Hi

I have a battery isolator switch fitted on the car - the type with a red plastic key. It has been on for about eight years.

When I came to start my car the other day, it was completely dead. I thought that the battery was discharged or kaput, so I took it out of the car and put it on charge to find out.

I had been renewing some of the wiring to the headlights the day before and could have introduced a fault, so I fetched a multimeter to seek it out. However, when I was setting up the multimeter, I happened to check the instrument by touching it across the isolation switch terminals - this showed an infinite resistance instead of zero! So the problem looked like the isolation switch! I returned the battery and bypassed the switch - everything was now working normally. So it was one of the few non-factory bits on the car that had suddenly failed!

The switch is rated at 100 amps - I wonder if the starter takes more than this?

I have now fitted a new switch to restore protection. I was very lucky to find it so easily.

Regards

Colin

Hello Colin,

I think that the accepted "round figure" for a 6v system starter current for a vehicle like our Austin Sevens is in the order of 300Amps at maximum instantaneous peak load.It is quite difficult to measure with simple ammeter type equipment for a range of reasons we need not go in to here.

But that is a maximum peak drain figure and your 100Amp rated switch should be well able to cope with that. The 100 amp rating would usually be at a definite time cycle of operation (possibly a continuous rating or maybe for some other operational period)Maybe if you were thinking about winding the car forward in gear on the starter it might not like it, but I think your battery and starter motor would give up before the switch.

It would be interesting to pull the switch apart and see just what has gone on inside it if that was possible.

At the sort of currents drawn on starter load, even thousanths of an ohm of resistance start to become important, and even three thousanths of an ohm would drop nearly a volt. That is why the starter circuit (and particularly the earth return) needs to be in absolutely top electrical condition, and must be comprised of sufficiently "heavy" cables to ensure precious volts are not frittered away.

On a personally sour note I have now thought on several occasions that my isolating switch, which sounds exactly like yours, has failed to switch on on (at least twice) Because it has come right on a second operation, in my sloth I have just carried on regardless.

I think I had better have a closer look.

Regards


Barry R.

A couple of points. No doubt these modern switches were designed for 12v electrics where the current draw on starting may be less (depends on the size of the engine) Secondly these switches were used on the racing car i helped build at uni, they were terribly unreliable even though they were purchased from reputable sources, we had to have two on the car and always had a couple spare. That said if the car started they never failed while in motion. Just keep a bolt and some insulation tape handy and you shouldnt ever get stuck!

Regards

Rob

The isolation switches with the red plastic key are made by Hella, but I have also seen "lookalike" ones which appear identical. Having had a problem with intermittent behaviour with one of these fitted by a previous owner, I took it apart and was not impressed by the contact design. I now use a 1000 amp rated key switch made by Roca and sourced from a marine chandlers. It is mounted downwards through the floor of the battery box and easily reached without raising the bonnet. No more starting problems.

These switches are unreliable, paticularly if they get some damp in them. At one stage, I got to the point where one of mine wouldn't work without a lot of effort if left switched off for a day or so. For a time, I resorted to leaving it switched on - which rather defeats the object of the exercise

STEVE

The strange thing is that most modern rally type
competitiion cars have had to have them fitted and have done for many many years (the alternator version incorporates a resistor to safeguard the alternator). This is a very harsh environment so you would think they would be OK?
Some research indicates several different types.....
Cheapy ones are available from the local boy racer factors, dear ones from reputable racing equipment suppliers, and a slightly different style like I use which is a replica of a Lucas unit and is black. You can also get solid state ones believe it or not which are remotely switched!.
Anyway back to the point in question, like Barry I would be interested in whats wrong and also the maker of the part? Was it a cheapy one or a more expensive model?

Steve

Hi.

Sorry, I don't usually bin any old parts, but I did this time - otherwise I could have had a look inside.

It was of the simplest type available, as shown below (from Google images):

Bought from the local general car parts shop, it seemed of reasonable quality. There is no name on the old red keys which I still have. The new one seems to be of inferior quality in terms of the plastic used but is exactly the same external design - it has now lasted a week!

There was no damage visible on the old part, it had never been wet, but it would not work even when an old or new key was pressed and worked about in the keyway. It had not failed whilst the car was running, but as I tried to start a cold engine with the starter motor, after 8 turns on the handle.

Regards

Colin

Just to respond to Steve, at this stage I don't know what make my switch is, as it came with the car, but I suspect that it is Hella.

In the past I have aways used the "screw-down" type which are a part of the termination to the battery terminal. They are obviosly slightly more inconvenient, and don't have the benefit of a removeable key, but really are foolproof electrically.

Will let you know after I have had a look at my current (no pun intended) switch over the next few days.

regards

Barry R.

I've had one of these in the car for 6 years and to date, no problems. In fact I'm of the opinion that the convienence of the switch for testing circuits etc without having to keep yaking the battery terminal clamp off each time is more than worth the ppossible problem of the switch failing, and yes I do have a spare. As stated in a previous reply if the switch fails it is easy to bypass and connect the two ends of the battery cable together with a small nut and bolt.

Re. Barry's comment on earthing the starter, both leads from my battery go direct to the starter moter, which has caused some Cornwall club members to think I have a 12v battery turning the starter motor over (fast...). I have a seperate earthing lead from the battery switch that goes to a busbar type system where all the other circuits are return earthed to (headlihgts, tailights, etc etc).

Back to the begining, I would not be without the battery isolator switch. I turn it off everytime I get out the car, so to say. I have a led light up by the wiper motor that shows when the switch is on, it is wired through the flasher indicator unit, and thus goes off (flashing) when the indicators are working - it was meant to be the other way around, but, but.....

I too would have been interested in why the switch failed.

Sandy

Just as an aside...
Becasue I have a Schottky diode as the cut out, there is some leakage current so I have no choice, I have to fit a cut out switch.

Steve

TYPICAL ---- never had a problem, read this thread, went outside and I had to have three attempts at turning the switch before connecting, last time I look at this Forum - well until the next time.....

Sandy

Gentles,

I have now had a proper look athe Battery Master Switch fitted to my Chummy.

It is a Hella, Model HL87181 (Rated at 100Amps continuous, 5000 Amps at 12volts for 10 Seconds). It is the type fitted with the removable red plastic handle.

I also think I have found the intermittent fault which I had previously sort of noticed once or twice and put out of my mind.

I found a corroded star washer under one of the attaching point of the lead going to the battery. They are a mongrel of a thing in electrical circuits and cannot be relied upon to give long term good conductivity.

There is a special fluid which is used on fishing boats down here for protecting an electrically cleaned conductive surface and I will visit the purveyors of same early this week and but a can.

I have also had a look at some “clone” type Battery Master Switches in my local “Cheapo Auto” or whatever emporium. Methinks you get what you pay for.

Regards


Barry R.