The warmth on my leg and the smell of frying bacon told me that something was wrong..

[Ade Ray]

As I looked down I spotted the resistance dump connected to the back of my new master switch was smoking as it tried to burn a hole in my trousers. I then did the the typical bloke thing of touching the resistance dump 'just to make sure' that it was the offending item and was rewarded with more frying bacon. I have not noticed the resistance dump getting warm before and I am sure that nothing in the cockpit of the car is supposed to get this hot.

 

The background:

 

Car is a 1600 xflow ex-scholarship with no fancy ignition stuff

 

replaced the master switch about 10 days ago, transferring each wire one at a time to ensure that everything got attached to the right terminal. Everything seemed fine although my idle revs were a bit low. I have so far put this down to my carbs needing adjusting.

 

The car was last used Monday morning and then tucked away in the garage whilst my wife and I had a few days away.

 

This morning I got in the car. Started OK (revs still low). The idle revs are usually better (but still low) once the car was warm but once on the way to work as I was pulling up to a set of traffic lights and also as I pulled into my parking space, the car cut out as I depressed the clutch. The oil pressure seems fine with 4.5 showing at speed and 2 at lower revs.

 

The car also has seemed slightly rough as I begin to apply revs and I was wondering if I may have some sort of electrical problem?

 

I would greatly apreciate it if anyone has any ideas.

 

Thanks

 

Ade

 

Make the world a better place, hug an estate agent

[Peter Carmichael]

You might have the wrong sort of master switch.

 

The master switch needs to be specified with at least one NORMALLY CLOSED(NC) auxiliary circuit. This means that in use with the switch ON, the main poles (normally open - NO) connect the CAR and BATTERY together and the battery current transfers across the switch. The NC pair of contacts are disconnected (OPEN) with the switch ON.

 

One side of the NC pair should be bridged to the CAR side of the main terminals. The other side of the NC pair should run to earth via the dump resistor. This means that when running, the resistor is not passing any current. When you switch the master switch OFF, simultaneously the battery is isolated and the car circuit gets connected to earth via the dump resistor.

 

I suggest you disconnect and thoroughly examine the switch with a continuity tester and make sure it has this behaviour. If your switch is not equipped with a NC set of terminals then you cannot wire in the dump resistor correctly without using a relay.

 

*thumbup*253 bhp, up and running *thumbup*New boingy bits

 

Edited by - Peter Carmichael on 2 Aug 2002 11:52:59

[Ade Ray]

Thanks Peter, I shall investigate. The master switch is identical to the one that came out of the car so I might be coming back and asking more questions later.

 

Cheers

Ade

 

Make the world a better place, hug an estate agent

[Richard Price]

Not wishing to shoot Peter down in flames, but the correct terminology for switches is that the "normal" state is the "not operated" state. Thus, the main contacts are "normally open" and are closed when the key is inserted and twisted, and the auxiliary contact in question is "normally closed" but held open when the key is operated.

Pardon me for "nit picking"

[Peter Carmichael]

Happy to be corrected. The meaning of the words was right, but the semantics were exactly wrong. I will edit the original in case the casual reader doesn't get down this far.

[stevefoster]

On a similar topic. How hot does the resistor get being used? i.e. when you turn off the batt key

with the ign still on... Just curious.

 

Steve

My racing pics here

Hants (North) and Berkshire area club site

here

 

[Graham Sewell]

It only runs for a couple of seconds, so does not get the chance to get hot!

 

Low tech luddite - xflow and proud!

[J.R.]

The problem you have is almost certainly the main isolator circuit (large leads) not disconnecting when the key is removed (usually sticky contacts) whereas the alternator bleed circuit (spade terminals) is making contact in the correct manner.

 

This would cause full battery current to flow through the wire wound resistor.

 

I have taken apart many after failure and found that the cheapest (about 3 quid) actually are the most reliable (but dont have the secondary circuit). the internal construction of the contacts taking the full starter current have to be seen to be believed.

 

Many man hours have been wasted on faultfinding and many a race has been lost due to these automotive chocolate teapots.

 

I now use a standard 2 pole automotive relay (cost 40p) and two small pushbuttons , one normally open, one normally closed; wired through a latching circuit to provide full circuit isolation and also anti theft.

 

Looks much smarter and 100% reliable.