Jumpy tacho needle

[Noel Mclaughlin]

Have recently purchased a Caterham S3 ford Xflow 1600 motor, and I am having great difficulty in curing a tacho that has a jumpy reading from idle to around 3000rpm or so. Instrument has been checked and tested by an instrument repairer and fault is said to be in the cars ignition wave form. On an oscilloscope it produces a "negative bounce" and this is the reason for the tacho prob(so I am told) Have fitted new points,condensor, coil, have fitted a resistor into pulse wire, with only marginal improvement. Has anyone a suggestion apart from a piece of cardboard taped over dial as one wit has suggested?

[StewartG]

Noel, I wonder if you're having the same problem as me. I would descibe it as a lack of damping causing the needle to bounce around the correct reading before settling. I had wondered if the tacho earth via the loom was intermittent as earthing seems to get blamed for all manner of se7en electrical ills but running a new earth had no effect. My problem also seems to come and go, with the tacho working perfectly sometimes and I'm coming to the conclusion that the tacho itself is faulty internally. I should also add that mine's running directly from the coil which is being driven by a new weber alpha ignition unit so I don't think the signal the tacho's getting has any funnies.

I suspect a new tacho will cure the problem and only costs about 75UKP but this present it's own problems as I think Caterham have changed the design slightly and a new one won't match my speedo and I'm not hopeful of the new one lasting that long, this seems a commonish problem.

A lovely new Stack one will do the job and be reliable but they're getting on for 200UKP

Last option is a rebuild by Speedy Cables in Islington at 50UKP, they aparently uprate the internal driver circuitry as the original is pretty crude and this cures the problem.

There is one more option. A little birdy told me that a Skoda tacho fits straight in and works brilliantly.If you can find one in a scrap yard the cost would probably be next to nothing.

[Pete]

Not sure if this will help but the innards of old Smiths tachos are not compatible with some types of electronic ignition such as Luminition (guess how I found out).

Speedy Cables will rebuild the unit with suitable modifications if you tell them which system you are using.

 

regards

 

Pete

[Super_Rich_Bernie]

I'll put in a good word for Speedy Cables. Excellent service, good prices and a quick turnaround.

 

Sorted out the same problem with the unit from a Lancia Fulvia I had.

 

 

Jonathan

 

 

[Noel Mclaughlin]

Thanks for your suggestions guys especially the one about Skoda Tachs. In Australia Skodas are so revered that wreckers

or breakers as you call them dissolve into tears of mirth at the mere mention of one. On a more serious note I have today CURED a problem that this car has had for years. By fitting an optically triggered ignition (Newtronic Pirahna) I have cured the tacho. The points and condensor I put in 900 kms ago were showing evidence of high voltage across them. Don't know if there is a more sinister reason for this or I got dodgy parts supplied to me