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Old 10-20-2001, 10:24 PM
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Join Date: Oct 2001
Location: Dallas, Texas
Posts: 19
1986 420SEL HVAC - Idle

I have a 1986 420SEL - a great car!! Here are a couple of items I have the car taken in for and they gave me a incredible story. First a rough idle - cold or hot - but only at idle - when I drive at normal or even low speeds - no notice no TACH fluctuations. I was told the plugs fouled caused the problem - I disagreed and said that they were changed and did not fix the problem. Then then said - some $1800 part was bad - I should have stayed with the fouled plugs!! I did not have the $1800 part replace - sounded crazy to me!! ANY IDEAS??

Second, the AC periodically starts putting out warmed air - for about 15 - 20 seconds then goes back to great cooling. The mechanics checked it and said all is OK! Well I know that is not true!!! Any ideas? Are these two related to a vacuum leak? The mechanics said they checked the vacuum - but they do not have any credibility at this point.

Lastly, does anybody know where I can get the fix for the Becker radio security lock. Seems every time I work on the car the radio locks out and I have to drive to the dealer to unlock the radio security - what a pain.

Thanks for the help up front!! What a great forum - just found it and have bookmarked it from this point on!!!

Mark Pavelka
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Old 10-20-2001, 10:54 PM
David C Klasse's Avatar
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Join Date: Jan 2001
Location: Mission Hills in the City of San Diego
Posts: 2,352
For the rough idle, I would suggest an inspection of the motor mounts (because of no tach fluctuations) firstly.

For the A/C problem... seems to be a vacuum leak, but I'm unfamiliar with that whole deal. But I think it's vacuum related.

As for when you disconnect the battery and get the CODE on the radio... you should go to the dealer and get the code for yourself for future use! It's a 5 digit code that you punch in and then press SC. Good luck!

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Old 10-20-2001, 10:59 PM
MikeTangas's Avatar
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Join Date: Apr 2000
Location: So. Cal
Posts: 4,430
Welcome to the best Benz site on the Net.

As far as maintaining power to the radio when you work on the car, try doing a search. Donald Swinford, I believe, posted a how-to where you attach a 9v battery at the cigarette lighter. This will keep the radio memory powered while you work.

The A/C could be the A/C compressor cycling, could be a vacuum leak or could the ACC control. (again try doing a search for A/C puts out warm air??).

The rough idle, always. Hmmm...seems that was recently discussed at length. As I recall the poster was given a bunch of different reasons/diagnosis and probable costs. I think he found out something that wasn't terribly expensive and it fixed his rough idle.

Can you guess what's next

Yup...try doing a search for 420SEL rough idle. I bet you find lots of good info. Unfortunately a near new model like a 1986 is not in my repertoire...yet.

Lots of luck, again welcome to the best Benz site on the Net. Someone more knowledgeable in the newer models may be able to give you more info than my simply pointing where it is .
Mike Tangas
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Old 10-21-2001, 08:31 AM
dpetryk's Avatar
Electrons can do anything
Join Date: Sep 2001
Location: Houston, Texas
Posts: 1,071
I have similar experiences with the AC. It turned out to be bad solder joints on the printed circuit board inside the climate control unit. MB uses a single sided non-plated thru hole printed circuit boards which suffer from vibration developed micro-cracks. If you are handy you can take the thing apart and re-solder the board.

As for the rough idle, I would check the fuel injectors. They are nortorious for causing that symptom.

Get the dealer to give you the 4 digit security code for the radio and simply key it in when it asks for it. It takes 10 seconds to enable the radio if you have the code.
I got too many cars!! Insurance eats me alive. Dave

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Old 10-21-2001, 10:56 AM
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Join Date: Jul 2000
Location: Toronto, CANADA
Posts: 1,193
I second the thought that your AC problems are related to the control unit in the dash. Keep us posted.
Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired
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