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Old 06-22-2010, 07:27 PM   #1
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Understanding how your Thermo cooling system works

Understanding how your Thermo cooling system works

Ok so I have been searching and searching and have to go through many threads to figure out my electrical fan problem and cooling system.
This is for a 1988 prelude 2ws B20A5
This thread should be your one stop fix thread. Majority of tests ran with the vehicle in the ON position not running.
There is also 3 thermo sensors that are located on my car and the wiring and system diagram I have only show 2 so for my thread purposes I labeled them my way.

I am not an expert on preludes so feel free to correct me or even add to this discussion. All prices are not exact just a estimate.

Always start withe easiest to check. fuses and relays and battery connection.


Condenser Fan Relay (under hood)
Radiator Fan Relay (under hood)
Fan Timer Unit
Fuse # 17 (15a) under dash
Fuse # 19 (70a), 20 (40), 39 (20a), 36 (15a) under-hood
Temp Sending Sensor (on Thermo housing)
3 Thermo Sensors (2 on thermo housing 1 on Radiator)
Ignition (white wire)

What Fan Works When

I dont have A/C but I was told that when the A/C is turned on BOTH fans turn on to keep the A/C cold. When the vehicle is turned off the Fan timer will kick on the fan in order to keep the engine cool shortly after the key is removed. The passenger side fan is the cooling fan and is double the size as the A/C fan, It turns on during street driving to prevent overheating since there isnt a constant wind blowing through the radiator.

Testing Your Fans

Testing your fans is easy. If you unplug your fans or even remove (it would be easier). Hook up on one side to a battery source and other to a ground source. If the fan fails to turn then your problem is at the fan.

Thermo Switch A

Testing the connection at the Thermo Switch (A) at the bottom of the radiator to the fan. Unplugging this connector and jumping it with a paper clip verifies that everything from the plug to the fan is working correctly and the problem lies with the switch. Part # 36426. $ 30.00
If your fan does not trigger on then that is where your problem is, anywhere between the switch and the fan i:e any of the listed components above.

Thermo Sensor B

This Thermo sensor is located at the housing of the Thermostat. It is an off yellow color plug with a yellow/green stripe and a green/white stripe wire. This tells the fan control unit when the coolant temp is >226*F.AFTER the key has cycled from on to off if the coolant is >226* the condenser fan will run until coolant temp <214*F.

You cant jump to test this wire like Switch A if you do there is a risk of disrupting your ECU. Part # 36426. $ 30.00

Thermo Sensor C

This Thermo sensor is located at the housing of the Thermostat. It is a Blue color plug with a white/green stripe and a black wire. This sensor is for the ECT. You cant jump to test this wire like Switch A if you do there is a risk of disrupting your ECU. Part # WT5009. $ 36.00

Thermo Temp Sending Unit

This little switch is located at the water neck also and is a single prong unit. This switch sends the temp to the gauge cluster. If this is in-op then you will have no reading on your gauge cluster but fans will still operate as normal. Part # WT622. $ 20.00

Understanding How A Relay Works

There is 4 prongs to these relays they are labeled A,B,C,D in your wiring diagrams according to http://media.honda.co.uk/car/owner/media/manuals/PreludeManual/index.html many people are unaware of how these majestic creatures work or even why there needed so hopefully I can break it down to understand.

The first picture is of a relay not energized. There is a C and B that are +. And D would be the negative to polarize the coil. A would be your Fan.

When the ground is applied to the coil it retracts the lever and connects power to A completing the circuit with power and actuating your fan.

Testing Your Relay

1. Remove Radiator or condenser relay in the under-hood relay box.

2. There should be continuity between the A & B terminals when the battery is connected to the C and D terminals. There should be no continuity when the battery is disconnected.

Purging the air in your Cooling System

Ok there are many different ways of doing this and Im sure they all get the job done. So Im going tell you how I did it and I will also display how the manual says to do it.

1. Take the radiator cap off and fill to the water neck.
2. Place cap back on
3. Start vehicle and let it warm up, and let the fan kick on twice
4. Use a rag or a large towel to open radiator cap. This will be under pressure but mine wasnt enough to burn me but whos to say yours wont be, so use caution.
5. Loosen the bleeder once again and fill to the water neck if needed. You should see bubbles or water dripping out of the bleeder bolt.
6. Tighten the bleeder bolt
7. Start the vehicle with the radiator cap off and fill if needed after fan kicks on twice again.
8. Install cap.

Fan Timer Unit

This box is located on the passenger side of the vehicle at the right foot. (picture below) this controls the driver side fan / condenser fan. This module tells the fan when the car is shut off and shortly after the fan kicks on. The purpose of this is to cool the engine when the car has come to a shut off point, because that is when the car is about to reach its hottest. So even though the water pump is not pumping there is still pressure momentum moving the fluid through the system and fluid still needs to be cooled to prevent engine damage.

Changing the Thermostat & How It Works

When the engine started cold, the thermostat is closed. The engine coolant / antifreeze does not circulate through the radiator. Instead it returns back into the engine through the by-pass tube. This helps to warm up the engine faster. As soon as the engine temperature reaches the normal range, the thermostat opens and the engine coolant starts circulating through the radiator where it cools down.

If the coolant temperature in the radiator drops too low, for example when driving on a freeway in a cold weather, the thermostat closes again to keep the temperature within operating range.

-Replacing The Thermostat
1. There are 2, 10mm bolts that are located on the housing that need to come out in order to uncover the Thermostat.
2. Once those are out wiggle the housing and it should pop right off. There will be some slight fluid leak from the connection of the housing you can avoid this by draining some of the antifreeze approximately 1qt or just count to 2onethousand.
3. Then you should have the Thermostat exposed, so just pop it out with your hands or pliers if needed (remember not to force anything).
4. Your new thermostat should include 2 parts, a rubber seal and the Thermostat
5. Now it is important to place the seal and the thermostat in correctly.
6. Looking in the empty housing to which you just popped the Thermostat out of you should notice 2 little notches.
7. Now inspecting the new Rubber seal you should notice there are 2 protruding notches that will fit right in the housing ones.
8. Now slip the Rubber seal on the Thermostat with the notchs facing the housing notches.

9. Pop the thermostat in and it should sit right in.

10. Place the Thermostat cover that is attached to the upper Radiator hose over and Bolt down the 2, 10mm bolts. The End

Thanks to pp.com member TSIAH for clearing things up for me and for future luders
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Last edited by beatngyou27; 06-28-2010 at 09:04 AM.
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