CBR250 Forums banner

Cooling System Explained

21417 Views 16 Replies 10 Participants Last post by  nk7
Before this goes into the Maintenance forum , thought i'd run it through u guys for corrections/additions !

thanks for reading ![:)]

PS- cooling fan mod below may help many of you "keep your cool"


1. Radiator

The flat structure with beehive looking fins located just behind the front forks and wheel fixed atop the header pipes, just ahead of the engine.

- its cooling fins increase the surface area of the cooling system allowing the coolant flowing thorugh it cool faster.

- keeping the radiator fins clean and preventing them from getting bent or blocked (refer to RPMs post in the Maintenance forum)
- HRC radiator available but involves slight bracket modifications as well as customised headers

2. Radiator Fan

The fan is situated behind the radiator fixed via 3 10mm bolts.

- it switches on at a pre-set temperature activated by the radiator thermostat switch thereby circulating air through the radiator cooling fins when the bike is stationary or in slow moving traffic.

- the metal bushings which act as bearings within the fan may need to be changed if the fan gets noisy during operation

3. Radiator Thermo Switch

Located on the left side bottom corner of the radiator (if astride the bike)

- it completes the electric circuit which switches on the radiator fan when the liquid in the cooling system reaches the specified temperature

- the stock Honda switch which normally turns the fan on when the needle on the coolant temperature gauge reaches the 3/4th mark can be replaced with a cheap replacement from any automobile spare store. The switch from most japanese cars can be used for this mod which enables the fan to switch on at the 1/2 mark rather than the 3/4th mark on the gauge.


- drain the cooling system
- disconnect the radiator switch from the wire connector
- open the OEM switch and take it as a sample for the thread size and source a replacement
- fit the replacement onto the radiator
- connect the wire connector to the terminal on the new switch (if the replacement has two wires, connect the second wire to an earthing point like one of the bolts which also earth the radiator fan to the body of the radiator) The OEM switch earths itself to the body of the radiator via its base.

4. Radiator Cap

Located under the fuel tank to the right on the MC-17 and MC-19, NC-23 and on the right hand top corner of the radiator in the MC-22 and NC-29.

- it pressurises the cooling system which helps raise the boiling point of the coolant in the cooling system.

- No modifications recommended but check at intervals if the rubber seal is not torn of hard and replace it at least once every two years since if the spring gets weak it will cause the hot water to escape into the coolant resovoir instead of circulating within the cooling system.
- make sure that the pressure rating of the replacement is the same as the original. The rating is printed on the cap

5. Coolant Resovoir

A plastic transparent container located under the riders seat in the MC-17 and MC-19 and just under the rear shock in the MC-22.

- it serves as a catchment tank for the coolant that escapes past the radiator cap and also serves as a top up resovoir when the level in the radiator has fallen wafter the bike has cooled down.

- No modifications recommended maintain the level of the coolant between the "Upper" and "Lower" level marks

6. Thermostat Switch

Located within the bulbous metal object under the tank to the right in the MC-17 and MC-19 and behind the pivot of the clutch cable connected to the engine casing just ahead of the rear shock in the MC-22

- its opens at a preset temperature allowing coolant to be pumped through the radiator and the entire cooling system. When the thermo switch is otherwise closed the coolant does not circulate which would lead to Övercooling"

- certain riders and race teams prefer to remove this switch altogether and allow the coolant to circulate at all times. But many others do not recommend this mod.

7. Temperature Sensor

Located on the thermostat switch (point number 6)

- sends the coolant temperature to the coolant temperature gauge on the instrumentation console

- none but the switch may need to be replaced in the event of failure

9. Coolant Temperature Gauge

Located on the extreme right of the instrumentation console

- displays the temperature of the coolant

- certain reports of a digital meter fitted on the 250's/400's

10. Coolant

- liquid used in the within the cooling system

- prevents corrosion within the cooling passages and helps prevents freezing in colder climates

- needs to be drained and replaced at periodic intervals
- pre-mixed coolant is recommended. Else use distilled water to dilute the coolant concentrate
- riders in warmer countries should avoid using coolant with anti-freeze

[blue]Real Men Ride 'Em Naked [/size=1][/blue]
See less See more
  • Like
Reactions: 1
1 - 17 of 17 Posts
For reference, this is where the radiator cap on an MC22 is. I had to go find it during a coolant flush.

You can get to it by just removing the air duct on the right hand side.
See less See more
Hi K916, I own a 1991 MC19 and after replacing the head gasket lately I was going to replace the coolant only to find that the pipe leading from the bottom of the expansion bottle under the seat is kinked and closed off. I presume a previous owner did this. Probably the reason the head gasket went in the first place. I'm told the radiator has no cap but your picture shows one. My cooling system is completely closed which is wrong so I was wondering could you shed any light. It would be much appreciated. I've seen some pictures of cooling diagrams and they show this pipe leading to the top of the radiator. I dont want to start the bike untill i fix this problem. Thanks.
^The MC19 has the rad cap under the fuel tank.

That being said the MC19 was only built until 1989, so I assume yours was imported in 1991 and that's the date you mean.

If yours was in fact a 1991 model, it would be an MC22 (See dual fornt discs, gullwing shaped swingarm, five spoke wheels etc..), the rad cap would then be under the right hand side false airduct.
Let us know how you get along
Yeah guys it's an import and registered in 1991. It only has a single front disc and the swing arm is straight. It was a breaker in Dublin who told me that model has no cap on the radiator so it's a bit of a pickle. Thanks for replying though.
For troubleshooting sakes:

Just to add to this, if your unsure whether your fan actually works or not, you can simply unplug the connection from the radiator thermo switch and ground the connection (to the block for example) to test make sure it works :) If it doesn't spin when doing this, its likely the fan motor is burnt out. If it works and your fan doesn't come on when the engine heats up, check your fuses next.
^ And if your fuses are fine, you may have an air bubble in your radiator which means the thermo switch isn't in contact with the hot coolant and therefore isn't getting hot enough to close. It could also mean that the thermo switch is faulty.
grifferello said:
Yeah guys it's an import and registered in 1991. It only has a single front disc and the swing arm is straight. It was a breaker in Dublin who told me that model has no cap on the radiator so it's a bit of a pickle. Thanks for replying though.
Well the point being, don't ever take what a wrecker says as gospel.

Here is a pic of under the tank, note the rad cap up the top near the coils :)

Also note Hessians mums undies used as blocks for shit going down the inlet ports..
See less See more
Hey there,
Cheers for the photo, i'm defo missing that cap. Even if i get one where does it plum into the water system? But i defo wouldn't take a breaker's word as gospel.
Thanks again guys.
If you were missing that cap that would spell big trouble. A depressurised cooling system, you would have had the cooling system pissing out eveywhere and no coolant in the engine jackets which means the fan would have been on pretty much everytime you start the bike and yo might have cooked your engine.

Are you 100% positive that the cap isn't there?

Remove fairings and tank and check the bike thoroughly. It has to be there if your bike is an MC14, MC17 or MC19.

That cap if it's the same as the MC22 plumbs into the cooling system straight into the rad, but also attaches next the pipe end of the cooling system once the coolant has completed a full circumnavigation of the rad, then engine jackets then enters back into the rad. There is also an overflow pipe there as wellI think.

If that doesn't make sense, which it probably doesn't see page 104 or section 5 of the manual.

Manual is available on here.
See less See more
Hi Jlyall,
I'm positive it isn't there as i've just done the head gasket and took the engine out to do so. I bought a cap/thermostat unit today and waiting for delivery. It has 5 outlets on it plus a sensor nipple. Outlet on the neck is the overflow you mentioned going back to the bottom of the expansion bottle under the seat. The largest outlet i presume goes to the top of the rad and I've no idea where the other 2 go. The last owner did some botch job on it. The nunce!!!! Still, I appreciate the help. I'm slowly getting there though. Thanks again.
Can't help you there as I don't have the same bike, but I know that one pipe should go to the engine jackets and one other to the rad as well as the small overflow to the expansion bottle.

Check the manual mate.
Also pics of the botch job? I'm curious.
The temp sensor that goes to the gauges is it this thing?

Wheres the ground for this too?
^ ground is the radiator itself. Fuck paying 60 pounds though, things like this are usually used in a lot of vehicles, that particular temp sensor is used in all of these:

Honda Part 37750-PC1-004
Temp sender unit.
Honda Part 37750-PC1-004

Vehicle applications:
ACCORD 3D/4D 82-83 ACCORD 3D/4D 84-85
CIVIC 4D 81-83 PRELUDE 83-87
CRX 84-87 CIVIC 3D/4D 84-87
ACCORD 3D/4D 86-89

Fit: Honda
Fit: Mazda
Fit: Rover
Check it
Delete it

ROVER 200 (RF) 1995/11 - 2000/03
200 (RF) 216 Si 16K4F 1588 112 Hatchback 11/95 - 03/00
ROVER 400 Hatchback (RT) 1995/05 - 2000/03
400 Hatchback (RT) 416 Si 16K4F 1588 112 Hatchback 05/95 - 03/00
ROVER 800 1986/10 - 1999/02
800 825 SI/Sterling (XS) C25A2 2494 167 Saloon 10/86 - 10/88
HONDA ACCORD II Hatchback (AC, AD) 1983/09 - 1985/10
ACCORD II Hatchback (AC,AD) 1.8 EX (AD) ET1 AD 1829 101 Hatchback 4 09/83 - 10/85
HONDA ACCORD II (AC, AD) 1983/09 - 1985/10
ACCORD II (AC,AD) 1.8 EX (AD) ET1 AD 1829 101 Saloon 4 09/83 - 10/85
HONDA QUINTET (SU) 1980/02 - 1984/12
QUINTET (SU) 1.6 EX EL SU 1602 80 Hatchback 4 02/80 - 12/84
HONDA ACCORD III (CA4, CA5) 1985/11 - 1989/12
ACCORD III (CA4,CA5) 1.6 L (CA4) A16A1 CA4 1598 88 Saloon 4 11/85 - 12/89
ACCORD III (CA4,CA5) 2.0 EXi (CA5) A20A3 CA5 1955 116 Saloon 4 01/86 - 12/89
HONDA CRX I (AF, AS) 1983/10 - 1987/10
CRX I (AF,AS) 1.6 i 16V (AS) ZC1 AS 1590 125 Coupe 4 03/86 - 10/87
HONDA JAZZ (AA) 1983/10 - 1986/10
JAZZ (AA) 45 1.2 ER2 AA 1231 45 Hatchback 4 02/84 - 10/86
HONDA LEGEND I (HS, KA) 1986/01 - 1990/12
LEGEND I (HS,KA) 2.5 i (HS) C25A1 HS 2494 150 Saloon 6 01/86 - 12/87
HONDA PRELUDE II (AB) 1983/11 - 1987/04
PRELUDE II (AB) 1.8 EX ET4 AB 1829 101 Coupe 4 11/85 - 04/87
HONDA PRELUDE III (BA) 1986/04 - 1992/01
PRELUDE III (BA) 2.0 i EX 16V (BA2) B20A1 BA2 1958 137 Coupe 4 04/86 - 12/87
PRELUDE III (BA) 2.0 i EX 16V B20A1 BA 1958 137 Coupe 4 04/86 - 12/87
MAZDA 121 I (DA) 1987/10 - 1990/10
121 I (DA) 1.1 B1 1139 57 Hatchback 10/87 - 10/90
121 I (DA) 1.3 B3 1324 60 Hatchback 10/87 - 10/90
MAZDA 323 II (BD) 1980/11 - 1989/10
323 II (BD) 1.3 1296 68 Saloon 11/80 - 10/89
MAZDA 323 III (BF) 1985/08 - 1990/04
323 III (BF) 1.3 E3 1296 60 Saloon 08/85 - 05/89
323 III (BF) 1.5 B5 1498 73 Saloon 09/87 - 10/89
323 III (BF) 1.6 GT B6 1598 105 Saloon 01/86 - 12/88
MAZDA 323 III Hatchback (BF) 1985/08 - 1991/11
323 III Hatchback (BF) 1.4 B3 1324 60 Hatchback 09/87 - 10/89
323 III Hatchback (BF) 1.5 B5 1498 73 Hatchback 09/87 - 10/89
323 III Hatchback (BF) 1.6 GT B6 1598 105 Hatchback 08/85 - 12/88
323 III Hatchback (BF) 1.6 GT Turbo (BF1) B6 1598 140 Hatchback 10/85 - 11/91
ROVER 200 (XH) 1985/03 - 1989/09
200 (XH) 213 S EV2 1342 73 Saloon 09/85 - 09/89
200 (XH) 216 16H 1598 83 Saloon 03/85 - 09/89
ROVER 200 Coupe (XW) 1992/10 - /
200 Coupe (XW) 216 D16A8 1590 122 Coupe 10/92 - 06/99
ROVER 200 Hatchback (XW) 1989/10 - 1995/10
200 Hatchback (XW) 216 GSi D16Z2 1590 112 Hatchback 10/89 - 10/95
200 Hatchback (XW) 216 GTi D16A8 1590 122 Hatchback 09/90 - 10/95
ROVER 400 (XW) 1990/04 - 1995/04
400 (XW) 416 GSI D16Z2 1590 112 Saloon 04/90 - 04/95
400 (XW) 416 GSI/GSE 1590 116 Saloon 04/90 - 04/95
400 (XW) 416 GTI/GTE D16A9 1590 131 Saloon 04/90 - 04/95
ROVER 400 Tourer (XW) 1993/09 - 1998/11
400 Tourer (XW) 1.6 i 1590 112 Estate 05/94 - 11/98

See less See more
Owning a bike is one thing but maintaining it is a totally different ball game.As with all the other expenses of daily life,the costs of bike repair are increasingly getting higher.The only feasible way of keeping tabs on bike maintenance expenses is by being able to do the repair tasks yourself.
I read out your interesting and informative post...This is a very useful thread that will be referenced long into the future.
Thought it was relevant so figured I might post here.

The part sir.b was referring to can be found on ebay here. Or search for 37750-PC1-004 on ebay (make sure you select "Include description" for more results. Only $13 delivered or pay $60+ from Honda. I bought 2.

Specs for the Honda 37750-PC1-004 water temp sensor can be found on this site.

Seems the ebay one has a 13mm socket head instead of the usual 12 though.

Also, Tridon has apparently an equivalent (#TTS003) also. Here is a pic for comparison:



See less See more
1 - 17 of 17 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.