JAGUAR 240 MKII CHANGING POLARITY
Q
I have a 1969 Jaguar 240 Mk2 which is still running as original on a positive earth circuit. I have decided that it would be wise to make the conversion to the negative earth system, but I am unsure how I should proceed. I have read that this is a simple process, but I would like further details before attempting the switch over.
Doug Hammond A
This should be easily achieved, but some Jaguars were fitted with an electronic tachometer that would need to be replaced. Most of the Jaguars had a separately energised unit that ran off a small generator on the camshaft via a gear – if yours has this type of tachometer, then it can be left in place and the swap from positive to negative earth is as follows. First disconnect the battery and reverse the connections, (you may need new terminals fitted as the positive and negative posts are different sizes). You will need to reverse the connections on the ignition coil, and if a radio suppressor is fitted to the coil this will need to be transferred to the positive feed.
I am presuming that as the vehicle is still positive earth it has the original dynamo fitted? This will need to be re-polarised as follows: Disconnect the dynamo terminals and, using a length of heavy cable, touch the battery live terminal (now positive) to the smaller field terminal on the dynamo for two to three seconds. This will produce a small blue flash, indicating that it has been energised for negative earth. It can now be reconnected in the normal way. However, if your Jaguar is fitted with an early positive earth alternator, this would need to be replaced. If a radio is fitted, this would need to be adjusted to cope with the polarity change, but all other equipment – wipers, starter and lights etc – will be happy to run under the negative earth system.
Another option you may consider if the Jaguar has the original dynamo is to replace it with a unit which, although looking like the original dynamo, is actually an alternator and will supply a higher output.