Friends:
I fear that this might be the onset of the W124 Wiring Harness Failure. Please advise if you think it might be something else (i.e. less expensive!):
--1995 E420 (W124, 8cyl, 117,000 miles). No damage history.
--Check engine light keeps coming on (I clear it, then 5-10 starts later, it comes on again)
--Error Code 26, which I understand to be the Electronic Control Module (also called the Elec Control Unit), the "brain" of the engine.
--However, if it is the brain, I am NOT experiencing any performance problems of note!
--But for the check engine light, I would not have cause to think that I have a problem.
--If it is the wiring harness, I understand the part number requires careful attention. Any guidance there is also appreciated.
Thoughts?
Thanks in advance.
RNGADNG

That code normally means that there is a vacuum problem in the up-shift delay for catalyst warm-up.

ON cold to cool mornings the up-shift cable is lengthened by a vacuum switchover signal & is designed to keep the trans engaged in 2nd gear longer to allow quicker warm-up.

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It is very possible that your looking at a failing harness. There is a thread about 140 harness failures but I hate to say it the 124 is just as prone to this problem.
In one week at the local Indy there were three (3) 124's all had wiring harness failures and all exhibited different symptoms. The most common are on the gassers under the hood and across the firewall. This is the result of bad insulation. If I had the harness info I would make looms out of Teflon wire and sell them all over the world
You can check the harness visibly by cutting open the jacket at specific points, its possible you have another problem too just checking the harness is not a bad idea.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

The wiring problem affects most Mercedes models in the early to mid 1990's. The replacement part takes care of this issue so there is no need to get your wire cutters out. If you want to manafacture wiring harnesses maybee you could help out the hundreds of thousands of other car owners who's car maker no longer support their cars and have no replacment harnesses. This problem affects many other European car makers and not just MB.

One of the luxuries of owning a Mercedes is being able to easily buy parts on a car that is considered obsolete. Most car makers only support their vehicles for 10 years before you can only buy parts in the aftermarket.

There's no reason to suspect a bad wiring harness. As MB Doc said you have an upshift delay problem. Code 26 has nothing to do with the ECU. You may need a new Bowden cable, not terribly expensive but a little tricky to get to.