Does anyone know how to do this purple wire trick?

No I do not. Perhaps you can provide a bit more info on your objective. Is this for a 300E? What year, etc.

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor

Right. 1988 300E. 50K miles.

There's a reference to this procedure in the Bentley E-Class Bible, but it's murky. Other posts are confusing about it as well. It seems the purple wire is under the fusebox but I'm unsure about its exact location. Raising the idle by 50rpm supposedly smoothes things out a bit.

There was a factory mod on the wiring at the fuel injection computer. Unplug harness and cut into the wiring, find the purple wire and shave insulation. Find solid brown wire[ground], cut into it and use a spare piece of wire to join the solid brown to the purple. This will raise rpm's by about 75.

Another explanation:
1. look for X26 connector (rectangular box with 12 pole connections) under the lower fuse box on the engine compartment on the drivers side

2. identify the pure violet (or purple) wire, unplugged the connector box, open the male connector side and remove the
violet wire-male connector from the connector box.

3. Plug back the connector box and ground the male connector violet wire to the ground.

the female connector is the hot wire from the ignition switch.

the male connector is line to the ECU and this wire is to be grounded to increase the idle by 50 to 100 rpm. if this wire is
unplugged and not grounded, you will note a high and fluctuating idle.

if you want to be sure if you got the correct wire, test for voltage and should have no voltage with ignition on.

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor

OK, where is this wire bundle located? Thanks.

The easiest place is to unplug from the computer and then cut into the wiring. The computer is black, the abs is silver which are behind the battery on the firewall..

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor

1. look for X26 connector (rectangular box with 12 pole connections) under the lower fuse box on the engine compartment on the drivers side.


Confused. Under fusebox or behind battery?

The information provided was obtained by performing a search in this forum, e.g. "purple"+"RPM".

There are two different methods suggested:

1. Splice the wires at the computer which is located behind the battery.

2. Splice the wires under the fuse box.

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor

Got it. Thanks, I'll give it a try. Appeciate your help.

I tried this trick and it worked a charm. Thanks.

Do enlighten us to the changes observed with the modification of the purpule wire,the most obvious being a rise in RMP ,how much?
thanks
mak

The rise is only ~50 rpm. The only way to notice it is by grounding/ungrounding the purple wire. Very slight but every little bit helps 'cause the factory idle rpm is quite low and unless your motor is PERFECT, the idle is never as smooth as it should be.

Just curious, what is your idle rpm's after the wiring modification?

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible

Not sure exactly but looking at the tach it's closer to 500 than 1000rpm.