Another Tacho bounce problem

[stuartgb100]

Must be my time for problems...

Sprint is fitted with a rebuilt tacho (by Speedycables) and has been fine, while running with copper-cored plug/coil leads and non-resistor plug caps (straight through --- not the 90 degree angle version).

Have just changed to new copper leads, and now with new NGK 90 degree angled plug caps (marked XB05F 5k ohm). Also changed the coil at the same time to a 'gold'.

Now tach bounces and reads low.
When up to temperature, it will now tick over at 400rpm !!!!!

Perhaps I should swap the tacho and leads to someone who's got a 'racing' tacho problem ?

Seriously though, any ideas?

In the meantime I'll remake the old leads and swap them out as a test.

Regards,
Stuart.

[Frank Howard]

Stuart,

I don't think the plugs or leads have much to do with the tach. What did the rebuilder do to it? Did he rebuild it, or convert it? Did he give you any special instructions as to how to hook it up? What kind of ignition system are you running?

[stuartgb100]

Hi Frank,

The tacho has been fine for a year or more, since the rebuild. May
be a coincidence, but this behaviour occurred after changing the leads and coil.

Regards,
Stuart.

[riverkeeper]

from memory I believe the coil is somehwere involved in the circuit to the tacho. As it's not much of a job I'd pop the old coil back on first just to see if that makes any difference. Likewise can't see the leads causing this...?

W.

[Frank Howard]

Stuart,

If you want good advice, then you have to answer the questions.

1. What kind if ignition are you running?
2. Did the rebuilder convert the tach?

The advice I give you will depend on your answers.

[stuartgb100]

Sorry Frank,

The tacho was converted to run with the conventional coil/points set up that I run with, but they also made it ok for use with electronic ( if I wish to go that way in the future).

Have just swapped out the new coil back to the previous one, and also to a coil from the PO's box of spare tricks .... no difference, just bounces around. When it settles, then at tickover it's showing 400 instead of 800 rpm.

Swapped back to my original leads and plug caps, and all is well again.

???

Regards,
Stuart.

[mikealdren]

Hard to imagine how the HT leads could interfere with the 12V supply line. Are the leads fully supressed to prevent RFI? Do you get radio interference with the new leads?
I have seen copper cored cables without proper suppression caps on the end. You could try putting a supression capacitor on the 12V side.
Are the leads running along the same path as the old ones and are they close to the tacho leads?
Just a few thoughts.
Good luck
Mike.

[stuartgb100]

Mike,

The new leads and plug caps are routed the same as the old were.
Both options give absolutely horrendous radio interference - if I was interested.

The original caps were not suppressed, the new NGK are suppressed.

I have not added nor removed any capacitors.

Regards,
Stuart.

[Frank Howard]

The tacho was converted to run with the conventional coil/points set up that I run with, but they also made it ok for use with electronic.

Stuart,

Boy, that's a strange one. I thought it was either/or, but not both. If they converted it, besides the red/white illumination wires, the green 12V positive, the ground, and the white ignition wires, there should be an additional wire coming from the tach. This wire should be connected to the negative side of the coil, not the positive side. If this wire exists, check which side of the coil it is connected to. Alternatively, if you inadvertantly reversed the connections on the coil, that would cause this additional wire to be connected to the wrong side. Hope this helps.

[stuartgb100]

Frank,

Can only say that Speedycables rang to ask me if I was running with a standard points setup, or electronic.

I said standard points. They then asked if I might run electronic in the future. I said that was likely, and asked why. They said it would affect how they modified the tacho.

I repeat that it has been fine for more than a year, now, and that the problem seems tied to these new resistive plug caps. Nevertheless, I will chase down that wiring and get back to you.

BTW Speedycables seem to have a pretty good reputation over here.

Regards and thanks,
Stuart.

[stuartgb100]

Ok, wiring is as follows:

A clean (ignition-switched) feed comes from the fusebox into the tacho, and a seperate wire goes out from the tacho to the +ve LV terminal on the coil (ie to the points).

Speedycables wiring instructions (which I believe I followed) said as follows:

Yellow wire to points side of ignition coil, or input signal from electronic ignition
Red wire to 12 volt switched feed pos (+)
Black wire to 12 volt neg (-)
If vehicle is positive earth, connect red wire to chassis and black wire to switched side of ignition switch.

BTW, kept the new coil but changed back to the old leads and plug caps. Just completed 100+ mile run without any tacho bounce.

Regards,
Stuart.

[Frank Howard]

Yellow wire to points side of ignition coil, or input signal from electronic ignition.

Stuart,

That's the extra wire I was talking about. Make sure the points side of the coil is the negative side. If not, the wires connected to the coil could be reversed. The yellow wire is needed for electronic ignition only. As you have conventional points, try disconnecting the yellow wire from the coil and see what happens. Good luck.