Rev Counter problem

[Deleted]

So I checked and rewired everything. The only difference to the original layout is that I use a blue Bosch coil without ballast resistor (but even installing one does't change anything).

The rev counter shows some 1700rpm when ideling so about two times the realistic figures.

Am I right that the instrument some sort of reads the number of interruptions in the current by induction via that loop in the wiring between the ignition switch and the coil.

So twice the reading you would expect if you use a 4 cyl. instrument on an 8 cyl. engine. But mine is marked "8 cyl"

I would suspect the instrument if the reading was somewhat irregular, but as it reads nearly exactly double I hope that the fault might be elsewhere.

But where? Any hints?

Michael

[Phil Nottingham]

I did understand that they work on pulses somehow but being no electronics expert cannot say. What I do know however is that any transister degrades over time and at different rates so if its original its well past its use by date so I suggest a rebuild

[stevem]

Michael, I have recently changed my rev counter as the old unit flickered all over the place and eventually used to stay hard against the end stop. The next time the car was used it would be OK.
Anyway, the new rev counter reads about 600 rpm at tickover but then seems to progress up the scale at about half the rate it should. Without blowing the engine to bits it will not go past 4000 revs.
The small code numbers on the face of the tacho are the same as those on the unit it replaced. I wonder if the tachos used for cars with the anti-theft coil are different to those without this type of coil??
Seems strange that yours reads double what it should and mine reads about half what it should.
Mine has the anti-theft coil, does yours?
Steve M

[Deleted]

Steve
mine had been fitted with an anti theft coil but it seemed to me that it had been fitted on an aftermarket basis because the way it was installed was rather crude. The legislation changed in Germany some years ago and you have to have some special sort of anti theft device. I took the anti theft coil out because the ignition key looked ugly on the dashboard and installed an hidden electric switch blocking the inhibitor switch instead. But I meassured the resistance of both coils and it is nearly the same. Now I found a problem with the contact breaker and if that will be solved I will come to the conclusion, that the Instrument itself is faulty.

Michael

[Deleted]

The later cars fitted with anti-theft ignition coil had the ignition switch on the steering column, early cars had the ignition in the dashboard, I think.

I recently had to replace my original "anti-theft" ignition coil and managed to fit a replacement and keep the wiring and ant-theft metal sleeving so it still retains the inbuilt ballast resisitor wiring.

Cheers

[Phil Nottingham]

Mine being a Dec 72 had the thief proof coil column set up which was a heath robinson affair to only satsify legislation on production run-off so it was not very satisfactory. I converted mine to the console set up like my earlier 1969 and 1964 models.

After much debate on the old forum its was concluded, with thanks to Tom Wilson who cannot use the new Forum that the thief proof set up did not even use a ballast like the earlier P5B's (but not the sixes)

The earlier P5B's had either a separate resister at the side of the coil with starter solonoid shunt or a pink temperature sensistive ballast feed and no shunt (I think).

The part numbers for the V8 rev counters werethe same for the ballast wire/thief proof coils

[p5tgc]

I can echo the earlier comment regarding the erratic nature of these "impulse" tachos. Even when new they could show some alarming inacurate readings.

I remember a blast along the M4 in my then bosses Ford Capri 3000E in the early '70s, when going east the rev counter read almost half what it should according to the speedo, yet on the return trip to Bristol the readings seemed OK.

On the other hand, maybe it was the speedo under reading, as I did get from Bristol to Brentwood in just over 2 hours!

[dorsetflyer]

Trevor I very much doubt if your speedo was under-reading. Having done nearly the same journey from Bristol to home in just under two hours on the M4 several times, I know what speed you must have been travelling at. Thank god speed cameras hadn't been thought of then. I was driving a Rover 216 Vitesse back then, and one passenger remarked that it went like s..t off a s....l
Strange you never ever see them up for sale these days.