voltage drop problem

Hi am building a Austin Healey replica with a Ford V6 motor space is a big problem so the radiator has to tilt at an angle. I have just installed an electric pull fan to help with cooling. The fan draws about 20 amps. I installed a 90 amp alternator. When the fan switches on there is a big voltage drop all the guages register a lower value and the volt meter drops to 10 volt but the charge light stays off so alternator still charging. Any ideas what the problem could be?

1) battery to small presently 60 ah
2) bad earth
3) faulty battery

If i disconnect the fan everthing returns to normal

When you have an electric fan............you should have a relay between the fan and switch. The relay takes the hit of electricity so the switch does not. Not an electric guru here but know enough to tell you this.

I agree, it sounds like you've got your fan powered from the Accessory Circuit. With a relay, the power comes direct from the battery, with the switch side of the relay the only Accessory load.

What gauge wire are you using? The lowest I would use is 12 gauge.

I think you need to put a relay in as well. Those fans aren't made to run without one.

+1 on the relay. I too am working on an Austin Healey replica (Sebring) and I can say that the original wiring was minimal. As I recall, the main supply to the harness is just 10 gauge wire (maybe 12 gauge) and this has to carry all the loads in the car. As these cars were built mostly back in the 80's and 90's, they were not really wired with electric cooling fans in mind. I will be rewiring mine with my own harness.

A 20 amp draw with a 90 amp supply shouldn't cause major problems provided the wiring is up to the task.

Run a separate wire with 30 amp fuse directly from the battery to the switched portion of the relay. This should be 12 gauge at a minimum, 10 gauge would be better to minimize voltage drop. The relay coil would be triggered by whatever switch you are using now. The fan ground would be better if run to the engine block or directly to the battery.

You didn't say if your car is a new build or a rebuild of an older car. If it's a rebuild, I would recommend that you look at the whole wiring harness carefully. It could have many poor or deteriorated connections in the harness (mine did). There are many street rod wiring harnesses available now that will have better wire, modern fuses and are built with things like fans in mind.

Ditto on the above

Here's the answer:
JEGS Performance Products 10560: Electric Fan Wiring Harness & Relay Kit | JEGS

I can make this up in my garage for about ten bucks, but I have a stash of #10 wire and a bunch of relays. Buy the kit and everything you need is there. Well spent $35.

Click on the instruction tab and there's a nice picture.
Glenn

What brand of 'lectric fan? I have a Cooling Components fan that claims 20 amps at steady state but the fine print says it pulls 70 amps at startup....Learned to read after I toasted a relay.....putting in a much larger capacity relay now.

I would add that the battery itself is not the best place to source your power for the relay.

There should be a power junction which the alternator feed wire connects to. This junction is a better source for the relay. It will allow the alternator to better sense the system load.