My Fuso truck has always had marginal AC. Number of guys on Expedition Portal complain about the AC problems. Last week, I took a non-contact thermometer on a road trip. Ambient temp was 90-95 degrees. At 62mph on the flat AC vent temp was 66 degrees. Later in the day in a headwind, uphill, temps rose to 72 degrees. When I pulled off the interstate and drove around town, vent temp would drop to 48 degrees within a minute or two and continued driving at low speeds around town temps got as low as 39 degrees. Back on the interstate, it jumped to 66 degrees again. 40 mph in 5th gear temp was 58 degrees.
Poster on Expedition Portal claimed problem was caused by the truck not having a valve to shut off coolant flow thru heater core. It just has a diverter flap in the ductwork. I installed a flow control valve in the heater hoses yesterday. If that does not solve the problem, does anyone know what might produce these odd symptoms of cold temps at low speeds and warm temps at high speed? Condensor is not in front of the radiator. It's off to the side and has it's own fan sucking air up thru it and that fan is working fine.

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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

That is an interesting problem. Is perhaps the condenser and associated fan located in what would be a low pressure area at highway speeds? In other words, are perhaps the aerodynamics of the truck (or lack thereof) reducing airflow across the condeser at higher speeds, hence causing the rise in vent temps?

Is the refrigerant charge correct? I could perhaps see an overcharge causing vent temps to rise as compressor RPM increases. But if other owners see the same operation at highway speeds, this seems unlikely.

Tom, most vehicles circulate coolant thru the heater core at all times, and use a blend door to regulate the temperature. If the seals ( usually foam) on the door deteriorate, it can allow excess hot air into the ductwork.
However, I doubt that the blend door is your problem as the seals are the same whether at high speed or low. I suspect something causing the engine to put more heat into the radiator, and the incoming air flow past the condenser is insufficient to take enough heat out of the A/C system.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags

Lots of people complain about the AC but I don't know anyone who's done a vent air temp test at varying speeds. I was wondering about air flow over the condensor too. It's a cab over truck. Condensor is mounted horizontally behind right front bumper with a fan sucking air up thru it. Bottom of the condensor has a plastic device with slanted fins designed to direct air up thru the condensor. Those fins are about 2 inches long. I don't know how hard it would be to suck air up thru that condensor at speed.

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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

It sounds normal to me. When more HP is needed, it turns the AC system off, which is a power robbing accessory. Every vehicle I've owned, to a point, suffers the same.

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1987 560SL
85,000 miles

I was wondering that too. Is there a switch which deactivates the AC under certain conditions? However, I don't think the AC is being completely deactivated because if it were, the vent temps would be even higher I think. And if it's deactivating it at 62mph on the flat, something is very wrong.

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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

When you get the lower temps are the engine revs higher? So when you are cruising on the highway your revs might be lower and thus the compressor is not turning as much as at lower speeds with a higher engine speed such as around town. I've noticed a similar problem with my wifes car where the temp seems to drop noticably at lower speeds/higher revs around town and then climb noticably at higher speeds/lower revs such as highway cruising.

- Peter.

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2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

When my wife's Tahoe is parked and at idle, the AC works, but not as cool as when it's being driven. When at highway speeds, it's also warmer than 35-40 mph.

The other day, while my wife and I were browsing the local community garage sales, I needed to sit for a while and take a break. I sat in the truck with the air on, but after a few minutes, it wasn't blowing as cold as I wanted. I noticed the button that recirculates the air in the cabin, and pressed it. Within 2 minutes, the air was so cold that I had to turn it down a few degrees.

Perhaps you should try that...see if you are able to recirculate the air in the cabin.

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1987 560SL
85,000 miles

Don't suppose you have approximate RPM readings to go with those temps? In any case, step one is to verify the charge level. This will probably mean pulling it all out, vacuuming the system, and weighing in a new charge, with accurate equipment. And of course, this requires that the system has a spec for a complete charge. Some trucks can be problematic in that regard. Slight differences in the length of lines, or differently sourced components (common in trucks, especially if dealer or builder installed A/C), can throw charges off by quite a bit.

Do you have access to a good set of gauges?

MV

Recirculate makes no difference. Believe me, I've tried every setting possible Yes rpm's do make a difference. High rpm's on the highway and very little cooling. I had concluded this was because the water pump was circulating a high volume of hot coolant thru the heater core and this was warming up the air from the AC. That's why I installed the coolant shut off valve. But I'm not sure this will solve the problem. I've had mixed results so far. There is about a 4" rubber flap that hangs down in front of the condenser below the bumper. Not sure why it's there. Maybe to protect the condenser from road debris. But it might be having a negative effect on air flow thru the condenser.

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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

No access to gauges and I'm AC ignorant. It could be the charge level but I doubt it's a result of differences in equipment. The guys complaining about it all have OE stuff on the truck.

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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

^^ Tell them "tough" your fathers didn't have A/C when they drove, and now, neither do you.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags

I carried out a similar vent air temp investigation on my 1994 80 Series Landcruiser today. The temperatures got colder the faster I drove. At 63mph it was putting out 22 degrees at the vents

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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

Ok, 22 is too low. You don't want to go below freezing, or you'll ice up the evaporator. Once it becomes a solid block of ice, it blocks the airflow and you get nothing. While you're climate might be a little low in the humidity dept. cooling air always pulls the moisture out of it.

But back to the truck, do you have a specification for the charge amount? You might just be a little low and it shows up when you spin the compressor fast. At any rate, verifying the system charge really needs to be your first step, as everything depends on that being correct.

MV

Also, what type of thermometer are you using, and is it properly calibrated?

MV