful troubleshooting guide, and here were my results.

1) Checking the Radiator Fan Relay Power Circuits.
Removed the radiator fan relay from the under-hood fuse box and verified that circuits 1 and 3 have power. They both have 12V.

2) Bypassing the Radiator Fan Relay.
After confirming that the radiator fan relay is getting juice, I jumpered circuits 1 and 2 with a jumper wire and the radiator fan motor comes on.

3) Bypassing the Radiator Fan Switch.
I removed the radiator fan switch harness from the coolant temp switch, and shorted the harness with a jumper wire to see if the radiator fan motor comes on (with key on engine off), and fans DO NOT come on.

Im not sure where that leaves me. Seems to point to something between the coolant temp switch and the relay, but I cant figure out what would be a good troubleshooting step to further diagnose this.

Can someone give me some suggestions on what I might do to figure this out?

Thank you.

tech8

You may be jumping the wrong temp. switch. See this other thread for info. on the correct fan switch A to jump.

https://honda-tech.com/forums/honda-...-help-3269335/

Zero1Niner

Tech8! You are incredible. Thank you. I was absolutely jumping the wrong one. Looks to be pointing toward the switch now.

THANK YOU!!

Zero1Niner

One more question. Since the car was fully overheating today, even with the AC running, would you guess the thermostat should be replaced? Seems like when the two fans were running with the AC on, the car should not have overheated, is that right?

holmesnmanny

Yes, that is correct.

The thermostat is bad if, when the car starts to overheat, or is simply warmed up all the way, the lower radiator hose is still cold.

Zero1Niner

So today I replaced the thermostat, and the fan switch, fired up the car, and the fans STILL did not come on, and the car started to overheat.

I pulled the plug off of the fan switch, jumped the fans again with a paperclip, and they fired right up.

I had a check engine light on for a while, but today I pulled the codes, and I have 4. Two of these certainly seem related to the problem.

P0117 - Engine Coolant Temperature (ECT) Circuit Low Input
P0118 - Engine Coolant Temperature Circuit High Input
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
P1456 - Evaporative Emissions Control System Leakage Fuel Tank

Im just starting to research the P0117 & P0118, but certainly related somehow.

Should I assume that new fan switch is bad since when I jump the connector it works?

holmesnmanny

Did you turn the ignition on without starting the car and jumper the thermostat fan switch connector with a paperclip? If the fans turn on then the fan switch is bad. If not then it's something else.

It's part 14 here.

WATER PUMP - SENSOR (V6) for 1999 Honda ACCORD SEDAN

Zero1Niner

Yes. Part 14 is the one I changed. I bought it from O'Reilly Auto (not sure if that was wise), and changed that switch as well as the thermostat (that part I bought from Honda for way too much).

When I jumped the fan, the key was in the on position, but car was off. The fans came on immediately.

So likely the NEW switch is defective?

holmesnmanny

Never buy any cooling system parts from anywhere but Honda, it's possible to get the wrong part, a defective part, or one that doesn't even last a year. It's not worth it.

holmesnmanny

I would take it back, try not to make it look damaged and say it was uninstalled. Then go buy one from Honda.

Zero1Niner

Ill do just that. Taking it out tonight.

Zero1Niner

These 2 engine codes seem to be pointing toward the ECT. Could that be causing the overheating/Fans not coming on?

Zero1Niner

Removed the aftermarket sensor, replaced it with a new Honda sensor. Still fan is not kicking on.

So to recap, I replaced the thermostat and the fan switch. I did all the troubleshooting and determined that the fans are working.

I turn the car on, it warms up, then keeps getting hotter until its on the H, and the fans dont kick on. I can turn the AC on, and the fans kick on, but not seeming to drop the temp at all.

Im clueless as to where to go from here. Could my brand new Honda thermostat be defective? Is there something else that I should be investigating?

Dr. Mike

Just out of curiosity, how good does the exhaust flow feel at the end of the tail pipe?
The reason I ask is because of the P0420 code you're getting " Catalyst System Efficiency Below Threshold (Bank 1)" If the catalyst is stopped up the engine will run hot to the point of doing severe damage in some cases.

Possibly a stopped up radiator as well ..... ?

Zero1Niner

Pressure seems fine from the exhaust from what I can tell.

I am feeling the hoses get warm. The top hose is more warm, then bottom is less warm, but both getting warm.

Dr. Mike

Maybe pull the thermostat back out of it, run it without & see what happens on the heat problem.

Zero1Niner

Thank you Mike, I was just going to try that out of frustration. Great suggestion.

Zero1Niner

Completely removed the thermostat, and it still overheated, and the fans never came on.

Clearly this should not be happening, right? I just turned on the car and let it idle, 15 minutes or so, and the temps just keep rising. Should that be happening with no thermostat? Or is it just that there is no air at all passing over the radiator to cool it down?

Seems like the gas smell is stronger than I remember previously. And also seem like the gas mileage in the days leading up to this was particularly bad.

Anyone have any suggestions what I can start troubleshooting next?

Zero1Niner

I found a service manual for the Accord, and I went through these troubleshooting steps and got the listed results.

Zero1Niner

I see there are a few other sensors that might be related to the coolant temp. Should I start looking at these? What about the ECT sensor? Could that be causing the overheating, and/or the fan not coming on?

Dr. Mike

With the thermostat removed & the radiator cap open/off ... & the radiator filled to within 1 -2" from the top .... you should be able to physically see the coolant fluid current moving at a swift rate ... can you see the flow ? I'm more interested in knowing the mechanical parts are doing as intended right now.

Zero1Niner

Hey Mike,

Its really hard to see the fluid. I cant say for sure. But looks to be moving through.

Im starting to think maybe I have two (or more) problems going on here. The fans are not kicking on unless the A/C is on. But once I let the car get really hot, and I kick the AC fans on, its not quickly cooling the car down. I would think it should cool it down quickly. Is that correct?

I tested the new thermostat. It opens and closes as it should. I also tested the fan switch (actually, I tested both). Switch is open when cold, and around 200* there is continuity, when it cools down its closed again. So Im pretty sure I just replaced two parts that were not causing my problem.

Is it possible that the water pump is going? But that wouldn't prevent the fan from coming on.

Thanks for the help.

holmesnmanny

"couldn't quite figure out what to do here" - It's asking you to check for continuity from the 1 side of the connector to ground(basically the negative battery terminal or any known metal connected to the chassis

"stopped at 2" - you have to find the ecu for your car and disconnect the 24 pin connector B

redo your testing and report back if you have any other questions

Dr. Mike

The fan should come on whenever the ac is turned on plain & simple ... or at least shortly afterwards to help prevent overheating from ever happening.

The reason I asked if you checked FLOW with the thermostat removed & radiator cap off is to make sure the coolant is circulating thru the system & it can't be done when it's pressurized.

Barring a head gasket failure or a cracked head a cooling system is relatively simple & only needs to be full of coolant, have a good thermostat that opens at the correct temp, a radiator cap that seals correctly @ the correct lbs. of pressure & not be full of air pockets.
The pump has simple blades designed to spin & pump fluid in one direction & a seal around a weep hole that usually goes bad & shows itself by leaking. Other than that the electric fan simply comes ON when the ac is turned ON to help the cooling process when the ac compressor adds an extra load to the engine or when a heat sensor say's the temp has reached a certain threshold.

It's a matter of ruling things out, got good flow thru the radiator etc. ?

Zero1Niner

Is this my ECM/PCM? Its under my center console, and Im accessing it from the passenger side. Which connector should I remove.