First Gear Start Module Interest?
 

Hello everyone,

A friend of mine on here (Hit Man X) asked me to looking to making a first gear start module for the W126. I think specifically for his 300SEL. I only have a 420SEL to do prototype work so we'll see how it all goes.

I was wondering if there was anyone else interested in such a thing should I be successful in getting it to fully function.

Most of the work I've done so far is just theory/preliminary stuff. My controller should arrive end of the week sometime so I can do more testing then.

Just thought I'd post this up and see if anyone else is interested in the results.

It basically clicks the kickdown switch for you when you come to a stop, right? Might be nice to have a switch somewhere in the car to select 1st or 2nd gear start.

-J

Thats the general idea. The controller will monitor the speedometer signal, using that as a trigger. It will engage the kickdown solenoid under some predetermined mph and release it afterwards so you dont hold first gear out fully.

Maybe 7-10 mph or so?

I was planning on including a switch to disable the entire unit as well.

Our 1990 300TE - with the S / E switch - has first gear start from the factory, if you set it to S, it rockets away, whereas it pulls away steadily in E.

US cars didn't get an E/S switch.


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Interested. Is it purely electrical or a valve body mod?

Purely electrical. I've tested the basic ideas on my 420SEL and it seems to work pretty well. From what Hit Man X has told me about his 300SEL, I don see why it wouldnt work on other 2nd gear start type MBs.

Would it work on my 1988 300E M103? Very interested.

I believe it would, I don't have a second gear start M103 to test it with (Hit Man X's 300SEL)

When you full-throttle the car, and hit the kickdown switch, does the car get into first gear? If so, then I don't see why it wouldnt work.

Yes it does but it will also start in 1st gear from a stop if you manually shift the lever down to 1st or even 2nd and then back to D. When the light turns green, off you go in first gear. That's why I believed it was the valve body and not purely electrical.

Thats what my 420SEL does as well. So it sounds like the module would work for you.

The module will simply engage the kickdown solenoid when the vehicles is below some predetermined speed, say 7-8 mph. Once it is over that speed it will disengage the solenoid and normal shifting will resume. Every time you slow down and come to under 7-8 mph it will engage the solenoid and back to first you go.

I'm guessing this pertains to the 4 speed non electronic controlled trans correct? Was there a 5 speed non electronic? ( My 97 sl320 has a 5 speed electronic shift with a Winter 2nd gear start / Sport 1 st gear start switch )

Some things to consider:

Build a bit of hystereses into the controller otherwise it might toggle if you drive at switching speed.

The KD solenoid might not be rated for continuos duty, adding current limiting ( pulling the voltage back, peak and hold ) or running PWM would help the situation. Finding a donor solenoid and running it at 14.7 V ish at over 200*F would be a good test.

The factory 1st gear switch sounds interesting, a look at the wiring diagram would be a good idea.

Moving the shifter to 2nd / 1st to engage 1st start sounds interesting and , if the trans is hydraulic controlled, you might only need to pulse the KD sol.

Anyone have a hydraulic diagram / apply chart of the trans in question?

Also a pressure chart is needed, the non factory 1st switch units might not have enough line pressure at light throttle to prevent slippage.

MSD makes a speed ( frequency ) operated switch as does Murphy. I've used the Murphy unit on industrial diesel engines that needed overspeed shutdown. ( diesels that work in a refinery / gas / oil well area need a overspeed air intake shutdown down because if there is a flammable gas leak they will run away. )

On my 300E you can do it with the electric kickdown switch by flooring it off the line, or by shifting to the lowest position. On my 420sel you can do pedal to floor, or drop to lowest selector position then click it to the right, this one must be completely electronic whereas the 300E is both valve body and electronic is my guess.

I would be interested in a 1st gear start for both of them. :D

Yes the original target was Hit Man X's 300SEL which is the 4 speed non-electronic controlled transmission. This should apply to any 722.3/4 series transmission. The 722.5 were electronically controlled I believe so this would not apply to them.

Good idea on the hysteresis, would not want the unit to be constantly trying to engage the kickdown solenoid.

I'll have to see if I can find out more information about the solenoid. Adding current limiting circuitry is a good idea. Arc suppression as well as inductive kickback prevention as well.

Can you elaborate more on the usage of PWM in this case?

The factory 1st gear switch sounds interesting, a look at the wiring diagram would be a good idea.

From my brief testing in the 420SEL the KD solenoid should be engaged the duration one wishes first gear to be engaged. When I disengaged my switch the transmission shifted immediately into the next gear.

Hit Man X recommended about 7 mph for the switching speed. I think this is pretty close to what the normal first gear start cars shift into second.

Some solenoids that need to generate high pulling power or have long throw, are wound with fewer turns / thicker wire ( more current draw / greater magnetic field ). This is OK for short duty cycles but they will burn up if kept at full power.

With PWM ( Pulse Width Modulation for others on this list ) , the circuit would apply full power to pull the solenoid in then , say after a second, rapidly pulse the output to reduce current ( and voltage ) If the pulses are rapid enough, the sol stays pulled in as the magnetic field to hold a sol in is usually lower than pull in.

PWM has a frequency ( how often the power is turned on and off ) and a duty cycle ( how long the power is on )

To clarify, the continuos duty of the KD sol I was speaking of would be when the car is idling at a stop light / sitting running warming up for who knows how long.