Rover 75 diesel pre heat unit
 

Have experienced a couple of owners recently complaining of drained batteries and tracing it to the pre heat unit staying on which can flatten a battery overnight.

As a replacement they are expensive, hard to find and can be a pain in the rear to change so recommend my solution which it appears no one uses.

The power to the pre heat is supplied by the orange cable which if you cut it and put in a relay normally open (closed on power up) will prevent the discharge.

Connect the cables that operate the relay on to earth and the seconf to the top right 20A fuse in the under bonnet fuse box which only becomes live on switching on the ignition, and subsequently switches on the unit when you need it. Also by the fact that it is active on ignition being turned on the engine management system does not treat this as a fault

I did this mod 3 winters ago and never looked back

:icon_lol:

9 times out of 10, all that is needed is to cut the K bus wire on pin 2 and tape it up. It is most unlikely that the heater is really running fully, all that happens is that the fan inside the heater sometimes runs for a period of time while the K bus activity winds down after locking the doors. Cutting the wire stops that happening, but a more permanent cure would be to repair the pcb inside the heater.

Yep, French mike does a superb job of repairing them.

I think young Phil has taken up the challenge as well...

Never looked back - FBH flat battery
 

Unless I have misread the above from Gorephoto, you are only using the FBH when the engine is running, The reason you have "never looked back" is because you will have not placed a direct drain on the battery before the engine has been started.

But,- you are not having the advantage of a pre-warmed car when you start the engine. If that is the case, you may as well have saved all your trouble and just use the basic 5 degrees or below temperature auto start, or the override method of manual start, dependent on which model FBH you have.

To improve on the above, The general Idea is to start the FBH about 15- 20 minutes before you need to start your journey. In that time, the FBH warms up the engine coolant, and after about 12 minutes the coolant is warm enough to be able to start the ATC, with the manual settings suitably set, to warm up the inside of the car. This then melts all the ice on the windows all round, and you drive away in a nice cosy car, while your neighbours are still scraping the ice away.

What you must cater for is a suitably timed switch off of the FBH so that it does not stay "switched on" until the next time you need your car.The favourite trick, developed on this Forum, is to incorporate a spare heated seat switch into the circuitry which has a 15 minute timer built in to shut off the FBH. Your car may be warm enough after the first 15 minute period. if not just run it again for another 15 minutes.

There are masses of information on on this Forum for the FBH and flat battery prevention. Again, apologies if I have read it wrong

:Snow: PS When I started my reply, none of the others had been entered.