Safety switch Issues

[bob_rich]

Hi All

I use a battery disconnect switch of the type shown in the picture attached to disconnect the electrics completely (except for the starter motor) when the car is parked up. I have just had a second one appear to fail in that when switch is operated the main contacts remain open circuit. When the first one failed about 2 years ago I stripped it down and was able to clean up the contacts and it was fine and found another application for it. Noe this one is showing the same problem. This time I did the usual "waggle and shake" and it would sometime make OK.

The feel of the mechanism is OK and with further experiments I found that if I switched the lights on before operating the switch that it would show the open circuit effect much less. This suggest that if the switch closes to a modest current demand it appears more reliable.

yesterday while out for a drive the voltmeter rose to a higher values that normal just under 15V. when I stopped the car at home it went dead almost immediately suggesting that although the switch was closed that the contact had gone open circuit.

the switch is mounted on the footwell with operating lever in the footwell so it can be operated easily form the driver seat while being out of site. the car is kept under a car port with a cover over it.

On the switch I repaired I noted that the contacts are copper to copper and I was wondering if there was an oxidising problem and might be causing the problem?

Has anyone else had problems with this type of switch ? they do seem widely used for battery disconnection application.

Would welcome any comments form other users

regards

bob

[Frogelan]

I have no personal experience of one of these ever failing, but on my old MG, I dropped this type of switch in favour of a modern one (from I think Cartek - it was about 18 years ago).

Steve Soper broke down in the pitstop in the middle of the Silverstone classic and this type of switch was reportedly to blame.

[mark030358]

maybe this..

In electrical engineering, wetting current (sometimes also spelled as whetting current in archaic sources) is the minimum electric current needing to flow through a contact to break through the surface film resistance at a contact. It is typically far below the contact's nominal maximum current rating. Could be the copper oxidizes and leads to a high resistence.

[nigelrbfurness]

We use exactly the same switch as the one in your picture on all our vintage buses (not the vw kind, the 35+ seat kind). They are all 24v systems and some are only used a few times per month. We have never had a failure of the kind described but all are parked in a dry shed when not in use. Maybe your car is in a damp atmosphere and contacts are oxidising?

[lotusfan]

I have used similar switches on 3 different cars without any problems. Maybe the whetting current suggestion is the problem. You say that the switch takes all the current except for the starter. I have always wired them into the battery lead so that ALL current flows through the switch, thus probably removing any oxidation every time you operate the starter.

[bob_rich]

Thanks all

I was beginning to think it may be an oxidising contact issue. The car does not appear to show any damp issues in other areas (Mould, condensation etc). For one reason or another it is sometimes a few weeks before It gets a run out. I note that the contacts do not wipe they are just pushed together. I had a plate arrangement to stop the key falling out. I have changed this just in case it was restricting the key movement.

It would be easy to set the windows to down then turn the switch this should give a decent current of around 20A to 30A. I will keep an eye on this. and see how it pans out.

thanks for all the useful comments

Bob

[69S4]

I've also got one of those switches fitted and taking all of the battery current. It's been there since around 1985 and apart from the plastic on the key breaking where you turn it it's been totally fault free - even when the car has sat in the garage for years at a time

[bob_rich]

hi All

I have taken the switch apart--They are quite easy to rebuild- and found something interesting

I have written up a note for my records and have posted a copy here.

Think I found the problem and suspect that after a repair it would probably be OK in a car
thanks for all the comments and hope you find my attached note useful

regards

Bob

[bob_rich]

Hi Again

I was a bit quick of the mark !

I have taken the moving contact apart now and it is held by a different mechanism that I first thought
It is easier now ( I think!!) to understand how the switch failed

I have update my note and attached it here

regards

Bob