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Discussion Starter #1 (Edited)
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.

Components

· Battery
· 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 don’t 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 isn’t 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 can’t 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 can’t 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 I’m sure they all get the job done. So I’m 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 wasn’t enough to burn me but who’s to say yours won’t 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 it’s 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 notch’s 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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Nice work. Pictures and looks easy to understand. When it is finished, send me a PM and let me know. I will take care of the FAQ sticky and like to keep it updated.
 

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Nice write up and keep it coming. This type of stuff is really helpful to your fellow luders. I repped you.
 

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good job and info chris. keep the good work up. shit if i have issues ill just send the lude to you when i want to be lazy. haha
 

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Discussion Starter #5
Nice work. Pictures and looks easy to understand. When it is finished, send me a PM and let me know. I will take care of the FAQ sticky and like to keep it updated.
will do. thank you

good job and info chris. keep the good work up. shit if i have issues ill just send the lude to you when i want to be lazy. haha
lol you gonna UPS your car to me :happy:
 

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i might. dont tempt me.
 

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I don't know what you did with the editing on this post but it made it very annoying to quote and post in different pieces... :razz:
You need to brush up on the difference between a sensor, switch and a sending unit. They do different things.

Thermo Sensor A

Testing the connection at the Thermo Sensor (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, At the fan.
If you jump this and the fan DOESN'T turn on, the problem could lie anywhere between the switch and the fan. IE: the relay, a fuse, any of the wiring, etc.​


I don’t have A/C but I was told that when the A/C is turned on BOTH fans turn on to prevent overheating.
This is to keep the A/C cold. Not to prevent overheating.




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 sensor tells the fan when the engine has cooled off enough and shuts the passenger side fan off. You can’t jump to test this wire like Sensor A.
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.

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 I believe tells the Transmission not to shift into high gear until the engine is warmed up. You can’t jump to test this wire like Sensor A.
That is for ECT. No you can not jump it like the other switches (its not a switch, its a sensor) you could damage the ECU if you jumped the wires together.

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 nor will any fans kick on to cool the engine.
This is not true. It is only wired to the gauge. If it doesn't work, you won't have a gauge. The fans will still work, the ECU will still know coolant temp.

for example...I don't have one.
I have an aftermarket gauge and sending unit. My fans work fine.




 

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Discussion Starter #9
I don't know what you did with the editing on this post but it made it very annoying to quote and post in different pieces... :razz:
You need to brush up on the difference between a sensor, switch and a sending unit. They do different things.
If you jump this and the fan DOESN'T turn on, the problem could lie anywhere between the switch and the fan. IE: the relay, a fuse, any of the wiring, etc.





This is to keep the A/C cold. Not to prevent overheating.






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.



That is for ECT. No you can not jump it like the other switches (its not a switch, its a sensor) you could damage the ECU if you jumped the wires together.



This is not true. It is only wired to the gauge. If it doesn't work, you won't have a gauge. The fans will still work, the ECU will still know coolant temp.

for example...I don't have one.
I have an aftermarket gauge and sending unit. My fans work fine.




i appreciate the constructive critizism and the reason i stated if the sending unit doesnt work neither will my fans is cuz i had that problem but im sure your right since your running no sensor it was probably linked to another problem i was having at the same time. i will update it tommorrow. thank you again
 

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Looks pretty complete. Good write up ;) Hopefully it will come up in future searches and we won't have a large number of repost threads.
 

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Hey I have a question. Today was a really hot day out (like 102*F locally). I used the AC for all my driving. I parked the lude and went into Walmart for 10 minutes. When I returned to the car, the condenser fan was running, but the radiator fan was not running. When I turned the ignition on, the condenser fan shut off. Is this ok or is there a problem?

Note: No problems with the radiator fan, it is just the condenser fan. Just wondering if a relay/switch/control-unit is failing, or if it was just a really hot day.
 

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Discussion Starter #13
I think its normal i just recently added my condensor fan so im a little behind lol. the guy i bought the car from had the plug for the condensor plugged into the radiator fan and the only time the fan would kick on was when the timer unit started it on ignition off. so since i just recently added my new fan now i have both but it seems like the condensor runs when engine is off and radiator fan when engine on. to me seems ok. sorry i wrote back so late ive been in hidding

Hey I have a question. Today was a really hot day out (like 102*F locally). I used the AC for all my driving. I parked the lude and went into Walmart for 10 minutes. When I returned to the car, the condenser fan was running, but the radiator fan was not running. When I turned the ignition on, the condenser fan shut off. Is this ok or is there a problem?

Note: No problems with the radiator fan, it is just the condenser fan. Just wondering if a relay/switch/control-unit is failing, or if it was just a really hot day.
 

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yeah I did the test in the shop manual. Everything was fine. The timer may be bad. by the way what you circled is the compressor control unit and not the fan timer. If memory serves me right the fan timer is lower and connection is in bottom of housing and not front.

Edit: also all three sensor switches are different. two are switches: one located in bottom of radiator is labled 90 and you should use that number when looking online at autoparts stores websites. The other is on the water neck and is labled 108. It is blue with blue connector. The third is not a switch and just a temp sending unit for ecu only. It is white with grey connector.
 

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Nice writeup. Sure not all the information is perfect but hell these cars are old and nothing ever goes right on the informational or mechanical side of things. Even in my line of work we still have problems like this...mostly because the Army won't get rid of their damn generators until they actually blow up...and then they rebuild them. If you think working on the Prelude is nuts try working on a 60Kw 400Hz generator that saw hell in Vietnam.


Now if only I could bring myself to dig into the wires on my Lude...
 

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So the coolant travels through the lower rad hose, up the radiator and back into the engine? Just want to clarify.
Well the water pump drives off the the timining belt which is powered by the crank and goes in the direction of clockwise. So coolant in the lower pipe on the backside of the engine feeds through the lower radiator hose and goes to the rear engine pipe through the timing belt water pump and up through the head of engine. It then goes through the upper head water neck for thermostat to upper radiadiator hose and upper radiator.
 

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just make sure to checkout my 108 degree or 90 degree celsius temp sensors and switches in many of my threads.
 

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^^ You say the engine turns "clockwise" . If you are looking at the engine from the timing belt side, drive belts side, the engine turns counter clockwise.

Our engines are "normally" thought of as turning counter clockwise.

Silly Cuda.

Is it just me or does anyone else find it hilarious that Konartis read thru this whole article and thought the coolant ran backwards. I guess the first thing beatngyou27 should have addressed was the direction of the coolant flow.

:):):)
 

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sorry dano I was tired when I posted that. That was central time. I had the coolant going in the right direction with the impellar just not the belt.
 
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