Hey folks. Here is something Redbull-1 and I wrote up a few months back.

I never got around to posting it as well; my life became hell shortly after writing this up. Many of you know that and I’d like to “Thank all of you” for the words and thoughts. Cam, I’d like to thank you as well for the advise you gave me, coming here and trying to help others is the only thing close to normal left in my life. So I’d had forgotten all about this until today when I was reading something.

Redbull-1- Thanks for your help with not only this but letting vent/rant in PM’s and that.

So here it is: Mod’s- please feel free to edit the content directly in this post that way we might end up with one post explaining the operation. This then might be able to “add” to what Jim already has written up.

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4th Gen Accords (90-93).

First a few assumptions. I’m using the diagram from this 91 Accord (4th Gen), others are similar; but, may differ slightly. To start with we will assume we are looking at this when the AC is off and ignition is ON/position II.

++++I’ll start with the Radiator fan relay which controls the “Radiator Fan Motor”.
The Yellow wire should be hot (+) going into the relay, this comes from the Ignition switch, thru the under dash fuse panel (#8 - 7.5 amp). The *White (*=assuming color based upon memory/history-not listed in the diagram) wire should also be hot (+) at all times since this is coming from the under hood fuse panel (#39 - 15 amp).

When Coolant Temp Switch A (Thermo-switch A located on/around the t-stat housing, lower radiator hose) gets to around ~194*, it closes allowing ground to be passed from ground location G101 thru the switch to the Blue wire into the relay. Thus creating the ground to the coil/magnet inside the relay (+ is from the yellow noted above). Once that coil is activated it pulls the relay closed. This allows the power (+) to flow from the *White wire into the relay thru to the BLU/BLK to the actual Radiator fan motor. Ground for the fan motor is thru G201. So you now have 12v in (BLU/BLK) and ground (BLK) at the motor.

++++So now to the Condenser fan motor relay….nothing has changed from the above conditions/assumptions.
The YEL/WHT wire should be hot (+) into the relay, this comes from the fan controller. The White is also hot (+) at all times into the relay since this comes from the under hood fuse panel (#29 - 15amp).
With the Coolant Temp switch closing at ~194*, it allows the ground to flow to the Blue wire of the relay. Thus, closing the relay which allows 12v to flow from the White wire to the BLU/GRN that goes to the condenser fan motor. Ground for the motor comes from the Black wire and ground location G281.

So when the coolant temperature at the thermo-switch A is ~194 degrees; both radiator fan and condenser fan should turn on. You can also test whether your two fans work, first with the engine off, disconnect the green connector on Thermo-switch A, jump the two terminals on the connector (not the thermo-switch) together with a small paperclip or wire, then turn the ignition to ON to see that both fans operate. If the fans works; but, they don’t come on when the coolant temperature is ~194 degrees, then you may have a faulty thermo-switch A.



-----------Changing the above conditions. Now the AC is on (assumption here is that you have a functioning AC System).
So the next question. How do they BOTH run when the AC is turned on? Basically, thru the same process. Instead the ECU is applying the ground thru the Blue wire which again grounds BOTH relays (radiator and condenser) allowing power to flow to the fan motors.

----------Changing conditions again-Now the key is OFF.
Then the last question I can think of. Why does only ONE fan run when the key is turned off? This time Coolant Temp Switch B (thermo-switch B-upper radiator hose) is “in play”. If Coolant Switch B detects temp above ~226* it closes and sends a ground signal to the Fan Timer thru the WHT/GRN wire. I’m not going to get into the internal workings of the fan timer (that is a can of worms in itself). But basically it sends a ground to the same Blue wire closing the Condenser fan relay.
You may ask why then doesn’t the Radiator fan relay do the same thing. Well, you have to remember that the 12v to the “coil” of that relay comes thru the Ignition switch which is off….therefore the relay will not close.

To recap: This is for cars with a working radiator fan and AC condenser fan. Whether the AC is on or not, if the coolant temperature reaches ~ 194 degrees and Thermo-switch A is working properly (switch on or near thermostat housing), both radiator and AC condenser fan should turn on.

When AC is on, even if coolant temperature at thermo-switch A is below ~ 194 degrees, both radiator fan and AC condenser fan will be on.

When the car is turned off, if the coolant temperature near Thermo-switch B is ~226 degrees, the radiator fan only should turn on and run for a short period of time.


5th Gens (94-97) operate with the same basic configuration/principal. Some of the wire colors are different (Ground to both relays-GREEN for example); fuse numbers and amperage, the temp settings on the coolant switches. If you can get a wire diagram for the fans this will help in following it and the operation.

 

nice write up
I just wanted to clarify about the one fan that runs after the car is turned off.
Its running when the car is off does not mean the car was too hot, correct?
My 97 runs the fan for a few mins every time I turn the car off and the car runs at a very normal temp.
That is correct, right?

 

Crispin-Running for a few minutes after the car is shut off, this time of year...hummm.

Mine and most will only do this in the "Summer" after a long drive. Don't stress, again if thermo B sees or detects temps at ~223* on the 5th, then the one fan will run when the car is off.....until the temp gets low enough or the timer "says" that
is enough.

So did someone replace thermo b with thermo a (turns on at ~199*). The fact it runs and then shuts off......again I'd move on but that is me.

EDIT: but that is long as you are sure there is not any air in the system.....

 

Are the 2 switches (thermo-A & thermo-B) physically interchangeable? Only difference is the higher setpoint? If so, that's good to know - for me that might be a suggestion when someone asks about it. (I don't own a Honda of that vintage any more)

Nice write-up.

 

Jim-I'm pretty sure they (thermo a and b) are interchangeable on the 90 thru 97.....can't say that for sure, just looking at them.....and as you stated the "set points" are different.

I'll take 85~90% credit for the write up, Redbull-1 gets the rest

Thanks

 

poorman212,

My overheating issues might be addressed by this great write-up, but I want to be sure I understand completely before jumping to conclusions. My radiator fan (one closest to the battery), kicks on when I turn the car off, which isn't a surprise, as the car has been running hot.

It is my understanding that when the A/C is turned on, BOTH fans will engage, and I observed this occur via my own testing earlier today. That eliminates the need to test the fans to the battery because I saw that they both work.

When I pulled into the drive this evening running hot, I popped the hood before turning the car off, and neither fan was running. The temp needle was moving further up, so I turned the car off, and a minute later the radiator fan kicked in.

I plan on flushing/filling the radiator and replacing the t-stat as you suggested in my original post, but after reading your write-up I wonder if I might have a faulty A switch. Thoughts?

 

Yes, kind of sounds like thermo a, on the t-stat housing (95% sure). If you have a volt meter you can test it. Once hot, 194*~196* it should "close"....continuity between the "pins" of the switch....if the switch tests good, jump the wires with the KO and see if the fans run...just to rule out the fan controller.

PS-Thanks as stated, Redbull helped...thinking of you, get well soon my friend.