Hints and Tips - Ignition Warning

From: Mike Harris
Sent: 12 July 1999
To: British Cars Pre War
Subject: Ignition warning lights

Can anyone tell me the approximate resistance of the wire which is wound round an ignition warning light on a 6 volt system. What is the approximate length and resistivity and current loading?

The special's ignition warning light failed and I found two broken wires amongst a jumble of other wiring and a resistor hanging loose. Completely non-standard A7, with Bosch dynamo conversion and cut-out. I am trying to unravel it literally!

Mike Harris

From: Carl Cederstrand
Sent: 15 July 1999
To: Mike Harris
cc: British-Cars-Pre-War
Subject: Re: Ignition warning lights

Dear Mr. Harris,

I know nothing about the value of the resistance Mr. Lucas chose for his 6 Volt warning lamps. I do however have related comments about his choice of resistance for the 12 volt warning lamps. Perhaps this information may be of use to you in reconstructing your 6 volt warning lamp.

The 12 volt warning lamps possess resistance wires of 65 to 85 ohms wrapped around their bakelite bodies. These wraps of resistance wire serve as a voltage dropping resistor. A resistance of this high a value drops the operating potential across the lamp to a value below that of its nominal operating potential. This serves to starve the lamp for current. The result of this lower current and thus lower filament temperature, is that the bulbs are dimmer and will last for decades, The original bulbs supplied were 2.5 volt, 0.2 ampere bulbs. I believe they (#970) were originally designed as flashlight bulbs. In flashlight service they would have been pushed hard because they would assumedly have been fitted to 2 cell flashlights (3 volts). I have always ASSUMED that this configuration (lamp plus series resistor) was also employed as the fuel warning lamp because the starvation current would result in a low current to be interrupted by the switch contacts. The switch, which is located in the fuel tank fuel cannot then ignite the gasoline vapor as the spark at the contacts does not attain the minimum energy required for ignition.

Assuming that your 6 volt A7 system, which I have never seen, also employs a 2.5 volt bulb, I would wrap the resistance wire to, say 30-40 ohms, and fit the afore mentioned 2.5 volt bulb. Where do you obtain suitable resistance wire? Attend the next automotive swap meet and buy one of the more plentiful 12 volt variants and simply clip off 1/2 of its resistance wire.

The resistance wires on the 12 volt units appear to be a Nichrome. You will have trouble soldering to this wire because of the chromic oxide that forms on the surface of the wire. Rosin core solder will not strip oxygen from chromic oxide so the soft solder cannot alloy with the chromium under the layer of oxide. Accordingly, go to a welding supply and purchase a bottle of stainless steel soldering flux. Now tin the ends of the wire using the stainless steel flux and solid core solder. This is to be accomplished before winding so that you can conveniently wash off the very acidic stainless steel flux. Once tinned, the ends of the resistance wire can be readily soft soldered. If you still have a problem soft soldering to the nichrome wire, drop me a note as there is a 'fail proof' method for electrolytic of decomposition of chromic oxide.

The above discussion assumes that the 6 and 12 volt units are the same except for their series resistances. When employed as an ignition warning lamp all this resistance wire adjustment becomes folderol and doesn't mean much. In this case you could fit a 12 volt bulb and omit the resistance wire. All the ignition warning lamp does is provide a starting current in the correct direction so that the dynamo has a faint magnetic field to initiate an induced E.M.F. in the correct direction.

Best wishes, Carl Cederstrand

Orange, California

From: TA Terry
Sent: 15 July 1999
To: Carl Cederstrand; Mike Harris
cc: British-Cars-Pre-War
Subject: Re: Ignition warning lights

Carl Cederstrand writes:

<< resistance wires of 65 to 85 ohms >>

Carl, why not just use a fixed resistor and forget the resistance wire? Inquiring minds want to know!!


From: Carl Cederstrand
Sent: 16 July 1999
To: TA Terry
cc: Mike Harris
Subject: Re: Ignition warning lights


A garden variety resistor would work fine. Be sure to calculate the power dissapation required for the resistor (I X I X R) -- I can't type I squared. This comes out to be about 2 watts for a 40 ohm resistor, so buy a 3 watt resistor.


From: Jack Hardy
Sent: 17 July 1999
To: Carl Cederstrand
cc: TA Terry; Mike Harris
Subject: Re: Ignition warning lights


I think possibly there are two reasons carmakers did not use "garden variety" resistors is: 1. there were no Radio Shacks to go to and buy resistors (or JPL electronic stores! shh, for that matter)! And the nichrome wire was compact and not sucsptable to vibration as a single resistor may be. The nichrome is well attached to the bulb and can not vibrate in the case of my MGPA. If you guys do use a "garden variety" resistor epoxy it to something substantial so that it can't vibrate and break its leads.

And that's my 2 pence worth


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