Making CSS pressure switches last longer

[merrill]

Gnuface,
I put the diode on the CSS, & I verified that the band on the diode was on the "+" side.

[Larry & Norma]

How did you verify + side?
The only sure way is to disconnect both wires, turn on ignition and test each wire
for 12volts connecting the other probe or test lamp lead to ground.
 Only one will have 12 volts and that is the one that the line on the diode must go to.
If you leave the wires connected to the CSS when you measure then they will both appear
to be at 12 Volts.

[merrill]

yep, did that, disconnected both wires & put my volt meter in line.
ground to ground strap and + to the wire.

strange

[Larry & Norma]

Maybe a bad diode?

[wwheeler]

I went to install the recommended diode on the CSS. So far it works as before and hopefully the switches will last longer!

I had to buy the assortment pack from Tandy in order to get the 002 diode. They didn't offer it seperately. Disconnected the wires at the CSS and found the + which was on the same side as Larry's. I installed the band side of the diode towards that + wire. I used two ring terminals and soldered the leads of the diode. I decided to "fancy it up" a bit and put heat shrink tubing around the assembly for protection and to disguise it.

I tested the diode before I soldered the leads and came up with .578. My meter has a diode setting and that is what I used. Not sure what units that is in. It wouldn't register with the leads reversed which is what I think is supposed to happen. I think a diode is like an electronic check valve. After soldering, the reading was the same .578 so I know I didn't screw it up. If Merrill soldered his diode, maybe it got too hot and failed? I had heat sinks on the leads when soldering.

Next are the diodes for the trans solenoid. I have some interesting info for that one.

[jacovdw]

Wallace,

The value of "0.578" you achieved with your multimeter implies the forward voltage drop across the diode when the red test lead is connected to the anode of the diode and the black test lead to the cathode.
The value is typically measured in Volts.

Basically how the diode test function works, is to pass a small known current through the diode and then measuring the voltage drop across the terminals.

Reversing the leads of the multimeter results in no current flowing through the diode and hence no reading or ".OL" displayed on the screen of your digital multimeter.

If you are using an ordinary (no diode test function) digital or analogue multimeter with the ohm meter function (resistance scale), then the values presented on the display has no real significance.

[wwheeler]

Very interesting. I know very little about electronics but just enough to be dangerous!

[wwheeler]

I had such good luck with the CSS diode (thanks Larry , I am hoping to install diodes at the terminal block for both the idle switch and the kick down switch but ran into some questions. As I said, I am not an electronics expert! In order to find the +, I did some tests and below is what I found:

With wires disconnected at the idle and kickdown switches and ignition on, I found 12 V at the wires feeding both switches. 1st question. According to the trans diagram, it shows these two switches to have feed wires hooked to the ground. Therefore they would be switching the ground just like the pressure switches do. Am I reading that correctly? But my switches clearly have +12V feeding them. I need help understanding that.

With the wires feeding the trans terminal block disconnected and with the ignition on, I found:
- The small terminal had 12V at kickdown and 0V at 1/2 throttle and at idle.
- The medium terminal had 12v at idle and 0V at 1/2 throttle and at kickdown.
- The large terminal never had 12V and was always grounded with the ignition on.

So to me, the two HOT (12V) terminals are the small (kickdown) and medium (idle) and the large is the ground. 2nd question. Larry has both bands of the diodes facing the medium (idle 12V) terminal. In my case, seems like you would want one diode band facing the medium (idle) and one facing the small (kickdown) terminals. Then both diode leads opposite the bands would be installed to the large terminal. Is that in anyway correct?

Maybe my W111 is wired differently than the 113 because the diagram just doesn't make sense to me. And I need help understanding why Larry installed the diodes the way he did.

Thanks for the help!!!!!  

[Larry & Norma]

Hi Wallace,
This is how I see it, the 3 position solenoid is two solenoids back to back with 12 volts going to the centre common point.
If either of the other wires are grounded (by kickdown or idle idle switch) then that solenoid is energised and moves the
rod in the relevant direction.
It is easy to become confused when measuring voltages as a solenoid is just a very long piece of wire coiled up so applying
12 volts to one end will measure 12 volts the other also if it is not connected to ground!
I would expect the terminal block to be wired the same as mine and as the diagram.
After taking note of where they go remove and seperate all the wires, only one will have 12 volts on with the ignition on.
(Make sure they do not touch any metalwork) The terminal with this wire on has, as before, the lines on the diodes going to
it. The other terminals must be the other ends of the solenoids. So on mine 12 volts supply was on the centre terminal.
Note that the idle switch is 'on' unless the throttle is moved off idle and the kickdown switch is 'off' unless operated.
It may be helpful to move the throttle off idle when measuring etc so that both sides act the same.
Hope this helps
Larry

[wwheeler]

Thanks for the reply Larry. I guess my issue goes all the way back to the switches. It sounds like you are saying (as the diagram) that the idle and kickdown switches switch the ground. My test (could be wrong), show them to switch the +.

What I think what I will do tonight is disconnect ALL of the wires to the switches and to the trans terminal block. I will then turn the ignition on and see if I am getting voltage to any of the wires at the two switches and the wires feeding the trans solenoid. If I get any voltage at the switches, then mine switch the +. If I get no voltage, then they switch the ground as the diagram. Does that sound like an accurate test?

It is possible that my previous tests on the switches have been compromised by the wires still being hooked to the soleniod and the above test should resolve that. I have had problems with the idle switch building up resistance over time and I think the diodes will resolve that. When the switch is clean (or new), I can hear the solenoid click on and off with just the ignition on and engine off and moving the throttle off idle. Over time the solenoid won't click with just the ignition on because the voltage is lower than with the engine running and because of the resistance build up. Once I take the switch apart and clean the contacts with micropaper, the solenoid will click again with just the ignition on. This cycle has repeated itself about five times now. I did find the solenoid to be "lazy" awhile back ago and was replaced. It is better now, but the resistance build up is still there. That is why I want to try the diodes so badly!   

Thanks again for the help!

[Larry & Norma]

Yes I think you adopting the correct procedure.
I will be surprised if your car does not switch the ground rather than 12volts.
Your car will be a give a good indication of the effectiveness of the diodes.
Let's hope your contact cleaning days are over at least not so often!
Larry

[wwheeler]

One thing I forgot to add about the idle switch. After it was on for about four months this last time, it started acting up. So I took it off and measured the OHMS on the 200 scale. It measured 20-30 when the switch was closed. To me thats 4000 to 6000 OHMS! After cleaning and on the same 200 scale, it was .6 to.9 or a little less than 200 OHM. I get the same results everytime I clean the contacts. The diodes will be a good long term test! 

[wwheeler]

I finished my test with a surprising result. My kickdown and idle switches are fed with the + and not the negative. Once I discovered that, I went on-line and found a diagram that had the  same layout as my '68 W111 (see #1 attached). It clearly shows the #55 (kickdown) and #52 (idle) switches are fed by the + and not grounded. The large terminal is grounded just as mine as well. The title of this document in german translates to: Exceptions additional cabling for automatic vehicles (for export vehicles particularly in the USA to be exceptions possible). It shows to be for W113 SLs in 1968 which is the same year as my W111. So which cars have this wiring and which have the grounded switches? I also attached the diagram found in the tech manual as reference.

I did my test with ALL of the wires disconnected at the switches and the terminal block with the ignition on.
- Idle switch: #1 wire 12V/ #2 wire 0V
- Kickdown switch: #1 wire 12V/ #2 wire 0V
- Trans terminal block: Small 0V/ Medium 0V/ Large 0V. (Note: Large terminal had continuity to ground with the igntion on!) These wires were the feed wires and NOT the ones leading to the solenoid itself.
The #2 switch wires go to the terminal block and I confirmed that using a continuity test on both ends.
That should prove that my '68 W111 has the + wired to the switches and not the negative. That almost matches the attached diagram perfectly. What do you think guys?

So now for adding the diodes, which way do they go? I would think the bands would point to the small and medium terminals while the other end of the diodes points toward the large terminal. Does that sound correct for my car? I guess if it isn't right, it will pop a fuse and possibly the diode.      

[Larry & Norma]

Hi Wallace,
Wow I wasn't expecting that!
On the diagram the terminal strip is labelled 54 and has 4 terminals.
The left most terminal (4.3mm) and the next to the right(5.3mm) should
each have the line on a diode going to them and the other end of both diodes go to
the next terminal to the right(6.3mm).
Good luck
Larry

[wwheeler]

That is what I was thinking but wanted to confirm with the master.

I don't like being the odd man out but that is the way this circut is wired. I would have thought it was a W111 thing but the schematic is titled for a W113. Your car is a '70 and the original diagram was back in the old "Starting 1959 BBB" and these have the switches grounded.

So I am not sure what cars would have this different way of wiring the switches. It would be interesting to hear from anyone else who has this wiring method to try to determine which cars are effected.

I will make the diode assembly tonight and try to get it installed.   

[wwheeler]

Success!!!!

The attached pictures show how I configured the diodes for my particular wiring setup. I just installed it tonight and it works just fine. With ignition on and the engine not running, I can hear the solenoid clicking from the kickdown switch and also from the idle switch when I move the throttle. The switches must have been wired as the earlier posted diagram, otherwise it would have popped a fuse.

Thanks to Larry for providing this information. This information really should end up in the technical manual.   

Long live the switches!

[Garry]

Neat job Wallace,

Now for you to write up a small blurb on it with photos and as a full member you are able to put it into the Tech Manual.  You can trial in the sand pit first before going live but I agree it would be a good thing to put in the Manual.

Garry

[Larry & Norma]

Nice job Wallace, looking forward to long term report on your switch operation.

[wwheeler]

Bad news on the throttle switch. It started to fail again with the key in run (engine off). I took it off and cleaned the points and started working again as usual.

Then a light bulb went off (in my head). All of the solenoids on the FIP are run through relays. The auto trans 3 way solenoid is much larger than the FIP solenoids so probably pulls more amps. AND the idle switch is constantly cycling whenever you let you foot off the gas. AND the throttle switch's throw is usually very small compared to the kickdown and pressure switches. All of which combines for a perfect senario of point burn. Because the switches actions are very hard to detect and only result in harder downshifts, I wonder if more switches are burned and few people know it? Many people complain about the harsh shifting Mercedes auto trans.... 

My solution is to wire in a (4) post Bosch relay. Very easy to do and only requires (1) 12 volt positive feed and (1) ground. My W111 had an extra spot for a relay to the left of the two relays on the firewall. The end result is a factory looking installation and hopefully a switch that will last a very long time.

I wonder why Mercedes never used a relay for the trans solenoid?

[Larry & Norma]

No solenoid relays on the Pagoda, I agree that relays would help preserve the switch contacts.