In a message dated 6/6/2001 5:29:40 PM Eastern Daylight Time,
nriedel@nextek.net writes:
> The car ran great but wouldn't stop when the ignition was switched off. If
> he tapped his brakes, the engine would shut off. (He said that was good
> British logic; if you want it to stop, hit the brakes.) He said the MDS
> module was installed as per instructions, wires directly to 12 volts and
> ground and also a wire to the white ignition wire at fuse block. ( I assume
> there was also a wire or two to the coil, didn't think to check.)
>
> There was obviously some source of 12 volt power to the ignition wire with
> series resistance such that it was loaded down by the brake lights (also
> powered from the white wire --- but after the fuse so it is the green wire).
> Checked out Dan Master's schematic for the '73 and noted the Alternator
> Warning Lamp (IGN) from the alternator voltage sense circuit connects to
> the white wire. We pulled the lamp and everything worked properly (except
> the lamp). We installed a diode in series with the warning lamp and
>
Nelson,
This is a problem of voltage division. When you turn off the ignition key,
all of the switched electrical loads are now wired in series with the
alternator warning lamp. Output voltage from the alternator is now divided
between the resistance of the lamp and the total load of the electrical
system. The resistance of the warning light is about 65 ohms. The resistance
of the brake lights (two lamps) is about 3 ohms (all approximate numbers).
This divides the voltage in a ratio of 65/3, which means that most of the 12
volts is dropped across the warning lamp, leaving little for the brake
lights. Evidently the resistance of the MDS module is much higher. With just
the MDS module in the circuit with the warning lamp, the voltage drop may be
50/50 or so. If the MDS will operate with 6 volts, then the engine won't
shutoff. When you hit the brakes, the combined resistance of the MDS and the
brake lights is less than the resistance of the either (ie., less than 3
ohms), so the voltage to the MDS drops to a point such that it will no longer
operate, killing the engine. Turning on the heater fan should have the same
results.
This is a bit difficult to cover well in an e-mail, but I think the material
in chapter one of my book will help to clarify it a bit. Or, ask "FT" ;-)
Putting a diode in the circuit works just fine, but there may be a better
way. I'm not familiar with the MDS hookup, but did you wire the "directly to
12 volts" wire to the battery? I gather that the MDS has two power feeds -
one for the electronics and one for the coil? Try hooking it up to the white
wire and see if that solves your problem without the diode. The coil has a
resistance of approximately 3 ohms, so it will not function in series with
the warning lamp.
If you can copy the MDS instructions and send them to me, I'd like to take a
look them. (For those concerned with copywrite protection, I won't be selling
them or otherwise make a profit from them - just trying to help a fellow
member solve a problem. As an author myself, I am very concious of copywrite
infringement, but I believe MDS would approve, as it may mean more sales for
them in the long run.)
Dan Masters
Alcoa, Tennessee
Triumph TR 250 - TR6 Electrical Maintenance Handbook:
http://members.aol.com/danmas6/
Stuffing a V8 into a small British sports car:
http://members.aol.com/danmas/
British V8 Newsletter:
http://members.aol.com/danmas4/mgv8.htm
|