I tested the center terminal on the potentiometer, I get a reading that fluctuates between .047V and .051V Disconnecting the harness from the unit has no effect on the car that I can detect and rotating the unit does not change the reading??? I didn't try the other tests that you just posted yet as I figured this was another problem.

Thats is not the voltage you want on the center pin. You have measured this voltage with respect to battery ground right ? In other words, the red terminal of the meter was on the middle pin and the black terminal was grounded somewhere.

Find out what is the voltage between terminal 3 on the air sensor connector and ground. It should be close to 5V. If you dont get this, this is another big problem. Keep engine on for this test.

For this test, engine can be off. Check wire continuity for the air sensor wires from the sensor connector (terminals 2 and 3 only) to the ECU connector (17 and 18 respectively).

Also you want to check that the wires are not shorted to ground anywhere. Disconnect the connector from the air sensor. Check the resistance between terminal 3 (in the sensor connector) and battery ground. You should not get close to zero resistance. Check the same for terminal 2 in the sensor connector.

If you do find continuity, and dont find a short, then the problem is with the ECU, it is not sending 5V on its terminal 18. You may even be able to test this right at the ECU. Keep ignition on start position (car not running), and check if there is 5V on this terminal with respect to battery ground.

It seems like your wiring harness from the ECU may have more problems than initially thought. If there is one more harness problem, this is the one part you should replace as soon as possible.

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Saumil S. Patel

Just to be clear about the first two sentences in the previous message:

You were testing the voltage on the middle terminal with alligator clips connected between all the 3 terminals on the connector and the corresponding terminals on the sensor. The connections between the cable connector and the sensor connector have to be in place for the middle pin to have 0.7V.

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Saumil S. Patel

OK, Well I gave up and brought it to one shop who charged me $500 to clean the fuel distributor and adjust the air/fuel mixture then tell me he couldn't fix it. The car was no longer tolerable so I brought it to another shop which successfully fixed it. The problem was the distributor was one tooth off. They fixed the timing and re-adjusted the mixture the correct way and now the car drives like new.

Hi there,
may be you can list the shop that finally fixed it in the "Good Shops" directory.
Take solace in the fact that you made everything else perfect,too.

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Reinhard Kreutzer
1987 420 SEL (126)

I am so happy to have stumbled upon this thread as now I feel there's hope for finally solving and fixing the ongoing frustration nightmare I've been having for months now with my 88 420SEL! RC, I swear it was like reading my own experience with everything you were doing and how nothing seemed to be fixing the hesitation issue. and everyone that's posted helping you along the way seem so knowledgable I'm so happy you were able to finally get it all fixed.
I've been going through similar issue as well. My Beastie has never really liked cold weather but this year has got to be her worst ever. Takes nearly 20-30 min just to warm her up to 80 and her RPM's in idle drop really low to almost dying, oil pressure gauge started fluctuating between 2 and under the 1 unless driving. starting out just to switch gears and either back up or start driving when barely press gas pedal lately she immediately stalls out unless I gradually apply pressure while in either park or neutral and Rev her up to the 3 and then pop her into drive. then she jerks and sputters some but starts slowly going. Loss of horsepower seems as well, when 40 degrees or lower am lucky if able to even reach 35MPH and even if push pedal to the floor she won't kick into high gear to climb hills or pass slow traffic. Yet when is warm she drives relatively smooth and can get at least 85MPH, but still have to do the slow rev starting out. I've changed the plugs, air, oil and fuel filters, fixed a fuel line as was leaking due to rubber cracked, next was about to clean the Idle Control Valve in hopes would help get her moving like before again. Am I on the right path? Thanks :-)

One should not ever be deterred by a fifteen year old thread.