Relay earthing question

[POPEYE]

OK i hope i can explain this properly! i've been wireing up the compressor, in the drawing mark sent he shows the neg lead from the pump motor going to the tank pressure switch and then to batt neg and the neg lead from terminal 86 on the relay connecting to neg, ok, i thought as the neg lead from the motor is heavy (3mm) and shown connecting to the thin lead at the pressure switch this may be a problem so what i've done is,, run a 3mm lead from the chassi (earth) to a terminal block, (MK type), run the motor neg to this, this means it's 3mm lead to and from the motor, then i've run the neg from T 86 on the relay to the pressure switch and then back to the neg terminal block, this means the neg is switching via the pressure switch on thin wires and will shut down the comp when the tanks up to pressure, (neg switches off, relay shuts down, motor stops),
Is this ok? or have i got it ass about face again
just thought there may be a prob with thick and thin wires on the motor neg side, thin one overheating etc!,

[POPEYE]

Bump

[Gamedawn]

Difficult to say without seeing the circuit, but I'm guessing that the relay is meant to be turning the compressor on and off. Therefore the pressure switch should be in the relay coil circuit (85 and 86) and the motor should be connected to the contact (30 and 87).

If you want to do negative switching then a 12V (fused) feed should connect to 86 and 30 of the relay. 85 should connect to one side of the pressure switch and the other side of the switch goes to ground. 87 should go to the positive side of the compressor motor and the other side of the compressor again to ground. You will need to ensure that the pressure switch contact you use is closed at low pressure.

[POPEYE]

Hi Rodger! thanks for replying, i can get the drawing i made scanned if it'll help but basicly it's as you describe in the first part of your post. it's the same relay set up as Marks drawing, the only difference is i've connected the tank pressure switch into the relay switching circuit and not in the actual pump's negetive lead, my thinking was that way i'm keeping all thick (heavy duty) wireing thick and all thin (low current) wireing thin,
the relay is connected up as follows,
Terminal 30, 3mm cable from battery via a fuse (30amp)
Terminal 87 3mm " to pump positive
Terminal 85 1.5mm cable from ign switched sourse
Terminal 86 1.5 mm " to tank pressure switch and then to earth,

T 30 and 87 being "switched" circuit and T85 and 86 being "switching" (energising) circuit,

3mm negitive cable from pump goes to earth,

so switch on ign, relay starts the pump, when the tanks up to pressure the pressure switch breaks the earth circuit from T86 and relay switches off the pump, the other way the tank switch would cut the pumps earth (stopping it) but the relay would still be energised, hope i'm makeing sense here mate,
soon see when i have the tank fitted and i try it out i guess,

BTW i think it was you who said it was a bad idea bolting the body to the chassi on the post i did regarding the side tube bracket's i made? well you were right it's not a movement problem but a noise one, their transmitting all the bl00dy noise from the chassi, engine, trans, tyre drumming, bumps in the road, i had a job to hear my sat nav when i went to marks and rodleach's, BUGGA!!!!

[MattF]

Well, I've tried reading it several times and it just ain't sinking in. The only high current wires are the ones feeding the motor, and you're using a relay to switch that feed? Is that correct?

[POPEYE]

Well, I've tried reading it several times and it just ain't sinking in. The only high current wires are the ones feeding the motor, and you're using a relay to switch that feed? Is that correct?

Yup thats right Matt, it's wired as you would normally wire a relay, only i've wired the air tank pressure switch (fitted in the tank with two "thin" wires comeing from it) into the T86 earth wire, wire runs from T86 to one of the tank switch wires and the other wire runs to earth, mark said it dont matter which of the pressure switch wires you run to earth, his drawing had this pressure switch wired into the neg (black) wire from the comp motor, this will also work BUT this means connecting a thick wire to the tanks switch thin wire and then back to a thick wire to eatrh, surely this creates a "weak spot"!
the wires from the motor are thick (3mm) for a reason surely,
i'll scan the drawing tonight, i just wanted a second opion sort of,
this is all assumeing the pressure switch is normally closed till pressure opens it and breaks the earth circuit of course, cant see it working any other way

[POPEYE]

OK heres the drawings, Matt/Rodger, can you see what i mean? lower one is the from the original, sorry it's sideways, i did "right" em on photobucket but the sods have comeout like this

[MattF]

From what I can glean from that second diagram, you're doing it wrong. The pressure switch should be in the feed to the relays coil. Nowhere else. You want it to break the feed to the relay coil when it trips, thereby toggling the relay and removing the feed to the compressor.

+12V----pressure switch----relay coil----0V

[POPEYE]

The second drawing is what mark sent (although it's not the original, i threw it out after i'd drawn it with a ruler) but it's how he drew it, and he said the pressure switch has to be switching on the neg side, thats why i figured it would be better to have it switching on the neg side of the relay as opposed to the neg side of the motor (which is a heavy duty cable, 3mm)
i am right in thinking that if you break the neg side of the coil (terminal 86) the relay will deactivate??, i thought you could switch either side of the coil circuit and the relay will work!,
in fact matt it's working in a similar way to the fan relay set up only without the five pin relay, those are switching on the neg side via the thermostat switches, i get a bit confused with these switches, the thermostat ones are normally closed right? so when the thing they controll gets up to temp they break and cut the circuit, no good if you want a fan to cut in at that temp but with the use of the 5 pin relay you can do it, the pressure switch must act in the same way allowing current through till the pressure breaks the circuit? and stops the pump, which is what you want it to do (without a 5 pin relay)
anyway i'll give it a go, i got a pressure gauge in the air circuit and the tank will be underneath the truck so if it goes bang i wont be near it!!!! it'll only destroy me fuel tank, prop shaft and exhaust,

[MattF]

i am right in thinking that if you break the neg side of the coil (terminal 86) the relay will deactivate??, i thought you could switch either side of the coil circuit and the relay will work!

Shouldn't matter whether it goes pos or neg side. Switches are neutral. It's just convention to show it connected on the pos side. Old habits die hard and such. Connect it either side. It won't affect operation. Also, yup. Break any part of that circuit and the relay will toggle.

in fact matt it's working in a similar way to the fan relay set up only without the five pin relay, those are switching on the neg side via the thermostat switches, i get a bit confused with these switches, the thermostat ones are normally closed right? so when the thing they controll gets up to temp they break and cut the circuit, no good if you want a fan to cut in at that temp but with the use of the 5 pin relay you can do it, the pressure switch must act in the same way allowing current through till the pressure breaks the circuit? and stops the pump, which is what you want it to do (without a 5 pin relay)

Yeah, you just alter specifics with regards to what you need. Standard type of four pin relay is N/O, so you need to apply voltage/current across the coil for the relay to work.

Just pop your meter on that switch and make sure it is reading short circuit by default. Then, (on either the pos or neg feed to the relay coil, although you might as well do neg to keep Snatch happy), pop that switch inline.

[POPEYE]

Cheer's mate, i just sort of wanted a second opinion, you've confirmed what i thought would work, but i always get jittery,