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Enable 1st Gear Start

on the 1984 500SEL (W126)

by pmckechnie


DIY 1st gear start, 80-85 S-Class

I was messing around with my 84 500SEL and found that it has a thing called "Moving off 1st gear" This is the system that makes it start off in 1st gear when the shifter is moved to 3 or 2. It didn't work on my car and I traced the problem to the NSS. I got to thinking, what if I just put a switch where I could get to it. Well, I used an old rear interior light switch and hooked the wire that normally goes to the NSS to one terminal and Ignition power to the other. Put the switch in the position used by the radio fader and I now have 1st GEAR START that can be disabled by turning off the added switch. Cost, nothing.
After checking I found that all 80-85 126 and 107 cars have this system. This is information I got from Alldata so it may not be correct, but I know that 84 and 85 500SELs have it. I'm sure it could be used on other models. It is a very simple system. There is a special relay or control unit that only has 4 wires. It only needs a speed signal from the speedometer, a ground, 12 volts that is switched on and off by the switch I added, and a wire down to the kickdown switch under the gas peddle.
It works great, can be disabled and very low cost if you can get the parts at the local PAP.

I have seen a lot of other things a lot more complicated on this site which were called DIY. On an 84 or 85 (the ones I know for a fact will work) there is only 2 wires and a switch that need to be added. One from the control unit very accessable, atached to the curise ontrol bracket just above the brake peddle and the other can be hooked to any ignition power such as the lighter socket in the ash tray. The control unit for this system has a 6 pin connector with only 4 wires used. The one used is a white wire and can be spliced into. The other wire is attached to the lighter in the ashtray which is black/green wire. The switch can be any type of on/off switch that the other ends of the two wires are connected to.
To me this is a DIY project. I realize that I do auto electric work as a living and building a new wireing harness is a DIY for me so each person needs to determine if it is a DIY for them. As far a adding the whole system to another car is a little more complicated but if you go to a PAP and get the control unit and it's harness is doable. I'll be glad to help anyone wanting to try this if I can.

I have been running this for a week or so and have not had any problems
or surprises. It works well and can be switched on or off.
I'm not good at explaining things but will try my best. If you have questions
just ask.

Step 1: Get an on/off switch. I used a rear interior light switch from an
85 SEL. Find a location for it and hook 2 wires to it to get the on/off function.
This is where I put mine.

Connect ign power to one wire (I spliced into the lighter socket blk/yel wire) and run
the other wire over to the "Moving off 1st gear" relay N23.
See photo
The black box in the center of this picture.

Step 2: Remove some of the outer covering from the harness at the N23 relay.
Splice the second wire from the switch to the WHITE wire at the N23 relay.
You only need pull the connector off the N23 relay and pull the plug down where you can get to it.

Step 3: Reconnect any plugs or wires that were disconnected. Road test in both
the on and off position. If everything works, reassemble and enjoy.

This unit (N23) could be removed from one of the above cars and added to any car
that has the kick-down solenoid on the transmission and doesn't have first gear
start already, for instance the 86 to 91 S-class. This will require a lot more wiring
but there are only 4 wires involved and shouldn't be too difficult for all the smart
people on this board.
This is a picture of the entire system that was removed from an 85 500SEL.

The 4 wires are:

Brown - ground

White - ign power from the added switch

Green - wire to the black wire at the kick down switch or the B switch in the shifter

Green/yellow - to speed signal from the speedometer. It can be found at the
speedometer or at the cruise module. The wire at the speedometer or cruise
controller is green/brown.

If anyone has any questions or needs more information, let me know and I will
try my best to help.

Discuss this DIY here.


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