Rev counter not accurate

[Guest]

Here is the problem.
The rev counter in my car is working but @ 80 mph in 5th gear the reading is 2000 RPM??!!
My Bentley manual says this should be about 2800 to 3000?? in 5th gear @80mph
I have followed another car to check my speed and the speedo is accurate.
Since buying the car 3 rears ago this condition has not changed.
I have pushed the car in lower gears and the highest reading on the tacho is around 4000 RPM.
Also I have noticed that when cranking te engine on start up the tacho needle oscilates from zero to 2000 RPM but once the engine starts the needle settles to a tick over of around 800 RPM when the engine is warm.
The performance of the engine is faultless and the mileage is 108K.

This was always on my list of things to sort out and it has now come to the top of the pile

So guys has anybody had a similar issue? and if so was the solution expensive??

Cheers
John

 

[bones]

Hi John, I recently drove for six months without a working speedo and it was a blast, I couldn't drive the 911 for six seconds without a fully operational tacho, get that thing fixed it's spoiling your enjoyment of the car.
It sounds like the tacho may well be toast and you can either replace it, probably get one at www.porsch-apart.co.uk or a similar place or repair it, (very costly). If you live local to a breakers ask them if you can pop by and try one out to make sure the dizzy isn't causing the problem. The clock is removed by pulling it out of the dash with your finger nails behind the rubber grommet you can just see. You can use a suitable instrument but be very careful or you will mark the dash in a flash. Fingers are better. Write down the wiring order.
Before you do all that You can do some simple checks to ensure you have a good 12volts to the tacho, if you have ever noticed the tacho swing from 0-infinity when you twist the key that is a sure sign of low voltage to the tacho. Make sure the battery is fully charged and serviceable and the contacts clean, check the connector with the single green wire that clips onto the distributor in the engine bay, waggle it around at tickover and see if you get any fluctuations , if so investigate the connectors there. If not repair or replace the tacho soon.

 

[Guest]

The clock is removed by pulling it out of the dash with your finger nails behind the rubber grommet you can just see.

Finger nails indeed !

The best way is the use the blade of a flat-bladed screwdriver, under the metal lip, with a large flat piece of plastic (or similar), between the shaft of the screwdriver and the dashboard, acting as a pivot. Moving the blade around the edge of the instument lets you easy it out slowly, and with no damage.

The only time that I get an oscillating speedo, is when the alternator is on the way out.
(Really amusing to watch the AA man, when it last happened, ask me where the alternator was after a 5 minute search. When I told him, he said that someone else could fix it !).

 

[Guest]

John,

I'm from the Government and I'm here to help []
(see John Bellringer's sig for full details)

There's an SC Tacho on eBay at the minute - click here

 

[bones]

John I'm a guitarist [] I think the post is about the tacho error not the speedo.

 

[Guest]

I think the post is about the tacho error not the speedo.

Humph ! Picky, or what ?

Notwithstanding, the same applies to all the instruments (he says, trying to recover from the fact that I read your bit about speedo, after reading the original).

 

[bones]

Brilliant, just heard an old friend died, so I was feeling a bit mis' till I read that, thanks John. []

 

[Guest]

Hi Bones,
Thanks for the tips especially the wire connection on the distributor I will check this. the needle does oscilate when cranking up to 2000 position and back to zero.
Looking at the rev counter removal it looks like the steering wheel drive shaft covers will need removing too before the tacho can come out? any comments.
Finger nails ( all 8 are available) will be tried first as I don't want to mark the facia.

Cheers
John

 

[Guest]

Hi John,
Thanks for the tip, will investigate.
Cheers
John

 

[Guest]

Looking at the rev counter removal it looks like the steering wheel drive shaft covers will need removing too before the tacho can come out? any comments.

You don't need to remove any part of the steering, but it is a tight fit as it comes out.

Mine are sufficiently tight fitting, that I would end up removing my fingernails if I tried that; the screwdriver/lever idea is much easier, as it makes the instrument more visible (no hands in the way, and not too much room behind the steering wheel), and it is easier to use/control, enabling gentle levering around the bottom half of the cowl.

When mine all came out (different dial colours), I made a Visio diagram of all the connections to all the instruments, complete with colour coding etc. Well, I was a bit bored at work one day - and it has proved useful since.

 

[bones]

Agree with John, once you've freed up the tacho tilt it as you pull slowly towards you, no need to remove anything else.

 

[Guest]

To Bones and all the guys.
Thanks for all your tips will let you all know how I get on.
Cheers for now.
PS
I am a guitarist too Gibson ES 335 TD to AC 30 but thats another story.

 

[Guest]

To all you helpful guy's.
I have just received my replacement Tacho from Porsche Apart. £60+VAT+postage I thought this was OK.
These guy's are very helpful. Steve at Porsche apart helped me check out the serial # to make sure the rev counter was the same model type etc.
For clarification the serial # is between the 3 and 4 on the rev counter (@ ocklock) but you can only see it viewed with your head on the drivers seat looking vertically upwards.
All numbers have checked out OK so will have a go at fitting it asap.
Thanks Guy's
John
Rev counter now fitted, it took all of 45 minutes thanks to all the tips from everybody.
Rev counter noe reading 3250 @ 80 MPH in 5th gear. Re checking with my 1983 service manual and this figure is where it ought to be.
Bones was correct the enjoyment factor has gone up, it seems that I have been changing gear to quick and not using all the power.

Thanks guys
John