Those relays are for the A/C control system (not related). The problem is usually with the fan timer unit, it develops cracked solder joints inside. You can open it up and try resoldering the contacts.

If you want a plug and play solution then I sell a bypass for it here: Fan Timer Unit Bypass Plug and Play Harness

 

I've tried to locate the fan timer unit (the internet say it is near the ECU) but I've found nothing resembling it
Mind you it's an european model, could it be located somewhere else?

 

Should be there:

If not then maybe someone has already messed with it in the past. Possibly your car didn't come with it in your market (Italy?)?

 

I've checked again and someone definitely messed up with it as you can see:

The hole in which the fan timer unit sits is empty (suggesting something should be there) and I have this connector dangling around...help me understand, if I buy the basic bypass I should be able to just plug it there and that's it?

 

No it won't work. That connector is completely different than what came on U.S. cars. I think you need to get a factory service manual that is specific to your market to figure out the wiring for this.

 

 

This is easy to fix. I will be back later tonight.

 

If you have a 1987-1988 model, then you can find the cooling fan control component location index on page 16-76:

https://www.hondapreludeclub.net/manuels_EN/3g/1988_HONDA_PRELUDE_3G_MANUAL_62SF100.pdf

If you have a 1989-1991 model, then you can find the cooling fan control component location index on page 16-28:

https://www.hondapreludeclub.net/manuels_EN/3g/1989_HONDA_PRELUDE_3G_MANUAL_62SF120.pdf

The shop manuals are missing a sensor in the component location index. There is a sensor near the thermostat housing which the cooling fan timer unit uses when the engine is off, not the one at the bottom of the radiator. If this sensor is faulty then it could be giving wrong readings by telling the cooling fan timer unit that its hot when its really not. After checking the fuses as per the troubleshooting guide in the workshop manual, I would try disconnecting the sensor to see if it stops the fan from starting, before investigating the cooling fan timer unit, because its an easy check.

You can find out exactly where the sensor is and what it looks like by looking at the radiator section in the 1987-1988 manual. Its referred to as a thermosensor in this section of the manual.

https://web.archive.org/web/2007022...http://media.honda.co.uk/car/owner/media/manuals/PreludeManual/62sf100/5-66.pdf

 

If you have a 1987-1988 model, then you can find the cooling fan control component location index on page 16-76:

https://www.hondapreludeclub.net/manuels_EN/3g/1988_HONDA_PRELUDE_3G_MANUAL_62SF100.pdf

If you have a 1989-1991 model, then you can find the cooling fan control component location index on page 16-28:

https://www.hondapreludeclub.net/manuels_EN/3g/1989_HONDA_PRELUDE_3G_MANUAL_62SF120.pdf

The shop manuals are missing a sensor in the component location index. There is a sensor near the thermostat housing which the cooling fan timer unit uses when the engine is off, not the one at the bottom of the radiator. If this sensor is faulty then it could be giving wrong readings by telling the cooling fan timer unit that its hot when its really not. After checking the fuses as per the troubleshooting guide in the workshop manual, I would try disconnecting the sensor to see if it stops the fan from starting, before investigating the cooling fan timer unit, because its an easy check.

You can find out exactly where the sensor is and what it looks like by looking at the radiator section in the 1987-1988 manual. Its referred to as a thermosensor in this section of the manual.

https://web.archive.org/web/2007022...http://media.honda.co.uk/car/owner/media/manuals/PreludeManual/62sf100/5-66.pdf

Mine is a 1989 model and as far as I can tell the cooling fan timer unit is located somewhere else, I'll have to check it out!
I will keep you posted as soon as I do it, in the meantime thank you!

 

UPDATE:
I checked the relay box and found out someone already put a bypass in it

As soon as I unplug it the fan stops. So I figured if I put the relay for the a/c in its place I would solve my problem: it didn't.

To recap: with the bypass inserted, the fan keeps going indefinitely. Without the bypass, the fan doesn't work (even when temps are high). Same thing when I tried using the a/c relay.
Here's where it got tricky: I can't seem to find the cooling fan timer unit. It's not in the location (next to the fuel filter) shown in the 1989 manual (my car is a 1988 model then?). So what's controlling the fan to stop? Is it the relay?

 

I think this is why you need to track down the wiring diagram. I believe in the U.S. the carbeureted 3rd gens didn't come with the fan timer, so your wiring may be the same as that.

Obviously the load side of the relay is working, so something on the control side ia bad.

The 2 pin switch at the bottom of the radiator is what normally switches on the fan. I would put the relay back in, key on, then jump the 2 pins at the switch connector. See if the fan comes on then.

 

Update:
Today I checked the continuity of the radiator bulb located at the bottom of the radiator

When cold no continuity, when I heated it up there was continuity so I'd assume it works just fine.
Then I tried to check the thermosensor located on the head near the thermostat:

No continuity, neither cold or hot. I'd assume there's a problem with it, still, I can't seem to understand why the fan doesn't shut off when I took it out (with the bypass in it).
It seems like my problem is related to the (missing?) relay rather than something else, no matter what I do or unplug the fan keeps going on when the bypass is in it and stops ONLY when I take the bypass out (reminder, it's that green plug). Even putting the a/c relay in its place (which should be the same right?) doesn't fix anything. I am running out of options (and patience).

 

I had the same issue on my 89 carbed euro lude, in the end it was just a loose/dirty connection in the fan timer unit, it is located on the passenger side of the dash by the footwell behind the plastic cover for the wiring. All 89 models should have it here in europe, if it doesn't then it might've been swapped to another Honda's system.

This is the Euro model wiring diagram for the radiator fan it is quite much the same across all models from what i understand.

Another issue to be checked is the TA switch as when I removed that the cooling fan timer did not work.

 

I've already checked there but I didn't find anything, I still don't know if it was removed or it isn't present at all
Where is the TA switch located?

 

Post a good pic of your engine bay so we can see what you have in that bay.

 

Sorry for the delay, here's the pics:

 

Ok so confirmed its the Si with ALB model.

 

UPDATE:
Turns out my prelude is a 1988 model, if I check amayama for spare parts there's no trace of a fan timer unit
Since I've already checked and tried everything else (thermostat, radiator bulb, thermosensor), there are only 2 options left:
-The green relay is stuck so it keeps the fan going (can't find a replacement anywhere)
-The black box I found has something to do with it but NO ONE knows what it is, it's missing from manuals and even dealerships know nothing about it

 

Not no one, just arm chair mechanics.

That black box is a fan control unit from an earlier make Honda.

The part is TL-40, the correct part for a third generation Prelude is TL-46.

https://www.preludepower.com/threads/resoldering-the-cooling-fan-timer-unit.468114/

You can see the newer the car, the higher the TL number.

Your 1988 model did not come with a cooling fan timer unit, only the 1989-1991 had them.

Your market didn't have the 4WS variant, only the Si model. I don't believe this would have come with an air conditioner.

I see you have a condenser fan, so it seems the dealership had it sent off for the optional air conditioner install. It was standard practice for optional air conditioners to be installed by a contractor. Mine was installed at the factory but the book for the air conditioner mentions if it was installed by a contractor.

It looks like they used an older fan control unit for yours because that's all they had on hand in 1988 in your market. Very different time back then, no internet and longer logistics. 1980's Honda cooling systems were all pretty much designed the same so almost any fan control unit would have done.

Look at the electrical tape job they did. That is not factory quality wrapped. And that sky blue wire on the fan control unit cannot be found anywhere else on this model. Its definitely not native. The Honda part code will be 37740-PK1-xxx. Its so old you won't find it.

Chances are its made by Shindegen. It uses the same style bracket and identical bolt to the other control modules in the Prelude.

For sure your problem is just broken solder joints. Open it up and resolder it. Nice big blobs with lead-free solder.

 

I am pretty sure all the Si preludes had ac. The s models I guess had ac as an option. At least for usdm cars. I have no clue about other markets.

 

There were three B20A varients in Europe. It would be helpful to see the factory plate.

 

Hi @WhateverMate, thanks for your precious input!
To make things clear, my model is a 4WS variant and I (highly) doubt that was an option that dealerships could add later on.
To recap, it's a 1988 4WS pre facelift Prelude with a B20A7 engine non-cat (exclusive to European market?) that was registered in Italy only in 1991 (so possibly an export from Austria/Germany since the dealership that brought it was based in northern Italy and as far as I recall those Preludes weren't sold directly by Honda itself).
I have found another 3rd gen prelude here in Italy with the same layout (green relay, "TL-40" black box and no fan timer unit) so it looks like it was standard procedure back then.
I can't really open it, I had to make some incision on the box in order to peak inside and it looks like the joints are submerged in a solid and rubbery compound:

I will have to check it again though, I stopped there because I didn't know what I was looking at.
By next week ill take a picture of the factory plate, in the meantime thanks again for the help!

 

Interesting. The information I got that 4WS was not available in your market was from a German owner, but if yours was imported by an independent retailer then who knows where it came from. We will see on your factory plate.

 

The bottom line is you have one off car in a market that never was offered. So now you are dealing the the aftermath of supporting a car that was never supported in your region.

So you are going to have to find the OEM region it came from and order a ETM from that region for that specific chassis.
Most all 3rd gens are the same world wide and the US got them all minus the RHD versions.
The differences are not that complicated as the car was not a unubtanium piece of technology.

So more work needs to be done on your end to get the correct info on your regional chassis.

 

The fan control unit housing can be found on Honda and Kawasaki motocycles ignition control modules from the 1970's in Italy and Germany and its made by Nippon Denso, this explains why the rubber is their to keep the water out. Honda must have been happy with the housing for cars as it was still suitable to house a circuit board for fan control.