12v Compressor blows fuses

[G-Dubya]

Hi all,
I have recently attempted to mount my 12v compressor and a 9 ltr air tank into the Nav. The compressor is mounted in a box in the tray. I already had some wiring coming from one of the front batteries to where the compressor is mounted that was previously used to run the 80ltr Waeco of the previous owner.

I have the power to the compressor going through a 5 pin relay (40a) which is activated by a switch in the cab (powered off a circuit activated by ignition so the compressor can't be activated without the engine running). I have the switch wire passing through a pressure switch before it gets to the relay.

I completed the wiring and tested the compressor before fitting it without any issues. The compressor filled the tank to the correct level and then shut down. When I purged enough air from the tank it kicked in again without any issues. I was using a 100ah deep cycle battery to complete this testing and it was fully charged prior to starting.

With the testing working as I wanted, I mounted the system into the Nav. The wiring was connected to the wiring as it was during the testing and now some issues have started. The issue being it is continually blowing fuses when the tank is filled to the correct pressure and the pressure switch cuts the power to the relay switch wire. I am using 40a fuses and the things are ending up black and melted when they go.

Can anyone give me an idea on what is happening and why the fuses are only blowing now it's mounted in the car?

 

[Old.Tony]

Cable length. As the voltage drops, the current needed rises, and it's current that blows the fuse.

There's a chance the cable used is a bit on the small side for long runs (long, in automotive high-current applications, means anything over 2m). For drawing 40A (and compressors can draw up near that) you'd really want a large cross-section of multistrand cable. I'd look at 4Ga as a minimum. That's 5mm diameter, 21 sq mm.

If you take a look at this page, you'll see that for 6+ metres with 40A, that's precisely the cable you'll need for that amount of current.

The fuse should only need to be near the battery - it is there to protect the car. With cable of that size, a short means a fire - nearly guaranteed. Very thin cables would just melt like a fuse, but 4Ga is serious stuff. Fuse it right off the battery before the relay and you can't go wrong.

 

[Navmania]

Check you have good earths to. If you are earthing to the tub make sure the tub has a earth lead to the chassis of the same size as the main power lead.

 

[TigerST]

What they or the save fuses get a readable manual or auto blade fuse

 

[tweak'e]

sounds like you have relay connected wrong and its shorting the power to earth when it switches the pump off.

 

[G-Dubya]

Thanks for the replies. It gives me some places to start to sort this out. I have removed it from the car and will recheck it.

 

[G-Dubya]

Hi all,
Just a follow up. I changed the cable that was in the car already and swapped it for 6BS. This has sorted the problem. There is minimal voltage drop through the cable and the compressor is now working as I wanted it to. Now I need to go for a run down the beach to drop the tyre pressures down and see if they fill a bit quicker than before.

A run down of the system is as follows:
6BS cable from battery to rear tray,
50 amp auto resetting circuit breaker,
12v Compressor (150psi, 150 ltr/min),
Pressure switch (on 120psi, off 150psi),
9 ltr air tank,
12v 120mm fan (to cool compressor, fan starts when in cab switch is activated)
230 psi Pressure Gauge
1 x Nitto Coupler
1 x Drain/Purge line
40A Relay

All fitted inside a carpeted ply box measuring 600mm (w) x 240mm (d) and 320mm (H).

The next project will be to add an auxiliary battery (130 ah), a 12v DC to DC charger (CTEK D250S Dual), multiple 12v Anderson plug input/outputs (probably 4) and a 4/6 switch panel. All in a box the same size as the compressor.

 

[longy509]

just out of curiosity what brand is the compressor i got a free one with 4x4 action and it blows fuses every time i use it

 

[scottybobcat]

Mine does the same thing Longy. Thinking of just putting a heavier sized fuse in it

 

[Old.Tony]

Scotty & Longy, how thick is the cable and how long is it?

The fuse should be rated for the draw of the motor, but it's not a surprise that they put a cable that's too small on it.

Cable rated for 40A means it is capable of carrying 40A. It doesn't mean it can carry 40A over 5m of cable - it's a common mistake. The longer the cable (for DC power applications), the heavier it needs to be.

 

[G-Dubya]

The compressor was an ebay special, Outbac (here is a link: http://www.ebay.com.au/itm/NEW-OUTB...=AU_Car_Parts_Accessories&hash=item20cd65b59d)