why switch ground

[rdssdi]

I am preparing to rewire my Elan +2.

In the course of familiarization with the wiring diagram, I have found that
the heater fan motor is switched on the ground side of the circuit. On high
speed both sides of the resistor are grounded.

Can any of the electrical engineers on the list tell me why it is so
designed? Why not switch the hot side?

Curious minds want to know!

Thanks.

Bob
1969 Elan +2

[elansprint71]

Bob, this has puzzled me too, it's not just the heater fan either. Common sense says put the fuse into the circuit as close to the supply as possible,followed by the switch. Lotus omits the fuse, runs yards of live wire, right through "the device" and then puts the switch right next to the ground connector- that's why they did it. Not safe but easy!

Cheers,
Pete
----- Original Message -----
From: ***@***.***
To: ***@***.***
Sent: Tuesday, February 01, 2005 1:29 AM
Subject: [LotusElan.net] why switch ground


I am preparing to rewire my Elan +2.

In the course of familiarization with the wiring diagram, I have found that
the heater fan motor is switched on the ground side of the circuit. On high
speed both sides of the resistor are grounded.

Can any of the electrical engineers on the list tell me why it is so
designed? Why not switch the hot side?

Curious minds want to know!

Thanks.

Bob
1969 Elan +2

















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***@***.***

[Rob_LaMoreaux]

That's interesting, the heater in the Elan Just 2 is power switched, and
about the only thing that is ground switch are the interior lights, which is
normal for a car. I just checked the wiring diagram and my two speed heater
box and both show power switched.

I just checked the Brooklands shop manual and it shows all the +2 heaters
power switched. I can't see why the heater box that was borrowed from a
metal car would be ground switched.

The Courtesy lights usually are ground switched since it is only one wire to
run to the plunger switch to ground in the door. Plus with lights themselves
will limit the current eliminating most of the danger of fire from the
courtesy lights switch shorting.

Are you sure you read the wiring diagram correctly?

Rob LaMoreaux
Ann Arbor, MI USA
(734)-971-5583
Cell (734)-604-9280
Email: ***@***.***
Too many Hobbies.... Too Little Time
1969 Lotus Elan....It's not a restoration, it's a never-ending adventure.

[rdssdi]

I read it correctly.

Keep in mind the motor case is not in connection with either wire leads. One
side is ground but apparently the case is not grounded.

Bob
1969 Elan +2

[rdssdi]

The Elan +2S diagram shows the hot side being switched.

If the switch shares the same internal logic as my +2, when on high speed
the resistor is also receiving current.

Bob

["e s"]

In answer to the actual question: It just doesn't matter a damn. Ou have electrical potential that must be switched, the electrons don't care. Think of positive ground cars.
----- Original Message -----
From: ***@***.***
To: ***@***.***
Subject: Re: [LotusElan.net] why switch ground
Date: Tue, 1 Feb 2005 09:25:38 EST


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["e s"]

Oh yeah, and electrons actually move from negative to positive not th'utherway round...
----- Original Message -----
From: "e s" <***@***.***>
To: ***@***.***
Subject: Re: [LotusElan.net] why switch ground
Date: Tue, 01 Feb 2005 10:32:30 -0500


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[poiuyt]

It doesn't matter which way the electrons flow. The best thing, as
someone previously noted, is to have the fuse and the switch as close
to the ungrounded side of the supply. Fuses are designed to protect
the wiring, not the equipment, from overloads. When the switch/fuse
is close to the grounded end of the circuit (positive or negative) it
leaves a large portion of the circuit unprotected from an accidental
short to ground. This will cause the wiring to melt, possibly
causing a fire. That is why the switch and fuse should be as close
to the non-grounded side of the supply as possible.

Steve B


--- In ***@***.***, "e s" <stupidemail5@l...> wrote:




lt.asp?SRC=lycos10

lt.asp?SRC=lycos10

["e s"]

fuse yes, switch doesn't matter. current is constant throughout the circuit. If there is a properly sized fuse in the circuit, it will protect the switch. Of course in a stock non S plus two there is no properly sized fuse anywhere, just a 17 amp for switched and one for unswitched, windows and headlights are on their own. We are also talking about fiberglas cars here, so 'short to ground' is unlikely. usually it is overload of the circuit.
So restated with out all of the silly terminology we apparently inherited from Ben Franklin: The fuse should be as close as possible to the side of the circuit which does not represent chassis; all components between the fuseand the opposite battery terminal are protected by the fuse, regardless oftheir location in the circuit.

I do not recall my car being wired with the ground switched. It would be an easy mistake, as the wires coming through the firewall just point in thegeneral direction of things.
----- Original Message -----
From: "Steve B" <***@***.***>
To: ***@***.***
Subject: Re: [LotusElan.net] why switch ground
Date: Tue, 01 Feb 2005 18:49:45 -0000


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["e s"]

Oh, I do recall the headlight microswitches were designed wrong. IIRC switched all ground for the relay, meaning the load also, assumed that's why they didn't last long. did a complete rewire at the time, wired it straight[IEswitching the positive]

Late wiring harnesses are available, and if you want to move the fusebox, get the four fuse style, simplest and most effective IMHO.
If you are rewiring in yourself I do have a point to point run list for theearly car, modified for 4 fuses in the engine bay and alternator. Finding it might be interesting.....


----- Original Message -----
From: "e s" <***@***.***>
To: ***@***.***
Subject: Re: [LotusElan.net] why switch ground
Date: Tue, 01 Feb 2005 15:43:49 -0500


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