Part12 Converting the Rev Counter

Perhaps now is the time to look at converting the TR7 tachometer to run with the V8 engine. The following was first published by previous TR8 Registrar, Paul Towle - I have amended and updated a few details. My thanks to Paul for a very comprehensive and well thought out article.

There are two reasons to change the rev counter (tacho). Firstly the new tachometer has to take account of the extra four cylinders that are being installed under the bonnet and secondly the red line occurs from 5500 to 7000 rpm on the TR8 version whereas the TR7 red line from 6500 to 7000 rpm.

The original TR8 tacho (BL Part No. TKC 2587) is no longer available. However, it is possible to convert the standard TR7 tacho for use in the TR8, the cost of the conversion being less than £5 in parts. This conversion should be well within the scope of most people, provided that it is done with care and in a good working environment.

The following items will be required for the conversion:-
1. A small tin of 'HUMBROL ENAMEL SATIN RED' paint No. 132 (This is available from most modelling shops).
2. 'HUMBROL' paint thinners.
3. A roll of 1" wide sellotape.
4. A small modeller's paint brush.
5. A scalpel or razor blade.
6. Tweezers.
7. A 15 - 25 Watt soldering iron with a 2mm bit (approx).
8. A small length of 'MULTICORE' solder, (suitable for electrical/electronic work).
9. A 47k (47 kilo ohm), 0.6 watt, 1% tolerance, metal film resistor. This can be bought from RS Components Part No.148-893.
10. A pair of fine wire cutters.
11. A jam jar lid (or similar) with a diameter of approx. 63 mm.

I must emphasize that it is essential to use a soldering iron with a fine bit as suggested above.
If you do not have one, try to borrow one. This job can not be done with a blowlamp!!

PROCEDURE

1. Assuming that you have already removed the tacho from the instrument pod.
a) Examine the rear of the tacho. It should be possible to see four components which look like 'liquorice all sorts' standing on legs!!!. (See Photo). These components are 'capacitors'. The capacitor on the right should have five coloured bands, brown, black, yellow, black and red, reading them from top to bottom. Locate the 'leg' of this capacitor which is closest to you when viewed as shown.
b) To the right of the capacitor located in a), there is a red wire which is soldered to the thick film substrate, (i.e. the ceramic base to which the components are soldered). Locate this connection.
Note that the 'substrate' or ceramic base is very brittle and can be damaged very easily if handled without care.