TQ: Little red light

PaulHere · May 21, 2005

After a drive today I switched the ignition off only to have the battery light stay on even with the key out of the ignition.

A quick ring to my local(ish) specialist revealed a faulty 'assimilator switch' somewhere.

I had no idea that Porsche used Borg technology?! [

]

924nutter · May 21, 2005

I dont know what to suggest Paul. I have never had that, anyone else?

924nutter · May 22, 2005

Try replacing the ignition switch. My 83 924 had a contact on the key slot somewhere. Key in, ignition off, the sun roof arcs retracted on activation of the switch, allowing the removal of the roof. Igniton on, the arcs extend, raising the roof. There could be some internal cross feed to terminal 15.

vincematthews · May 22, 2005

Yes I'd go along with that. The ignition switch, the bit behind the ignition key barrel, is made of a plastic that doesn't age well and cracks. This allows contacts that shouldn't be allowed to, to make contact, and contacts that should connect, to fail. Can produce some unexpected effects, in my case not starting at all. I'm told its a fairly common 924 fault. Its not difficult change, I got a replacement from the Porscheshop for about a tenner.

924nutter · May 23, 2005

I wouldnt have said its that common, in my experience, more that it happens. I have 3 924s on the go and have only replaced the switch in one, and that one had done over 160,000 miles. I guess it just depends on how much use it has been subject to, after all, the switch does pass a lot of current with all the accessories on. What do you reckon Vince, upwards of 40 amps for a full house?

vincematthews · May 25, 2005

The 'common problem' was the impression I got from the guy at the Porscheshop. I would think you're not far out on the amps - I forgot to look at the starter motor last time it was out to see what it was rated at.

I had another look at the switch I took out and its not the amps wot done it in . . . its mechanical failure (like most electro-mechanical devices [there I go, generalising again [

] ] ). There is a small hole to the side of the recess that the key barrel plugs into, I would guess it takes the end of the spring that returns the key back off the Start position. The plastic body has cracked straight through this hole and the crack has continued into the key barrel recess. This allows the 'shaft' to move sideways and give poor connection between the contacts inside.

PaulHere · May 28, 2005

The fault seems to be quite random - the light can be on when I stop in my drive, but sort itself out by the time it get into the garage - still going to get it checked out.

Thanks for the advice everyone [

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924nutter · May 29, 2005

This could be the be heralding the beginning of the end of the regulator in the alternator. According to the wiring diagram, there is the possibility of an earth return via the fuel gauge, if the battery can back-feed through the regulator to the lamp and then to earth. Checking the ign switch is easier in terms of accesibilty, but from my experience with prices, the regulator is half the price of a new ignition switch.

924nutter · May 29, 2005

ORIGINAL: vincematthews

...... I forgot to look at the starter motor last time it was out to see what it was rated at.

For members who perhaps are not so well versed on vehicle electrics;

The igntion switch per se does not supply current for the starter motor windings, this comes from a special relay called a solenoid, which had two sets of windings, pull in and hold in. When the contact is made for the starter to rotate, the voltage drop across the starter motor is so great that a much heavier set of windings, the hold in windings are necessary to stop the solenoid from "chattering". A typical half horsepower starter will draw 31.7 amps at 12 volts for 377 watts, but this is the theoretical no load current. ( 1 horsepower =745 watts) Load it up with a 2.0 litre engine and the current rises to about 250 amps, which explains why the cable is so large in cross section, and why the battery flattens so quickly. A 50 amp hour battery would, theoretically, be dead in twelve minutes, but polarisation of the plates ( the collection of gas bubbles on the positive and negative parts of the plates) insulates them from the electrolyte (battery acid) and reduces its capacity further. That's why if you leave a battery for a few minutes it seems to recharge itself, as the bubbles dissipate.