Rev Counter Upgrade
The electrical rev counter on XK engines is notoriously inaccurate, usually reading much lower than the actual rpm. There is no adjustment to correct this and it's a common fault of the design. The rev counter operates by reading the voltage being output by an AC generator which is driven off the rear of the inlet camshaft; when new this gives about 1 volt for each 100 rpm of the engine. This signal is fed to the moving coil meter via a precision wound resistor and a bridge rectifier, the resistor reduces the incoming voltage and the rectifier converts the voltage to dc to feed the meter. The rotor of the generator is constructed from permanent magnets and these lose strength with age resulting in low indication of engine rpm. I have heard that new generators are expensive and variable so will not necessarily restore the accuracy. Checking this rev counter against a digital timing light showed it was reading around 20% low so at an indicated 4500 rpm my engine was close to the red line. I wonder how many XK engines have been “blown” because of faulty electrical tachometers?
Comparing the modified rev counter with the original at 1000 and 2000 rpm.
The tach generator gives a sine wave output of 4 Hz for each revolution of the camshaft or 2 Hz for each crank revolution. The amplitude of the generator output is unreliable but its frequency is directly related to rpm so converting the frequency to a voltage and using that to drive the meter provides a good solution to the problem. The electronics to achieve this can be mounted inside the meter and apart from providing a fused +12 volts supply to the circuit no changes to the car are necessary. The circuit is based on a LM2917 frequency to voltage converter IC, the circuit and a suggested board layout are shown here, click on the image for a larger view. This circuit only works on negative earth.
Remove the resistor and rectifier from the original SRBP board, just cut the wires leaving the blade terminals in place. Take care not to pull on the red and black wires connected to the meter movement and tuck them out of the way until ready to connect them to the new board. Fitting the PCB with short 4BA screws is tricky but it can be done without removing the instrument face and pointer. The +12 volt supply wire can be fed out through the hole in the SRBP board created when removing the rectifier.
To adjust the trim potentiometer use the .mp3 sine wave files at the bottom of this page with one channel of the PC audio output connected to the blade terminals of the instrument. Alternatively a function generator may be used; amplitude is not critical.
Having setup the circuit at 133 Hz to indicate 4000 rpm I find the reading is accurate to within a few percent from tick over to 6000 rpm.
The modified gauge is installed and it is now responsive and accurate. Click on the picture for a .wmv file of it in action.
Measuring the ac voltage output of a working generator at 600 rpm. The tacho board upgrade will provide an accurate reading of RPM as long as the generator output is greater than 0.2 volts at tickover.
There has been a lot of interest in this modification so I have made a small run of complete boards. These are not commercially made but are good quality, tested and include fitting instructions and 4BA screws. Including shipping within UK the cost is £45 and £47 overseas. E-mail to the address on the home page if you want one. Note that they are only suitable for cars with the camshaft driven generator; late XK150s, Mk 2s and Series 1 E-types and are negative earth only.
600 RPM.mp3 1000 RPM.mp3 2000 RPM.mp3 3000 RPM.mp3 4000 RPM.mp3 5000 RPM.mp3 6000 RPM.mp3