126 Fast Idel Problem
 

I have 500 SEL 1984 with 97K miles on it, recently at start up it is taking a little longer to turn over (6-8 Seconds) additionally intermittently it will go from idling its normal 600 RPM to 1500-1600 RPM and will not kick down. It will continue to idel this way for some time. This is an intermittent problem does not occur every time.The engine is spotless all linkage is clean and working properly. Up until thNo major changes have been done to the engine to cause this to happen. Does anyone have any suggestions it would be greatly appreciated.

Have you changed the Overvoltage proterction relay? It is among other things responsible for idle control. It is the little silver box to the left of your fuse box. It has a red top with a 10 amp fuse in it. Check the fuse first - dealer gave me a replacement for free, and if that doesn't work replace the relay, it simply pulls out and plugs in. Cost me about $120 Canadian and it is the best repair/replacement I have made.

I too had the same problem. I changed the OVP and that didn't do it. I cleaned the Idle valve and that did't do it either although it is a good idea to do a through cleaning of this valve. I even re-soldered the joints in the idle control module. That worked for a short time. Finally I bit the bullet and ordered a new module. That cured the problem. My guess is that you too will need a new idle speed control module. E-mail me if you need a good source.

2 factors to consider:

1. Idle Valve Control that may need replacing or dirt (if intermitten); or
2. Accelerator cable link is jammed and needs to be lubed or perhaps looked into. The 126, unlike the 124, does not run on cables and has links: K-jet which can be lubed.

I don't suspect an OVP problem here.

This thread may not line up exactly with your situation, but it won't heard to check it out.

http://www.peachparts.com/shopforum/tech-help/10452-idle-too-fast-wont-start.html?highlight=%2Afast+idle%2A

I had the same problem. It was fixed by replacing the idle speed control module.

I also had the smae problem cured by the replacement of the idle control module in an '88 420SEL -- expensive part: $300+.

Where is the IDLE SPEED CONTROL MODULE on the 89 420?

I am not sure about the 420 series but I bet it is in the same location as my 380. Check on the firewall about half-way of the engine. It is a black box about 4 by 11/2 inches and is held in place by a 10 mm screw. Loosen the nut( careful not to drop it) and pull the module and its harness out. It pulls away from the harness. The solder joints in these things are brittle and constantly fail. You can try to re-solder them or just buy a new one. I paid $250. It was worth it.

On the 420 series it is located on the inside of the passenger compartment on the firewall passenger side.