Non-standard overdrive switch and cables - what capacity?

[MilesA]

Two questions relating to overdrive electrics.

 

Switch

I need to replace my car's non-original overdrive toggle switch and I am not very confident with electrical matters.

 

It has a J type box (I think - o'drive in 3rd and 4th) installed during its rebuild. The present switch operates the overdrive through a relay. No point installing an original pattern switch - the car is not original and the present hole in the dash is in the wrong location.

 

Options are a replacement of the existing toggle but the question for those who know is, 5amp (as used on the electric fan via a relay) or 18amp?

 

Cables

The 2 cables from the overdrive to the relay* simply rise vertically from the gearbox cover behind the gear lever, are secured under the dash and then routed to the relay located behind the switch. There is very little slack and so to avoid catching them by accident and ripping them out I would like to replace with longer cables and re- route under the carpet on the gearbox, along the bulkhead and across to the relay out of harm's way. Again, what capacity cable should I use?

 

Thanks - Miles

* Think they both go to the relay from memory but cannot check as I am not with the car.

[ianc]

Miles - the switch, which energises the relay, will see only a very small current, so you can use small cable (3-5 amp is more than sufficient).

I've pasted below measurements which I made some 3 years ago on a solenoid, from which you'll see that the solenoid draws about 17 amps for a very short time as the overdrive is engaged, so you need cable capable of carrying that current (if you use a lower rating cable, you'll lose a little in the cable itself, and that's undesirable).

Ian Cornish

 

Pull-in (Pull-in and Hold-in coils in parallel) 0.8 ohms, drawing about 15-17 amps.

Hold-in (pull-in circuit disconnected by plunger operating the switch within the top of the solenoid) 12.5 ohms, drawing about 1 amp.

 

For the technically-minded, this means that the Pull-in coil has a resistance of about 0.85 ohms, and takes about 14-16 amps.

The current drawn will depend upon the state of charge of the battery, which is usually between 12-13.4 volts.

[RogerH]

Hi Miles,

as Ian states the solenoid itself takes betwen 1 amp and 17 amps depending on its position. The relay coil takes apprx 150mA.

In theory the switch needs to be 'only big enough to carry this very small current.

However small current switches tend to be delicate so go for the bigger version of switch but you can go easy on the cable.

TRy and use colour coded cable the same as the car that way you will reduce any areas where disaster may lay.

 

 

Roger

Edited September 4, 2012 by RogerH

[MilesA]

Thank you Ian and Roger. That is all very clear and Roger I agree entirely about using the correct colour coded cable to avoid future complications.

 

Regards - Miles