At 09:54 AM 4/8/99 +0000, Larry wrote:
>Hi John, I was wondering as to why the need for a relay. I have not had any
>problems with the way I wired mine up. Though I have not been able to drive
>her much. I put a switch beside the air gauge which is located on the lower
>left side of the dash, just to the left of the steering wheel and down. I
>ran a wire directly to the fuse box which had an empty outlet on it. I ran a
>fuse in-between to the switch. I then ran a wire from the switch to an inline
>fuse just before the compressor, then to a switch, then to the compressor.
>What does the relay do and why is it needed and if I do need to put one in
>where in the line should I put it? Now that is a lot of questions in just
>one sentence.
Larry,
You may not really need a relay. I don't know how much current your
compressor takes. But my rational for using a relay is:
1. I'm assuming that the compressor takes quite a bit of current. Say
better than 10 amps. Now, keep in mine that I have an electronics
backround and I'm used to several hundreds to thousands of volts and
very little current.
2. If assumption in (1) is correct, I don't want to run the risk of damaging
the wiring harness by trying to pull too much current through it. Also
depending on where you actually pulled the power from in the fuse box,
most of the power there comes through the igniton switch. Do I want to
pull a lot of current through there? Again, you run the risk of melting
the connector at the ignition switch, burning up the ignition switch,
etc. You would really have to look at how much current the wire and
switch are rated for.
3. Any dirty connection between the battery and the take off on the fuse box
will create a voltage drop. This will do 2 things, a) reduce the current
and voltage (consiquently power) available to the compressor - reduced
compressor efficiency, and possibly damage the compressor motor, and
b) generate heat - again possibly melting connector contacts, or eventually
starting an electrical fire. (This last one about the fire is remote but
possible.)
By using the relay, you can run a seperate large wire directly to the battery
to supply power to the compressor (via a fuse of course). Thus reducint the
load to the wiring harness and the ignition switch. You use a very low
current control signal which can easily be tapped off of the fuse block
without having to worry about anything in (3) above.
The relay should be mounted relatively close to the point of use. For the
air compressor, there is plenty of room to mount a relay along the front
chassis crossmember, if that is where the compressor was mounted. If you
do install a relay, I'd go to Radio Shack or your local parts store and
look for a 12V relay that has contacts rated at about 2 times the current
required to run the compressor. Remember that the starting current can be
several times the running current!
Most of the newer cars are running horn and headlight relays for that reason.
In addition, many of the hobbiests are modifying their older cars to put
relays in these high current circuits for these reasons. Especially
the headlight circuits.
It's possibly a manufacturing thing now also. Again they can reduce the
current through the various connections, use cheaper switches and components
on the control side, have longer life expectency from ALL control switches
involved.
As a case in point, on the Bricklin (and all AMCs) the high current for the
heater blower motor is run through the connector on the back of the fan
switch. I strongly recommend that owners look at trying to clean the
contacts of this connector and putting some of the electrical connections
compound on this connector. This connector is PRONE to melting and buring
up some of the wires or the switches.
Hope this answers your questions.
John
John T. Blair WA4OHZ email: jblair@exis.net
Va. Beach, Va Phone: (757) 495-8229
48 TR1800 65 Morgan 4/4 Series V
71 Saab Sonett III 75 Bricklin SV1 77 Spitfire
www.team.net/www/morgan bricklin.shel.olsy-na.com/bi
|