The AC on my Fuso FG is not that cold. A poster on another forum mentioned that a dealer told him (apparently--the poster's native language is not English) that the problem is that the heater core always gets coolant flow even when the AC is on. The system is supposed to operate by diverting air away from the core while on AC, but it doesn't really work. I'm thinking that putting a valve in the coolant line, cutting off flow to the heater core during the summer should solve the problem.
Anyone know if cutting off flow to a circuit like that would have an adverse effect on other parts of the coolant flow or the water pump? Most vehicles I'm familiar with, cut off the flow of coolant to the heater when the AC is on.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Without seeing how the system is plumbed, I can't say for sure, but putting a valve in to control flow through the heater core should work fine.

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83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
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99 Mazda Miata 183,xxx miles.

How certain are you that the hot coolant is always run into the heater core? Having coolant constantly flowing inside the cabin doesn't seem correct.

I first learned of it when the poster wrote the dealer said this is the problem with the Fuso AC. I then checked the FSM. Sure enough no valves to control coolant flow in the system.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

My '03 Dodge works the same way. The coolant also serves one of the transmission coolers on its way back to the block. While blocking the hose may solve your problem, it could create another.

If coolant always circulates through the heater it's possible the cooling system is designed to have that continuous flow. You can bypass the heater and maintain flow without worry.
The heater in my Miata works this way. Coolant flowing through the heater is part of the engine cooling system and if blocked off the engine can overheat.

The a/c worked when the car was new. That it doesn't work now indicates that something has changed. On cars with a constantly circulating heater there is a door that opens and closed to allow air across the heating coil. If your a/c system works but you are not getting cold air in the cap I would venture to say that system that controls the heater door is not working. In older cars this door is often controlled by a sliding lever on the dash. This lever is connected directly to the door by a cable. If you have this type system then the only think that can go wrong is that either the lever is frozen or the end came loose from the door.

In newer cars this door is controlled by a vacuum pot. If the pot doesn't work you might have a vacuum leak, a bad pot, or a plugged hose.

In even newer cars this door maybe controlled electrically. I don't work on new cars so I don't know.

Apparently not. Fuso has had a lot of complaints about the AC. It was FUSO USA which provided the solution to the dealer described on the other forum. They changed the design in 2007.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Try just clamping the hose Kerry. A clamp is not a permanent change & can be easily reversed without much fuss.

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1980 300D now parts car 800k miles
1984 300D 500k miles
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2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
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I've seen both setups in the VW's I've worked on.

the early watercooleds had a valve in the coolant line, you shut it with the 'temperature' lever and that shut off the heat.

The MK3(?) and later watercooleds had coolant circulating all the time, and a door blocking the air.

from the construction, it was pretty apparent that after 20 years it was more likely that the water valve would leak than the air flapper.

To answer your question ' have I seen this ?' - yes.

Can you put a valve in: probably yes.
Unless like a previous poster talked about coolant flow from the heater core to another heat exchanger...
Put a valve in there and be done with it.

Just remember a few things
1) defogging the windshield will get tougher =)
2) Filling and venting the cooling system will get tougher if you have a closed valve somewhere. Not sure how often you do this, but if you insert a valve you are more likely to trap an air bubble in the system, which will complicate the coolant re-fill if you ever drain it. (fix- open your valve when you refill the coolant)

If I had a coolant flow diagram, and knew there wasnt anything downstream of the heater core - I'd put a valve in.

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My 94 explorer has a shut off valve thats vacuum operated and im pretty sure it recirculates hte water back to the block when closed. 94 was the first year of r134 and the cooling is not that great so i think they used them for the same reason.
Head down to Colorado Auto Wrecking its a pick a part just off Sante fe and have a look around. youll find some good ideas there.

Lots of dodges, my Audi, and probably other cars do have a shutoff valve on the coolant line going to the heater core, but it's only activated when the air resirc door is closed.

VW went to an air blend door setup in 1984 for the Golf / Jetta, it allows quicker, more precise control of cabin temp (at least that's what the tech brochure said )

2x on getting a coolant flow diagram. That will let you know if the heater loop is just part of the bypass circuit, or the only path from the bypass circuit.

-J

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The poster on the other forum who had this modification done by the dealer is supposed to be posting pictures of what they did. I should have a better idea of the options once I see those pictures. From what others have noted, I think it's a pretty simple heater circuit that can have a shut off valve installed.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13