Ignition switches, how they fail Options

[porschetub]

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.

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.

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.

[jim_hoyland]

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

[Spoke]

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



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.