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.

http://www.peachparts.com/shopforum/showthread.php?t=5169&highlight=470+ohm

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

fahrgewehr2 · #7 07-13-2005, 11:44 PM

Check the mixture readings next. Easy to do, and it sets a good baseline.

DOes the car start easily? How are the warm up charactersitics? WHEN exactly does it hesitate?

giewont · #8 07-14-2005, 09:47 AM

It starts fine COLD but needs a little gas when WARM. The hesitation is only present when WARM along with the low RPMs. When cold and in gear it runs about 700 - 900 RPMs but after it gets to temp the RPMs hover above 500. I will look at the mixture tonight along with the OVP. I read on one of the threads that if the OVP was Austrian it should be replaced.

-Chris

giewont · #9 07-19-2005, 10:35 PM

Just an update and maybe some more help if possible. I replaced the fuel filter as it looked like it was original. It still had the Blue plastic insulator same as the the fuel pump. No improvement in the "in gear Idle". Next located the OVP relay and pulled it out did the rattle test and it failed. Replace it with an OEM from the dealer and still no change. Started the "testing and regulating engine" procedure 07.3-110 and found the dwell at 14% and when I check the ignition coil on Jack 5 terminal 15 I read battery voltage but on Jack 4 terminal 1 I get 4.85 volts. According to the procedure it should be equal to terminal 15 reading on jack 5. There is another procedure (15-563 ignition system) that is referenced if it is way off. I tried braingears for that procedure but the server is up and down. Anyone have this procedure they can email me? Or does anyone have any suggestions as to way the readings are off? Did I miss something? I know my Hondas well but these gas powered MB are a little more then I bargained for.

sbourg · #10 07-20-2005, 09:51 AM

If you're idle is erratic AND acceleration off idle poor when warm, I would absolutely check the airflow meter pot before continuing. These potentiometers will eventually wear out the resistance element where they spend the most time - at idle. When that happens, it will fail in it's main function, which is to alert the ECU of a rapid change in airflow. Since the wear is at the idle airflow position, that is where you will notice it.

At idle, this pot will also send erratic info to the ECU, due to poor contact, causing the ECU to vary mixture via the EHA, which causes erratic idle. This does NOT occur with a cold engine because idle speed is higher, and the pot is functioning in a range where contact is still good.

The pot is located on the side of the airflow meter unit, where the pivot shaft of the meter vane is located. It is fastened with two screws through slots, so it can be rotated for calibration. Before removal, it should be marked for position so it or a new one can be installed in the same position, for rough calibration. It can also be rotated slightly to see if that will change the symptom of off-idle hesitation and unstable idle.

Moving the pot will temporarily upset idle speed, but it is not used to determine stable idle speed. It is not available from Mercedes - pity - but can be found at Bosch electronics suppliers. Info available searching posts here.

Steve

giewont · #11 07-20-2005, 10:50 AM

Steve,

I know I haven't check that and you have already suggested that once before, Sorry. I will test it tonight. But since it is a potentiometer it should have a certain amount of resistance ideally, correct? If so, what is that value?

Bruno_300TE · #12 07-20-2005, 11:18 AM

I remember that I found this thread useful when I exchanged the potentiometer:
http://www.peachparts.com/shopforum/showthread.php?threadid=62133
In case you need a new one: I bought it from europartsdirect.com for about 45$ (Fastlane does not sell it to my knowledge).
Good luck, Bruno

Lenny Lipscomb · #13 07-20-2005, 12:42 PM

The idle speed control will cylce up and down if it is working properly (50 rpm + - should be acceptable). The 103 engines in the 88-91 time frame have what MB technicians call a "twitch" at idle. I replaced every component in the fuel system with excepion of the distributor and air flow meter. I still have the twitch. You can get rid of the hesitation by playing around with the mixture screw which is accessible through the top of the air cleaner. If you have it set by a mechanic using an exhaust gas analyzer it will continue to hesitate.

The A/C compressors in these cars have a difficult time keeping with the cooling load when everything is to specs. I suspect that you would be better off going back to R12. There is supposedly a compressor oil now available which is compatible with both refrigerants.

sbourg · #14 07-20-2005, 06:12 PM

Quote:

Originally Posted by giewont

But since it is a potentiometer it should have a certain amount of resistance ideally, correct? If so, what is that value?

You cannot measure its resistance accurately, since there is a large series resistance with the output, and the wear only occurs under the contacts. A good one or bad one will measure about the same on a resistance scale. You can SEE the wear if you remove the pot, and you could measure the erratic voltage probably if you disconnect the EHA plug. However, when you remove the pot, the wiper stays captive to the airflow meter, so the center terminal of the pot will measure open circuit. Still, inspection is your best bet, and replacement.

Steve

giewont · #15 07-25-2005, 11:02 AM

Steve,

I finally got to check the air flow potentiometer this weekend along with completing the r134 to r12 conversion. Man the A/C is alot better now then it ever was with the r134. Anyway, getting back to the potentimeter. I did not remove it to check the wiper arm but did check the voltages. Between pins 1 and 3 I read 4.85 V and between pins 1 and 2 it was .83v and as high as .95v. It never was lower then the .83 though. According to the 0.73-121 procedure it is out of spec and the air flow meter needs to be replace. I have a potentiometer on order and it should be in this evening but I wanted to ask you about the body alignment vs the potentimeter adjustment screw. What does the body aligment effect vs the screw ? I wasn't clear and I read several of the posting back from 2003 you had replied to.

Thanks,

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