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Discussion Starter #1
Hello all, my '89 420 SEL is giving me climate fits!

In summer, AC works, but system switches from AC to heat and back to AC.
AC comes back on just before heat becomes uncomfortable.

In colder weather, heat comes on but AC cycles on and off as well.
Noticed that even when heat is on the vent that blows into the back seat blows cool air.

Exchanged climate control unit with no improvement.

Any ideas? I'm at a loss on this one....

Thanks in advance.
 

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2004 SL500, 1993 S420, 1990 420 SEL, 1985 500SE, 1994 Toyota Supra Aerotop
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753 Posts
Hello all, my '89 420 SEL is giving me climate fits!

In summer, AC works, but system switches from AC to heat and back to AC.
AC comes back on just before heat becomes uncomfortable.

In colder weather, heat comes on but AC cycles on and off as well.
Noticed that even when heat is on the vent that blows into the back seat blows cool air.

Exchanged climate control unit with no improvement.

Any ideas? I'm at a loss on this one....

Thanks in advance.
Where did you source the new climate control unit from - is it a brand new one? Can you confirm that the AC compressor is going on and off, or it could just be the air is being routed through the heater core when it shouldn't be, which perhaps is either a faulty switchover valve or a bad vacuum element.

The default failure mode is defrost and heat I believe, so is it when you accelerate or putting the engine to use that the heat comes on? If so this would point to a vacuum leak/broken heater element.

Otherwise, if it's not this and the system holds vacuum then it may be a problem with the A/C pressure or sensors. Time to do some tests!
 

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1990 Mecedes-Benz 420SEL
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1,217 Posts
Try replacing the monovalve... it's located between the firewalls near the battery. You can either work fast to avoid much coolant seeping out while you change it, or drain some coolant out so you can work slower. This is usually the problem when you get this cycling effect.

Regards,
Dave

PS: My location info is based on a LHD Series 2 car!
 

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Discussion Starter #4
Where did you source the new climate control unit from - is it a brand new one? Can you confirm that the AC compressor is going on and off, or it could just be the air is being routed through the heater core when it shouldn't be, which perhaps is either a faulty switchover valve or a bad vacuum element.

The default failure mode is defrost and heat I believe, so is it when you accelerate or putting the engine to use that the heat comes on? If so this would point to a vacuum leak/broken heater element.

Otherwise, if it's not this and the system holds vacuum then it may be a problem with the A/C pressure or sensors. Time to do some tests!
I tried a used climate control unit but it operates exactly as the one in the car. Compressor works, but not sure if it turns on and off. Intermittent heat at all driving circumstances. Is switchover valve same as mono valve?
Thanks for your help.
 

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Discussion Starter #5
Try replacing the monovalve... it's located between the firewalls near the battery. You can either work fast to avoid much coolant seeping out while you change it, or drain some coolant out so you can work slower. This is usually the problem when you get this cycling effect.

Regards,
Dave

PS: My location info is based on a LHD Series 2 car!
Thanks for your suggestion. I'll take a look at the car and decide whether to try replacing mono valve myself taking it to the shop.
 
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