410 Air Conditioning

[David C]

Further investigation regarding fitting air conditioning to a 410 has made me think about the current draw that this adds - around 15 Amps. While an uprated alternator will easily provide the extra current, the ammeter and its original wiring were not designed with this load in mind.

Conversion to LED headlamps should lessen the load enough to prevent this becoming a problem. Otherwise I suggest wiring the air conditioning so that it disables the heated rear screen when in use. It is unlikely that both would be needed at the same time.

[Sam410]

I find AC a very useful aid to demisting, so don't discount using both together. My rear screen heater has never worked, so I don't have this problem. Maybe the new loom going in will fix that.

[David C]

Good point well made.

I think I will probably fit a newer alternator of at least 90 Amps with integral regulation. They look all but identical to the original. As I need to get the dashboard refurbished I will change from ammeter to voltmeter at the same time so that load in the wires to the dashboard can be reduced to a minimum.

I had a look at the 411 wiring diagram to see how Bristol changed the wiring as more toys were added and current consumption increased. An interesting anomaly to my understanding is that the 411 engine fans are fed directly from the battery if controlled by the Otter thermostat but through the ignition switch in manual override. Unless anyone can give me a good reason I think I will keep that sort of load away from the ignition switch.

[Roger Morrall]

Originally Posted by David C

I had a look at the 411 wiring diagram to see how Bristol changed the wiring as more toys were added and current consumption increased. An interesting anomaly to my understanding is that the 411 engine fans are fed directly from the battery if controlled by the Otter thermostat but through the ignition switch in manual override.

Seems to me to be unlikely, the comparatively unsophisticated wiring-wise 410 already had a relay mounted under the bonnet that had parallel trigger inputs from the otter switch and the over-ride switch and I believe the (later?) 411 wiring and fusing arrangements were further improved. So the load isn’t actually taken directly by either the otter switch or the ignition switch

Originally Posted by David C

Unless anyone can give me a good reason I think I will keep that sort of load away from the ignition switch.

I alarmed myself considerably when I tested and added up all the other potential loads on the poor little ignition switch (the windscreen wiper stall current on a dry windscreen was particularly amazing) and rearranged things so that the ignition switch drove a very heavy duty relay (sometimes known as a contactor) to take those loads.

[David C]

I agree it's unlikely, but it doesn't necessarily mean it isn't so. My interpretation of the 411 wiring diagram on the BODA site is as stated. See attached and let me know if I have got it wrong.

It looks as though the thermostat switched relay is fed via a fuse from the battery while the manually switched relay is fed by an in line fuse powered through the ignition switch.

http://www.bristolcars.info/forums/a...1&d=1699814779

[Sam410]

I haven't checked the diagram or the car itself, but the 410 handbook does state that the fuse protects the "manual control switch for fans" and is controlled by the ignition switch, so certainly implies that there are two sources of power to the fans. Incidentally I believe that running the AC on my car will also switch on the fans regardless of engine temp.

[David C]

To summarise:

The 410 has a single fan relay which is operated by the Otter bimetallic switch in the top of the radiator. According to the wiring diagram the manual switch passes full current and does not operate the relay. Both options, automatic and manual operation are fed from the ignition switch.

The 411 has two relays, one for automatic thermostatic control and another for manual override. The relay for automatic control is fed directly from the battery and the relay for manual control is fed from the ignition switch.

My feeling is that I would rather not have this much current flowing through the ignition switch. I have seen melted switches and wires on early V8s. It's much easier to prevent the smoke coming out than to get it back in.

The ammeter and wiring on the early V8s coped - just - with the original load but as we add more power hungry accessories it is important not to add more load to the ammeter or to the ignition switch. This is easily achieved with relays fed directly from the battery controlled by feeds supplied from the ignition switch but this will not show up on the ammeter. Much better to ditch the ammeter and replace with a voltmeter per 411 which will give a better overall idea of what is going on.

[Roger Morrall]

Originally Posted by David C

I agree it's unlikely, but it doesn't necessarily mean it isn't so. My interpretation of the 411 wiring diagram on the BODA site is as stated. See attached and let me know if I have got it wrong.

It looks as though the thermostat switched relay is fed via a fuse from the battery while the manually switched relay is fed by an in line fuse powered through the ignition switch.

http://www.bristolcars.info/forums/a...1&d=1699814779

My apologies, on checking the 410 wiring diagram I found my memory of the standard 410 arrangement to be faulty. You are absolutely right

[David C]

Thank you, Roger. Good to have confirmation that I'm not losing the plot! I think what you thought you had remembered is a better arrangement but leaves the fan running with the ignition off unless the relay coil has a separate switched live feed.