I am having problems with either my idle control unit, or the wiring. However I am going full EFI on the car, as the old system is trash anyway, it was probably good when new. But I am sure not going to keep a car with a system that is closing 40+ years. So I won't waste any money on the original system. My question is though, have anyone had problems with their wiring harness? Did it need replacing or was it something that caused it be like that?

Are you sure it's the wiring that's the problem? The idle control board is known to develop solder faults (cracked joints, poor connections, etc) and cause some truly random idle issues.

If you're going full EFI, why are you bothering to troubleshoot it now?

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Because I don't plan to do it here, the plan is to give it to a shop. The EFI kit is a little expensive, but so are all the parts I need to change related to the K-jet. So if I can quick fix it, its better, will have a closer look tomorrow. But no I am not sure it's the wiring, the idle control computer has been replaced by the previous owner though, and he made home made connectors and wire going to components from that unit. The home made wires got a bunch of cracks, especially the wire going to the temp sensor. The unit in mine is from 87 500SEL, mine is a 88 560, and from what I understand, i need a us spec 560 controll unit for it to function properly, but don't know if that my case, since the wires don't have any power. The green stuff on the control board itself, did have a few minor scratches, and plugging it in, made little to no difference on the idle.

My idle problems resolved when I replaced the idle control valve and its associated rubber air flow parts.

Note there seems to be many different part numbers for these idle control valves, need to get the correct one.