Mercedes-Benz Forum banner

1 - 17 of 17 Posts

·
Registered
Joined
·
24 Posts
Discussion Starter #1
I drive a 1988 420SEL. Recently, I noted that when I would come to a stop at a light the ac would warm quite a bit. Not heat - just not cold ac. When I would start to drive the air would cool again. I check the following:

  • freon levels good
  • auxiliary fan works
I took it to a shop and the tech agreed that the freon was fine and the fan was working. He also state the evaporator was good and that the AC is functioning as it should and to just embrace the 80s inefficiency. But the AC used to be great ....

Now I notice, even when driving, the ac seems to be significantly warmer than is to be expected. Now...I'm not really sure who to take it to since the mechanic was my "mercedes" guy.

Any ideas of what to check? Could this be a vacuum issue if all else seems ok? Any help would be appreciated.

Sent from my SM-G960U using Tapatalk
 

·
Registered
1989 300SE 232k miles
Joined
·
1,449 Posts
So many places to start...

Are you still on r12? Or has the car been changed to r134a?

How did he check the evaporator? That’s deep inside the dash and requires a lot of work to physically check.

Did he look at the condenser? That’s the “radiator” in front of the radiator. It’s where the aux fans blow.

...welcome to my world. Where my car blows warm(er) in motion (50-60F) and then reaaaaal warm (70-80F) at idle.

I have to double check my vacuum pods. I think mine may be the scoop flap not working properly. This mixes the hot and cold airs.

I also plan to flush the different components (evaporator, condenser, hoses) individually. If I find a singular one, I’ll let you know.
 

·
Registered
Joined
·
24 Posts
Discussion Starter #3
Hmmm....good question about how he checked the evaporator. I plan on diving in next week myself to check the pods at least. Your mention of them not closing and allowing hot air is interesting....I'll let ya know .

Sent from my SM-G960U using Tapatalk
 

·
Registered
1967 250 S (sold) 1986 560 SEC 1987 560 SL
Joined
·
1,315 Posts
The expansion valve would be my guess. I had another car that had symptoms similar to yours and it was the expansion valve. On the 126 they are a PITA to remove ( under driver dash) and I used crows' feet on extensions. Prob is that you have to evacuate the system first...
Wishing you success!
 

·
Moderator
1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
Joined
·
4,653 Posts
Hey Red,

I hear you, man...

Here are several ideas regarding AC.

You haven't said yet whether its R12 or R134a we are talking about yet.

My '87 560 has R12 still and a couple of years ago, I found the evaporator upstream face to be partially/greatly covered with a grey 'fuzzy' dust kitty schmutz that was apparently attenuating airflow from the Fresh Air & Recirc flap. ALL the airflow into the ACC box was restricted. Once cleaned, airflow was way better and my A/C work [hose, TEV, drier, etc.] still appreciated. So maybe the airflow across through the system might be something to look at?

My '91 has factory R134a fill and after a leaky compressor & receiver/drier change.. ICE cold.

Years ago, [many] we used to charge the then R12 systems at 2500 RPM, both front doors open, fan High.. until there were NO bubbles in the drier sight glass.
This was on NEW installations & repairs.

I have done & can do that on my R12 560 system & it works great.

My 500 with R134a, I cannot. With a fully charged system [by weight this time] I get a wavering foam across the sight glass at the same engine speeds. It's darn cold, so who am I to complain?

If there are dead insects & other road stuff in the airstream between the condenser/radiator fins.. there goes the airflow.

My S70 Volvo does not cool the A/C at very low speed as higher ones. Its system is good with many recent new components compressor, accumulator, TEV, etc. R134a charged by weight. I take that as normal.

Sadly, I have not driven either of my SECs since late October, but I'm pretty sure they both are less efficient at idle & low engine speeds.

I may be way out of touch, but I guess your 88 is probably low on freon, give the metric just cited.

Good luck man

MBL
 

·
Outstanding Contributor
350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
Joined
·
3,304 Posts
I'm guessing your guy checked the pressures on the system. First reason for it to not cool at low speeds is a failing compressor, but the pressures would be off.

Second reason is insufficient airflow across the condenser at low speeds. Is the aux fan coming on? It should be running at low speed if you're idling on a warm day with the AC on.

Third reason is something wrong with the air control doors, but I wouldn't expect it to be speed sensitive.

Last one I can think of is a long shot: your heater is staying on and you also have a bad monovalve, so you only get heat at low speeds.
 

·
Registered
1989 300SE 232k miles
Joined
·
1,449 Posts
I'm guessing your guy checked the pressures on the system. First reason for it to not cool at low speeds is a failing compressor, but the pressures would be off.

Second reason is insufficient airflow across the condenser at low speeds. Is the aux fan coming on? It should be running at low speed if you're idling on a warm day with the AC on.

Third reason is something wrong with the air control doors, but I wouldn't expect it to be speed sensitive.

Last one I can think of is a long shot: your heater is staying on and you also have a bad monovalve, so you only get heat at low speeds.
As I’m having similar issues in my 300SE, I have 2 questions from this.

1) I’ve jumped my aux fan when idling and the blowing temp increases. So I’m looking elsewhere. But, I’m pretty sure my aux fan does not run at it’s low speed setting. What do I check/replace for that?

2) I tested my car’s temperature dial yesterday, at 75F ambient. When moving, the air out of my center vents blows the same no matter what temp I set it at, except max heat. I set it at 62F, it blew 60F. I set it at 75F, it blew 60F. I set it to 82F it blew 60F. I set it to max heat, it blew 90F. I gather that’s not normal?
 

·
Registered
Joined
·
24 Posts
Discussion Starter #9
Hey Red,

I hear you, man...

Here are several ideas regarding AC.

You haven't said yet whether its R12 or R134a we are talking about yet.

My '87 560 has R12 still and a couple of years ago, I found the evaporator upstream face to be partially/greatly covered with a grey 'fuzzy' dust kitty schmutz that was apparently attenuating airflow from the Fresh Air & Recirc flap. ALL the airflow into the ACC box was restricted. Once cleaned, airflow was way better and my A/C work [hose, TEV, drier, etc.] still appreciated. So maybe the airflow across through the system might be something to look at?

My '91 has factory R134a fill and after a leaky compressor & receiver/drier change.. ICE cold.

Years ago, [many] we used to charge the then R12 systems at 2500 RPM, both front doors open, fan High.. until there were NO bubbles in the drier sight glass.
This was on NEW installations & repairs.

I have done & can do that on my R12 560 system & it works great.

My 500 with R134a, I cannot. With a fully charged system [by weight this time] I get a wavering foam across the sight glass at the same engine speeds. It's darn cold, so who am I to complain?

If there are dead insects & other road stuff in the airstream between the condenser/radiator fins.. there goes the airflow.

My S70 Volvo does not cool the A/C at very low speed as higher ones. Its system is good with many recent new components compressor, accumulator, TEV, etc. R134a charged by weight. I take that as normal.

Sadly, I have not driven either of my SECs since late October, but I'm pretty sure they both are less efficient at idle & low engine speeds.

I may be way out of touch, but I guess your 88 is probably low on freon, give the metric just cited.

Good luck man

MBL
Thanks for the tips ! It is r134 converted. Gonna dive in tommorow and use your suggestions.

Sent from my SM-G960U using Tapatalk
 

·
Registered
2008 E350 110k, 2000 E320 143k, 1983 380SEC 240k
Joined
·
957 Posts
I found the evaporator upstream face to be partially/greatly covered with a grey 'fuzzy' dust kitty schmutz that was apparently attenuating airflow from the Fresh Air & Recirc flap.
How did you clean this?
 

·
Moderator
1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
Joined
·
4,653 Posts
Hey nugent,

Initially, I was looking for evidence of refrigerant oil/evap. leakage & I found another issue instead.

Here are the gory details from 2016.


It took a fair amount of time and proved amazingly effective.
I did get quite a bit more airflow through the entire system after my efforts, and yes, the output was cooler too.

Stay safe all.

M
 

·
Registered
1989 300SE 232k miles
Joined
·
1,449 Posts
That thread was a fascinating read. I need to test my climate control vacuum solenoid, so I’ll be right at the fan. Now that I know how, gotta take a look at the top of the expansion tank.
 

·
Registered
Joined
·
24 Posts
Discussion Starter #13
So, for those of you with similar issues I got the problem resolved! I had always noticed intermittent heat issues and so I thought maybe the monvlovalve was allowing hot fluids in when unwanted. I tackled that first.

Upon removal I did confirm the rubber diaphragm was awful with huge tears present. I ordered a cheap one and grafted the new diaphragm onto the factory Mercedes part. The ac issue wasn't resolved, or only partially resolved but now the heat worked perfectly!

Part 2: check the vacuum pods for the fresh air flap behind the glove compartment. Sure enough, the short throw one on the right had failed and the fresh air door was permanently open. I pulled it off and plugged the vacuum ports and boom - the flap opened for recirculation and I had cold air again! The new pod was ordered!



Sent from my SM-G960U using Tapatalk
 

·
Registered
1989 300SE 232k miles
Joined
·
1,449 Posts
So, for those of you with similar issues I got the problem resolved! I had always noticed intermittent heat issues and so I thought maybe the monvlovalve was allowing hot fluids in when unwanted. I tackled that first.

Upon removal I did confirm the rubber diaphragm was awful with huge tears present. I ordered a cheap one and grafted the new diaphragm onto the factory Mercedes part. The ac issue wasn't resolved, or only partially resolved but now the heat worked perfectly!

Part 2: check the vacuum pods for the fresh air flap behind the glove compartment. Sure enough, the short throw one on the right had failed and the fresh air door was permanently open. I pulled it off and plugged the vacuum ports and boom - the flap opened for recirculation and I had cold air again! The new pod was ordered!



Sent from my SM-G960U using Tapatalk
This is exciting! I flushed my condenser and evaporator. And I used a fiber optic camera to look at my evaporator, it’s clean. I’m blowing in the 40’s F when moving, and it climbs up to 55 - 60 F at idle during the day. Maybe I’ll pull my mono-valve and look at it anyway.

Good job! Particularly rebuilding your mono-valve!
 

·
Registered
Joined
·
24 Posts
Discussion Starter #15
This is exciting! I flushed my condenser and evaporator. And I used a fiber optic camera to look at my evaporator, it’s clean. I’m blowing in the 40’s F when moving, and it climbs up to 55 - 60 F at idle during the day. Maybe I’ll pull my mono-valve and look at it anyway.

Good job! Particularly rebuilding your mono-valve!
Thank God for this forum! There is a really good posting about it somewhere! It was definitely more of the vacuum than the mono I think.

Sent from my SM-G960U using Tapatalk
 

·
Registered
1989 300SE 232k miles
Joined
·
1,449 Posts
Thank God for this forum! There is a really good posting about it somewhere! It was definitely more of the vacuum than the mono I think.

Sent from my SM-G960U using Tapatalk
Interesting. Good to know. Because I’ve already rebuilt my vacuum pods. It did make a huge difference, using recirculated air vs fresh.
 

·
Registered
1989 300SE 232k miles
Joined
·
1,449 Posts
One other thing, with the vacuum pods, when you replace the pod check that you can turn off the defrost via the climate control unit. If you hit the air flow buttons and you always have air blowing up defrosting the windshield, there’s another vacuum leak somewhere. Blowing defrost is the default, “safety,” mode.
 
1 - 17 of 17 Posts
Top