Ignition Switch Durability - VW Golf 3

Hello.

Da ich bei längeren Ampelstopps und Gefällstrecken öfters den Motor ausschalte und mir dabei das ewige Rumkurbeln am Zündschlüssel lästig ist, hab ich mir 2 Taster installiert. Einen zum Motorabstellen, und einen zum Anlassen. KlappIt's also quite good.

By pressing the start button, I activate a changeover relay that redirects the power from the X-output of the ignition switch to the starter motor. This effectively turns off the low beam headlights, blower motor, rear window heater, etc., as if they were normally switched off during starting.

I have now measured the input resistance of the starter motor (terminal 50). The approximately 0.3 ohms suggest activation current peaks of around 40 amperes.
The ignition switch contact is designed to withstand that kind of stress.
According to my wiring diagram, only the low beam or high beam headlights are directly powered through contact point X, drawing approximately 10 amps. All other devices connected to the X-terminal receive power through the X-bypass relay.

According to this, the X-contact in the ignition switch would only need to be rated for approximately 15 amps with some margin, and in this case, I would likely overload it if I activate the starter motor through it (see above -> approximately 40 amps).

My question: Has anyone ever seen the internals of a G3 diesel ignition switch and can tell me if the "X" contact is the same size as the starter contact?

I don't really feel like disassembling everything again to install another relay that would draw the starter activation current directly from the constant positive terminal .
But before I permanently damage the X-contact, I would probably install the second relay instead.

Hello Ulf,

I'm a bit surprised by the resistance of the magnetic switch. Are you sure it's 0.3 ohms? When I touch the probes of my multimeter together, it still shows around 0.1-0.2 ohms. If you then also consider the cable resistance and the contact resistances, there's almost nothing left for the starter switch.

It's the way the magnetic switch on the starter motor works: it switches the high current of the starter (in my Golf 2 GTD, I once measured around 80 amps). To switch that current, it certainly doesn't need 12 volts x 40 amps = 480 watts. Based on my feeling, I would estimate that the magnetic switch needs a maximum of 5-10 amps (corresponding to 1-2 ohms). Also, the control wire leading to the magnetic switch is not designed for much more than that.
Could it be that the magnetic switch isn't working against ground, but against positive voltage? I also had a similar strange horn circuit in my GTD.

But let me ask a different question: Why do you turn off the engine on downhill sections? Doesn't it already have a fuel cutoff in gliding mode to increase the braking effect of the engine?
In addition, I suspect that the increased wear and tear on the starter motor and battery would offset the potential fuel savings achieved through engine shutdowns.

Regards,

Eike.

Hi there,

Switches and relays are typically specified with two values.

the maximum current and the maximum switching current.

Small relay coils, for example, can be loaded with 1.5A, but can only switch 0.3A.

Therefore, I'm honestly not too worried. The X-connector is definitely designed to handle more than 15A, because incandescent bulbs, in particular, have very high inrush currents (even if they are brief).

In your case, you're not actually closing the circuit, so this type of load doesn't exist.

You shouldn't factor in those 0.3 ohms either. The magnetic switch represents an inductance that initially slows down the current increase. I'll try to find a measuring device and then record the current with the oscilloscope.

CU Gremlin.

Hi.

@eike

My multimeter shows 0.1 ohms when the probes are shorted together, and 0.4 ohms between the "small" terminal on the starter and ground. This indicates approximately 0.3 ohms for the starter.

"Additionally, I briefly connected a 3.9 Ohm high-resistance resistor from the battery positive terminal to the starter motor. I measured approximately 920 mV between terminal 50 and ground, which, with a battery voltage of 12.6 volts, also suggests an input resistance of around 0.3 Ohms for terminal 50."

Quote:

It's the case that the magnetic switch on the starter motor switches the high current of the starter (in my Golf 2 GTD, I once measured around 80 amps). To switch this current, it would never need 12 volts x 40 amps = 480 watts. Roughly speaking, I would estimate that the magnetic switch needs a maximum of 5-10 amps (corresponding to 1-2 ohms).

However, he also has to simultaneously push the starter pinion gear into the crankshaft sprocket, which requires quite a bit of force.

Quote:

Could it be that the magnetic switch doesn't work against ground, but against positive voltage? I also had a similar strange horn circuit in my GTD.

I don't think so: if the magnetic switch receives power from the ignition switch's positive terminal and its other winding end is connected, for example, to a constant positive terminal, how can there be any current flow to activate the "main switch"?

Quote:

But let's talk about something else. Why do you turn off the engine on downhill sections? Doesn't it already have a fuel cutoff in "glide" mode to increase the braking effect of the engine? In addition, I suspect that the increased starter motor and battery wear

would offset the potential savings achieved through engine shutdowns

Quote:

. Schubabschaltung ja, aber ich will das Auto möglichst weit (aus)rollen lassen. Meine täglichen Gefällstrecken enden teilweise fast flach in Dörfern, und da kann ich im Getriebeleerlauf prima bis zum 1. KreIsel ranrollen. With the gear engaged, the engine would slow the car down much earlier, so that I would have to use engine power again for the last part of the journey

.

Anlasser und Batterie haben übrigens nicht viel mehr Streß, denn ich lasse den Motor meist wieder im 5. Groll up.

And I'm happy to forgo the engine braking effect, preferring to keep my brakes "in practice" instead.
Before I realized it, the brake discs rusted away faster than the brake pads wore out.


@gremlin:

}You shouldn't factor in the 0.3 ohms either. The magnetic switch represents an inductance that initially slows down the current increase. I'll try to find a measuring device and then record the current with the oscilloscope. }

I measured the inductance at approximately 460 µH, which, combined with 0.3 Ohms, results in a time constant of around 1.5 milliseconds... so there's not a significant delay.
I already understand that the current is zero at the very first instant.

However, the thought about the headlight bulbs has calmed me down a bit. A H4 filament has a resistance of approximately 0.3 ohms when cold, which means the inrush current for two bulbs is theoretically 80 amps.

In practice, the power is even switched on via the ignition switch. For example, if you start the engine with the low beam headlights on, the X-contact receives power again via the ignition switch after the engine starts.
However, the filaments probably won't be completely cool yet, so they will likely start with a lower amperage.
And a few µH of inductance are also present in the wiring between the battery and the headlights, which should help to reduce the inrush current peak.