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[Doctor Gollum]

Didn't run them long enough to get hot

The 40 mile thing is the key, seats only on for so long as they have built in timers.

If the fans, all three, don't dismiss the light switch

That was from the '99 model year facelift diagrams. Haynes is OK for fundamental guide, but there are differences 

[STEMO]

I'm no mechanic/auto sparks, but it seems a bit idiotic to me to have all those things running through one fuse.

[Doctor Gollum]

Most of that lot are relay feeds. The other side of the relays are also fused

[LC0112G]

I'm no mechanic/auto sparks, but it seems a bit idiotic to me to have all those things running through one fuse.

I agree. With probably 20-30 relays, plus fans, heaters and motors it's virtually impossible to diagnose what is wrong quickly/easily. I'm seriously considering wiring a separate feed to the instruments because that's the one system I really need to work - can't go driving about for long without functioning speedo and fuel gauge. The rest I can live without for now.

[STEMO]

I'm no mechanic/auto sparks, but it seems a bit idiotic to me to have all those things running through one fuse.

I agree. With probably 20-30 relays, plus fans, heaters and motors it's virtually impossible to diagnose what is wrong quickly/easily. I'm seriously considering wiring a separate feed to the instruments because that's the one system I really need to work - can't go driving about for long without functioning speedo and fuel gauge. The rest I can live without for now.

You could use the STEMO method of diagnosis. Remove fuse, replace with piece of metal bar, start engine and wait until something starts smoking. You have found your faulty part. 🙂

[LC0112G]

I'm no mechanic/auto sparks, but it seems a bit idiotic to me to have all those things running through one fuse.

I agree. With probably 20-30 relays, plus fans, heaters and motors it's virtually impossible to diagnose what is wrong quickly/easily. I'm seriously considering wiring a separate feed to the instruments because that's the one system I really need to work - can't go driving about for long without functioning speedo and fuel gauge. The rest I can live without for now.

You could use the STEMO method of diagnosis. Remove fuse, replace with piece of metal bar, start engine and wait until something starts smoking. You have found your faulty part. 🙂

I've already upped the fuse from 20A (yellow) to 25A (white) to 30A (green), and they all blow sooner or later. I get the feeling that going any higher is just going to test the amount of smoke left in the cables - probably at the worst possible time.

Unplugged the Cruise control today, and replaced the fuse again. So far so good, but I've only driven for 10 minutes or so. I don't really believe it's the CC but I never use it anyway and it's relatively easy to unplug.

Looked under the drivers seat for the seat memory widget. Trouble is to get to it you have to have the seat in the fully up position. If I then unplug the wiring the seat probably won't go down again. I'm 6'7 and need the seat in the down position to drive it so I'm a bit snookered.

However, given that the fuse blows after a few minutes, not immediately, I'm starting to think its got to be something that engages/turns on kind of psuedo randomly once the engine is warm. In order to draw >30A its got to be a heavy consumer. Fans, Motors, Pumps, Heaters etc. Can't see it being a relay coil. Window motors still work with F15 blown, so not them, and in any case the fuse blows when these are not being operated. Got the feeling it'll be either fans like DG suggests, or something to do with climate control.

[LC0112G]

Didn't run them long enough to get hot

The 40 mile thing is the key, seats only on for so long as they have built in timers.

If the fans, all three, don't dismiss the light switch

That was from the '99 model year facelift diagrams. Haynes is OK for fundamental guide, but there are differences 

The 40 mile thing...

The first time it blew was on leaving the petrol station at Chambery France on the way up to the alps. That was after a 9 hour drive from London via the Chunnel. We then continued for another 2 hours up the hill to Tignes praying it wasn't really the alternator.

During the 2 week stay in Tignes I replaced the fuse. Seemed Ok for the 10 minutes I ran the engine in the covered car park. It blew again after about 30 minutes on the drive down the hill on the way home (before Bourg).

I replaced the fuse again at Chambery petrol station, and it blew after only a couple of minutes.

Whilst in the Chunnel on the way home I got the Haynes manual out, turned the front seat heaters to '0' and replaced the fuse. Turned ignition on, and the fuse popped after perhaps 2 minutes. Engine wasn't running.

Replaced the fuse before a drive up to London (from Somerset) on Christmas day and got as far as Boscombe Down before it popped.

Replaced the fuse in London/Barnet before returning to Somerset on 27th and the fuse blew before reaching the M3.

So on the basis of abut 10 fuses, it blows quicker when the engine is hot. It takes 30-40 minutes to blow on a cold engine.

[LC0112G]

Duplicate

[STEMO]

Hmm.....cable looms heating and expanding, causing a short. Vibration or movement, causing a short. Water ingress, causing a short. If it's inside a loom somewhere, I can see you losing your mind. Sorry, Malcolm. 🤪

[Doctor Gollum]

Recheck the fans hot and cold and also the relays.

My money is on the front fans

[LC0112G]

Fuse is still intact, but I've only managed to do 2 short runs today so the engine never really got up to temp. However, tomorrow is another trip up to London, so plenty of opportunity for problems.

I've taken DG's advice and investigated the fans. As far as I can tell from Haynes, the only fan that is actually powered by F15 is M28, via relay K48. M28 seems to be one of the two A/C fans at the front of the rad - dunno which one though. F15 does power the relay coils for other fans, but not the actual fan itself. So on the assumption the relays aren't short circuit on their coils I've pulled (what I think is) relay K48, and will run up to the smoke without it.

Sweep stake on how far I get? Mudford, Marston Magna, Queen Camel, Sparkford, Wincanton, A350, Stonehenge, Boscombe, Andover, A34, M3, Fleet, M25, M4, M40, M1, A1, Barnet   

[Doctor Gollum]

Stop referring to Haynes.

The list I gave is the definitive list and that fuse feeds every relay in the cooling and aircon systems

The relay you mention actually controls the blower to cool the engine bay relay box and has absolutely sweet fanny Adams to do with the radiator fans

Unless I wasted a couple of hours of a day off trawling through the diagrams of course...

[Doctor Gollum]

Here's a definitive list of circuits to check:

S2 Light switch.
S7 Reversing lamp switch.
Instrument cluster.
MK engine cooling - relay box blower M28
MK engine cooling - radiator blower M4
MK engine cooling - cooling/aircon relays: coolant pump, coolant pump timer, radiator blower M11/M12/M54/M4/M28, aircon compressor, fan management.
Telephone.
Warning buzzer.
TID/MID.
Aircon.
Electronic Climate Control.
Car Level Control - control unit/sensor (same unit connected mechanically to the right hand trailing arm).
Glove box light.
Headlight levelling.
Interior lamp (all).
Sun roof.
Window lifters.
Exterior mirrors.
Seat heating.
Seat/mirror memory control relay (under driver seat).

Listed as they appear and are described on the wiring diagram, NOT in order of importance. Although everything unplugged so far has either been unlikely or not on the above list

[BazaJT]

According to fan test[detailed in maintenance section] the fan relays are K26-all fans on low or aux on low depending on test being carried out,K28/K52 aux2 on high and K67 main on high.

[Doctor Gollum]

Fuse 15 directly feeds the following Cooling/Aircon relays:

1. K60(Relay - Compressor, Aircon) which controls K87(Relay - Blower, radiator) and controls M4(Motor - Blower, radiator).

2. K52(Relay - Blower, radiator) controls M11(Motor - Blower, radiator) and is fed from M4.

3. K28(Relay - Blower, radiator) controls M11.

4. K48(Relay - Blower, relay box) feeds M28(Motor - Blower, relay box) and is switched by K26(Relay - Blower, radiator).

5. K67(Relay - Blower, radiator) feeds M12(Motor- Blower, radiator).

6. K22(Relay - Pump, coolant) controls M54(Motor - Pump, coolant on the radiator).

K60 also feeds S128(Switch - Temperature, coolant on the radiator) which energises K52 and K28.

K26 feeds S88(Switch - Temperature, coolant on the radiator) which energises K48 and K67. It also provides power to M54.

S20(Switch - pressure on the aircon plumbing) has three parts: S20.1(Switch - Compressor, low pressure), S20.2(Switch - Compressor, high pressure), S20.3(Switch - Revolution, acceleration). S20.3 has a diode connection from both K28 and K48.

M4 and M11 are the front fans, M12 is the main fan.