[W126 Coupe] CIS-E Blues

[Robert Karl Stonjek]

Hi,
I've been trying to solve a CIS-E problem for weeks with no success.

The original symptoms were:
1)    Missing at high revs (4,000+) in third gear;
2)    sudden dip in power above 4,000RPM (all gears);
3)    excessive fuel useage - 11mpg city, 18mpg highway (9 and 15 miles per US gallon).

The car is a 1987 560SEC with 90,000 miles on the clock.  It is the 220HP RUF dual manifold/no cat version.

I was told by two experts, including the local Mercedes dealers, that the problem had to be the Fuel Distributor.  But I have found problems in every electrical system in the car that I have looked at thus far, so suspected something electrical.

I started by removing and examining the EHA, which I passed (diaphragm moves freely, coil resistance correct).  I then checked the EHAs operation by measuring the mA and got the following readings: 68.8 with ignition on, motor not running (75 specified); +20 engine cold, running down to +3.2 hot (specified +/- 3 2 when hot).  But when revving, the needle dipped to the minus and continued down to -9 at 3,000RPM.  It is supposed to rise to +6 at 5,000.

I reasoned that if the leaning of the mixture was the cause of my problems then disconnecting the EHA should resolve them and this was exactly the case.  This led me to the CIS-E.

I examined the CIS-E and replaced the electrolytic capacitors (they dry out with time).  Could see no dry solder joints, no sign of water damage, no shorts etc.

Next I examined all the peripheral devices such as the altitude sensor, temperature sensors, idle speed control unit etc.  All passed.  I reasoned that even though the car does not have an oxygen sensor, changing the voltage of pin 8 should also change the rich/lean mixture.  No effect.  Same with other peripherals - the CIS is not listening.

I noted that while the engine is heating up I get positive (enriching) readout with revs.  But as soon as its warm it swings to negative.  In fact I can plug the EHA lead in backwards and the car runs fine (except that it won't start cold).

I assume that in the closed loop operation it is failing.  What I need is a circuit diagram of the CIS-E so that I can repair it or modify it so that it stays in open loop mode even when hot.  New CIS-E devices are very expensive ($1,200+US), which is a lot for a computer with a brain smaller in power than the average modern cellphone or digital camera.  Besides, a new unit would almost certainly be a version designed for use with a CAT and O2 sensor, and so would have to be modified for the high power model, which means I'd need the circuit diagram anyway.

If anyone can help me out here I'd be extremely grateful.

BTW the two pipes that come from each bank of cylinders join up under the driver's and passenger's seat and give a meaty but dignified roar as the revs rise - like you just ran over a lion's tail :)

Kind Regards
Robert Karl Stonjek
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