Tacho needle dipping

[jimj]

For ages, maybe all the time I`ve had the car, as the indicators blink on the electrical surge (?) causes the tacho needle to dip in time with the flash. Of late when I put, say, the wipers on, the tacho needle drops to zero. Sometimes it even does it when I`m indicating and put the brake (lights) on. What`s going on?
The car starts OK but, at this time of year, could it be that the battery is not up to full charge and a trickle charge will fix it? Maybe I should have tried that.
Jim

[Higs]

Hi,

You probably have an RVI rev counter (it should say this in small letters at the bottom of the gauge). This works by sensing the ignition pulses as the points open. A wire loops around a block on the back of the gauge and this "excites" the electronics by induction.

The gauge is powered by a separate wire with 12v.

I do not think anything is wrong with the sensing wiring - it would not be disturbed by additional pulses at the rate of the indicators. What is more likely is that the 12v feed is being compromised when the indicators / wiper / breaks are on. As the indicators are on and then off, this probably just causes the needle to dip and then recover. When the wipers are on, then the needle goes to zero.

This leads to the probable cause (the same cause for much of the Elan's electrical problems) being a poor earth. Somehow, the earth for the indicators/wipers is not working and it is earthing through the rev counter.

The earthing point is low down behind the dash on one of the bolts that secures the body to the chassis. Have a look at this - undo it and clean it up.

After this, it is a question of patiently problem solving with a multimeter. It is not too scary - just laborious. The advantage of our cars is that they were built in an era when V = I x R was about as complex as the electrics got - no printed circuits or other modern stuff to worry about.

So, see what voltage you have going to the rev counter (should be 12v) and then see if it changes when the indicators or wipers are on. This will confirm the problem.

Once you have done this, test the indicator circuits and then the wiper circuit. Test the continuity between the earth leads on the switches and something you know is earthed (the earth side of the battery or the base of the coil are two options). This will all be easiest if you undo the 6 bolts holding the dashboard on and pull it forward. This will give you access to the wiring.

Good luck.

Richard

[10kph]

hello Jim,
Because your engine starts ok your rear earth to the chassis will be OK and the front earth to the engine will be OK. The main earth for the internal instruments is behind the lower part of the centre dash. What a stupid place to form an earth connection, so string a tempory wire from these earth wires to say a cylinder head bolt . If you have some small crockodile clips on the wire ends then the job is made easy. If this sorts your problem then redo the earth screw behind the dash if not then move that crockodile to other black wires behind the dash to find a poor connection.
Good hunting
Tony