89 300SE Idle problem along with A/C woes - PeachParts Mercedes-Benz Forum

giewont · #1 07-12-2005, 07:19 PM

I needed some guidance on where to begin troubleshooting a idle problem my Dad's 1989 300SE W126 has. First off, the car has 47K original miles and I just replaced the cap, rotor, spark plugs, air filter, Oil Filter.

What I am seeing (or feeling) is a slight 50 rpm fluctuation when car is in gear (550- 650rpm). It idles around 800rpm in park. NOt sure if this normal or what. ALso, it sometimes has a slight hestitation when starting from a stop light but then launches rapidly. One thing I was going to replace next was the Fuel filter but not sure after that. From the searches I have done on the forum, some suggest the Fuel Accumilator.

The other problem is the A/C. It was converted to 134 but it is no where near cold enough to defeat the Central TX heat. I have been considering converting it back to R12 and wanted to know the tips, tricks, and warnings the experts have here on the forum. I know the system needs to be flushed and the oil changed back to Ester from the PAG installed for the 134 conversion. Any info would be appreciated.

giewont · #2 07-13-2005, 09:33 AM

So after doing some more searching on this forum, it seems there are several things that could cause this issue on the w126 platform. One that keeps coming up is the OVP relay. Can someone tell me where it is located? The 123 I had wasn't near as complicated....

Mike Murrell · #3 07-13-2005, 04:28 PM

The following thread discusses a 470 ohm resistor that I believe was used to get an extra 50 or so RPMs. I believe this may have been applicable to your '89, although the thread may indicate otherwise.

470 ohm resistor trick

There's been talk in other threads about this resistor mod to bump rpms. Try the SEARCH function

The OVP is along the firewall, about midway across. Here's what it looks like. This device among other things provides power to the idle controller.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1EQ0PTV2S1KD0Z5JCU&year=1989&make=MB&model=300-SEL-006&category=P&part=Overload+Relay

Same for the conversion back to R-12. Use SEARCH facility. Many discussions. Larry Bible has had much to say on this subject.

giewont · #4 07-13-2005, 05:40 PM

Mike,

Thanks for the reply.

sbourg · #5 07-13-2005, 07:22 PM

I don't think fuel pump or accumulator would cause your problem. The OVP could be the problem, but age of the car, hesitation off-idle AND erratic idle sounds to me like the airflow meter potentiometer. Check plugs too to see if improper mixture could be the issue.

Steve

giewont · #6 07-13-2005, 11:35 PM

Steve,

Yes I checked the plugs last night and they all look good. Even wear and normal light brown, grey firing end.

-Chris

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