Author Topic: Making CSS pressure switches last longer (Read 22322 times)

Larry & Norma · « **on:** October 20, 2011, 11:45:24 »

I removed the faulty pressure switch and carefully cut the top off. (photo 1)
Removing the two screws revealed the moving contact (photo 2)
and the static contact (photo 3). The contacts were blackened and pitted so I
removed the CSS (Constant Speed Solenoid) from the car and connected a relay and switch so that I could
operate it from a bench power suppy. I noticed that the relay contacts sparked and arced
when removing power from the CSS. This is caused by the back emf generated by the
collapse of the magnetic field of the CSS. ( much the same as an ignition system works ).
The normal way to stop this in electronics is to put a reverse biased diode across the coil
which i did and the arcing no longer occured. Photo 4 shows my diode across the CSS.
I used a '1N4002' from Tandy or Maplins stores. Be sure to have the line on the diode body to the positive
terminal on the CSS.
This should help to prolong the life of the pressure switches.

Cees Klumper · « **Reply #1 on:** October 21, 2011, 20:35:31 »

Thanks for researching this - I would like to install a diode on mine - do you know whether the positive terminal is always on the right hand side as in your photo (probably yes, but as I am an electronic zero I'd like to be sure before I install the diode)?

wwheeler · « **Reply #2 on:** October 21, 2011, 21:09:33 »

Would the same thing apply to the venturi and kick down switches for the automatic transmission double acting solenoid? I have had problems in the past with the venturi or throttle switch pitting its points. Seems like any switch connected to a solenoid that doesn't have a relay in-line would have the same problem. This would be especially a problem with switches/solenoids that are constantly used.

Very interesting about the diode!!! So you just bridge the two wire connections as you have shown? I guess the easiest way to do this is put the diode leads on top of the existing ring terminals and wrap around the hold down scew. Sound correct?

As Cees said, thanks for looking into this. It is always interesting to see what is hidden inside these parts.

Larry & Norma · « **Reply #3 on:** October 22, 2011, 09:13:04 »

Hi Cees, you are correct in that the positive is the right one on mine. To be
really sure use a meter when the CSS is operating as it looks easy to install
the wires the other way around. Wallace the diode leads can just be clamped under
the ring terminals as you suggest.
This back emf effect is true whenever a coil is de-energised to a greater or
lesser degree depending on the size of the coil. So a relay has a coil and
would produce the same effect but not such a strong spark. In the electronics
world it is common to have a diode across all relays to protect the switching devices.
I guess the switches etc on the car were engineered to handle this but not for
40+ years!

tel76 · « **Reply #4 on:** October 22, 2011, 10:27:48 »

Correct me if i am wrong but is there not 12v positive to both sides of the switch and the only time it will operate is when the either the rh or lh switches on the gearbox earths the system?

stickandrudderman · « **Reply #5 on:** October 22, 2011, 11:12:55 »

Well, if you test with a test light then yes. However, only one is the voltage supply.

Larry & Norma · « **Reply #6 on:** October 22, 2011, 13:56:27 »

If you disconnect both wires from the CSS and turn on the ignition then,
using a voltmeter or test lamp touch each wire in turn (ground the other
meter probe or test lamp lead) then only one of the wires will show 12 Volts
or light the lamp. This will be the one on the right in my photograph and
needs the end of the diode with the line connected to it.
If you connect the diode the wrong way round then you will short out
the 12 volt supply to the CSS when the switches operate, this will possibly
take out a fuse/destroy the diode!!

For completeness, the back emf produced is the reverse polarity to the applied voltage
which is why the diode will conduct and short out this brief spike.

Naj ✝︎ · « **Reply #7 on:** October 22, 2011, 17:04:39 »

Hi, Larry,

Do you think a diode across the ign coil would work to reduce the back emf spark at the points in the distributor?

Naj

Larry & Norma · « **Reply #8 on:** October 22, 2011, 17:54:06 »

Hi Naj,
Interesting point, obviously we are talking about the low tension side as we need
the HT to create the spark at the plugs. I think if we use a diode here then we effectively
destroy the back emf of the LT which then will not induce the HT. This is perhaps
why they use a condensor (capacitor) to help absorb the arcing energy across the points and
not a diode.
In all honesty I don't know but will give it some thought $:-\$

Cees Klumper · « **Reply #9 on:** October 22, 2011, 18:43:05 »

Quote from: Gnuface on October 22, 2011, 13:56:27

For completeness, the back emf produced is the reverse polarity to the applied voltage
which is why the diode will conduct and short out this brief spike.

I hope I am not the only one who can read the words but fail to see a meaning!

But I now know what I need to do, thanks!

Larry & Norma · « **Reply #10 on:** October 22, 2011, 18:50:21 »

Am I correct in thinking the spark at the sparkplugs occur when the points open?
If this is true then you need the back emf created by the collapsing field to
induce the HT. This is a case where the back emf is desirable and so should
not be destroyed with a diode, I think $:-\$

tel76 · « **Reply #11 on:** October 23, 2011, 07:31:39 »

Hello Larry,
Would the micro switch on the induction benefit from the same diode?
I have two wireing diagrams of the Pagoda and niether has any reference to this circuit and niether does the card inside the fuse box.
Do you know if this circuit is fused and if so which fuse?
If the circuit is not fused and someone wires up the diode incorrectly they could damage there wireing loom or worse.

Larry & Norma · « **Reply #12 on:** October 23, 2011, 08:28:24 »

Hi Eric,
I have attached the circuit you are looking for.
Yes I do believe a diode across the idling switch would be beneficial,
the kickdown switch is probably beefy enough to handle the voltage.
If you are unsure about fitting the diode correctly then temporarily put a fuse
in circuit with the diode, say 1/4 or 1/2 amp, if it blows then reverse the diode.
(the diode is good for 2amps).Always connect the end of the diode with the line
marked round it to the positive side of the switch.
I will add a diode to my idling switch.

wwheeler · « **Reply #13 on:** October 23, 2011, 23:42:33 »

Hi Larry,

I understand the diode connections to CSS, but am confused about how to apply it to the idle switch. A switch wouldn't have a positive and negative side. The switch breaks either the positive OR negative but not both. Wouldn't the diode have to be connected to leads going to the transmission solenoid where there is a positive and negative lead? Maybe at the 3 pole terminal block?

Thanks again for your reasearch on this!

hands_aus · « **Reply #14 on:** October 24, 2011, 06:27:17 »

Isn't the back emf only produced in a coil / winding and not a switch?

Larry & Norma · « **Reply #15 on:** October 24, 2011, 07:26:31 »

You are both correct but my theory is that the back emf is created when the
switch opens and will try to reach ground through the open switch, hence the spark
across the contacts. If a diode is put across the switch (the correct way round)
then this emf will have an easier route to ground and therefore no spark.
So the diode can be either across the coil or switch. I will try this in my garage today
and report the results.

Larry & Norma · « **Reply #16 on:** October 24, 2011, 09:49:39 »

I guess my theory was not that good

A diode across the switch is not very effective, so it will
have to go across the correct coil of the three way solenoid.
This can be done on the terminal block on the side of the
gearbox.
However a capacitor across the idle switch does absorb the spark.
A value of 0.5 or 1 microfarad non electrolytic works very well.
This is what I will do as it does not involve getting under the car!

Naj ✝︎ · « **Reply #17 on:** October 24, 2011, 11:15:02 »

Quote from: Gnuface on October 22, 2011, 18:50:21

Am I correct in thinking the spark at the sparkplugs occur when the points open?
If this is true then you need the back emf created by the collapsing field to
induce the HT. This is a case where the back emf is desirable and so should
not be destroyed with a diode, I think $:-\$

Hi, Larry,

I agree that the spark is created when the points open, but this useful spark is from the secondary winding in the coil.
A back emf occours in the primary windings as well which results in the spark at the points (about 125 V) from what I've read.

May be worth a try?

Naj

Larry & Norma · « **Reply #18 on:** October 24, 2011, 13:54:49 »

Hi Naj,
I don't think you will do any harm by trying it, the engine will either run or not!
I have a 123 ignition installed so cannot experiment.

wwheeler · « **Reply #19 on:** October 24, 2011, 19:58:12 »

A capacitor is easy enough. Do you have a part number you can reference to make sure I get the right one?

The idle switches are NLA from MB but rebuilt ones are available with exchange. Of course they aren't cheap. Any protection you can get for these switches is well worth the time.

I have taken my old one apart many times to clean and can see how easy it is for them to arc across the points. The moving contact rotates away from the stationary contact and therefore takes longer to create a gap wide enough not to arc. Couple that with the fact that you can have a throttle position while driving so that the points are constantly opening and closing. Its a wonder these switches last as long as they do.

tel76 · « **Reply #20 on:** October 24, 2011, 20:54:05 »

With reference to the induction micro-switch, what is the correct procedure to adjust it correctly ?

jacovdw · « **Reply #21 on:** October 25, 2011, 08:30:28 »

Quote from: tel76 on October 24, 2011, 20:54:05

With reference to the induction micro-switch, what is the correct procedure to adjust it correctly ?

Eric,

Have a look at this thread about the throttle switch where I explained how to adjust it: http://www.sl113.org/forums/index.php?topic=13583.0

Quote from: hands_aus on October 24, 2011, 06:27:17

Isn't the back emf only produced in a coil / winding and not a switch?

That is correct.

The arcing you see across the switch is the physical manifestation of the back EMF produced by the coil when the magnetic field in the coil collapses.

Quote from: Gnuface on October 22, 2011, 18:50:21

Am I correct in thinking the spark at the sparkplugs occur when the points open?
If this is true then you need the back emf created by the collapsing field to
induce the HT. This is a case where the back emf is desirable and so should
not be destroyed with a diode...

That is also correct.

When the ignition points open, it results in the magnetic field in the primary winding of the ignition coil to collapse.
It is the collapse of this magnetic field that induces current in the secondary winding of the ignition coil.

The ignition coil is nothing more than a transformer, meaning the secondary winding has more turns of wire around the core than the primary winding.
In short, more turns in the secondary = higher induced voltage when the magnetic field in the primary winding collapses.

Unfortunately some arcing across the points would occur due to some back EMF being generated, but that is taken care of by the condensor. Some of the energy stored in the condensor when the points open is fed back into the primary winding when the points close again to build up the magnetic field in the primary.

In theory it could work as most electronic circuits that employ a power transistor/MOSFET to switch inductive loads have a fly back diode mounted across the current carrying path protect said transistor/MOSFET.

Larry & Norma · « **Reply #22 on:** October 25, 2011, 13:19:10 »

A capacitor does effectively reduce the arcing across a switch when tested on the bench.
A suitable capacitor is shown in the first photo. You may have to solder longer leads to attach
to the switch terminals.
However I have decided to go the diode route as I believe it is a more efficient way

merrill · « **Reply #23 on:** October 29, 2011, 02:06:58 »

I installed a diode as instructed and i blew a fuse...?

removed diode and no problem.

Larry & Norma · « **Reply #24 on:** October 29, 2011, 07:57:51 »

You must get the diode the correct way round or it will be like a short circuit
and take out a fuse and may also destroy the diode!
Which solenoid did you put diode across, CSS or three 3 position solenoid?

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Author Topic: Making CSS pressure switches last longer (Read 22322 times)

jacovdw