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> Ignition switches, how they fail
porschetub
post Aug 12 2017, 04:25 PM
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Had one fail in my 914,there seems to be a common failure point around the top casing that fits to the main assembly,this design is the the same for early Golf,944,T 1 and T2 aircooled Audi and I would expect many other in "the family''.
The return spring mounts via a tab on the spring into a hole in the top assembly,repeated use causes the area to crack.
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The result is that the green plastic centre section loses tension and pops upward which in turn causes the metal wiper plate inside the switch to make a poor contact as it moves through its arc.
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The end result of this poor contact is complete or partial failure,the crack in the housing shown in picture 2 is from my early 944 switch this was enough to stop injector pulse due to insuffient power to my ECU.
I pulled a known bad switch from another VW I had and found no crack in the housing but the contacts inside were all gummed up as the grease had gone bad and the contact pins were oxidized,cleaned it and it worked in the 944 till the correct replacement arrived.

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jim_hoyland
post Aug 12 2017, 07:53 PM
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Thanks ! That is a terrific explanation. Can you elaborate on the use/need of a suppression diode on the hot start relay,,, (IMG:style_emoticons/default/smile.gif)

Tom had mentioned this in a threaf


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Spoke
post Aug 13 2017, 07:57 AM
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QUOTE(jim_hoyland @ Aug 12 2017, 09:53 PM) *

Can you elaborate on the use/need of a suppression diode on the hot start relay,,, (IMG:style_emoticons/default/smile.gif)


The diode is sometimes referred to as a snubber since it prevents a large voltage from developing across a coil when the coil is turned off. The large voltage can damage transistors and cause switches to arc and shorten the switch life.

The diode snubs the inductive kick when the coil is turned off.

A coil is an inductor. An inductor stores energy in the form of current and resists changes in current. The voltage across an inductor is given by the following formula:

V = L x di/dt

where

L = coil inductance in Henrys
di = change in current in Amps
dt = change in time in Seconds

How large can the voltage across a coil get when turned off? Here's an example.

Suppose a switch opens up to turn a coil off like happens when starting the car and the starter is turned off.

Let:

Coil inductance L = 1H
Current I = 1Amp
Say the switch turns off in 1ms (0.001 sec); dt = 0.001 sec
Therefore the change in current: di = -1 Amp

Put these numbers in the inductance formula:

V = L x di/dt = 1H x -1A/0.001sec = -1000V

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This is the inductive kick everyone refers to. Without the diode, the coil will kick back and may cause switch contacts to arc thus shortening it life. If controlled by a transistor, the high voltage may damage the transistor.

Placing the diode across the coil will give the current a path back to the inductor. The resistance in the coil, wiring, and diode will cause the current to decay to zero in a relatively short time.


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Posts in this topic
porschetub   Ignition switches   Aug 12 2017, 04:25 PM
914_teener   Yep.... Very common. My switch failed when it got...   Aug 12 2017, 04:38 PM
porschetub   Yep.... Very common. My switch failed when it go...   Aug 12 2017, 05:00 PM
porschetub   Part 2 of my rant :rotfl: ,you can tell I had not...   Aug 12 2017, 04:58 PM
ndfrigi   same what happened with my previous 75.   Aug 12 2017, 05:25 PM
nditiz1   So what is the fix?? I just bought a supposed ...   Aug 12 2017, 05:46 PM
porschetub   Hi Noel,that switch is a standard failure ,I reall...   Aug 12 2017, 05:50 PM
jim_hoyland   Is it true that a remote hot-start relay takes the...   Aug 12 2017, 05:59 PM
Mark Henry   Is it true that a remote hot-start relay takes th...   Aug 12 2017, 07:15 PM
porschetub   Is it true that a remote hot-start relay takes t...   Aug 12 2017, 08:18 PM
Spoke   Is it true that a remote hot-start relay takes th...   Aug 12 2017, 07:23 PM
r_towle   Thankfully, it's the same switch on mid to lat...   Aug 12 2017, 06:06 PM
porschetub   Thankfully, it's the same switch on mid to la...   Aug 12 2017, 08:46 PM
bbrock   That's why I asked if anyone had used the gen...   Aug 12 2017, 09:11 PM
jim_hoyland   Thanks ! That is a terrific explanation. Can y...   Aug 12 2017, 07:53 PM
Spoke   Can you elaborate on the use/need of a suppressio...   Aug 13 2017, 07:57 AM
nditiz1   Note that the genuine porsche one from PP does not...   Aug 12 2017, 09:11 PM
porschetub   Note that the genuine porsche one from PP does no...   Aug 12 2017, 11:52 PM
r_towle   Pretty sure they are all made in the same factory....   Aug 13 2017, 12:33 AM
Mark Henry   Pretty sure they are all made in the same factory...   Aug 13 2017, 07:22 AM
McMark   As 3D printing has evolved I've wondered if so...   Aug 13 2017, 08:24 AM
Mark Henry   As 3D printing has evolved I've wondered if s...   Aug 13 2017, 11:30 AM
rhodyguy   Anyone with first hand experience on the German on...   Aug 13 2017, 11:37 AM
mepstein   Anyone with first hand experience on the German o...   Aug 13 2017, 12:10 PM
Porschef   Anyone with first hand experience on the German o...   Aug 13 2017, 01:12 PM
porschetub   [quote name='rhodyguy' post='2516510' date='Aug 1...   Aug 13 2017, 06:23 PM
Mark Henry   [quote name='rhodyguy' post='2516510' date='Aug ...   Aug 13 2017, 06:48 PM
barefoot   [quote name='rhodyguy' post='2516510' date='Aug 1...   Aug 14 2017, 11:35 AM
Jeff Bowlsby   Porsche OEM only, the cheap knockoffs do not last....   Aug 13 2017, 04:18 PM
porschetub   Yea have seen that metal switch Mark ,got to be be...   Aug 13 2017, 06:08 PM
nditiz1   I did call the local dealership, but the guy on th...   Aug 13 2017, 06:16 PM
mepstein   I did call the local dealership, but the guy on t...   Aug 13 2017, 06:23 PM
Mark Henry   111-905-865 L Type one 1974 to 1998, I assume the...   Aug 13 2017, 06:57 PM
Porschef   I don't know what the part numbers are, all I ...   Aug 14 2017, 03:50 AM
saigon71   Thanks for the info on the "Genuine Porsche...   Aug 14 2017, 05:39 AM
nditiz1   Does anyone know if the part number above will wor...   Aug 14 2017, 06:02 AM
michael7810   Does anyone know if the part number above will wo...   Aug 14 2017, 07:16 AM
nditiz1   Coming from aircooled beetles I am wondering how t...   Aug 14 2017, 08:22 AM


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