Dancing Tacho

[the dodger]

After changing my plug leads to Magnecor and new heatshield adaptors, and replacing the dizzy cap, rotor arm and coil my tacho has started playing up. It's got very erratic and doesn't show a steady reading, getting worse up the rev range. Stranger still is that when travelling around 70mph (tacho dancing between 2200 - 2600) and I lift off the tacho increases to about 3000 - 3200 momentarily. It shows this increase of about 500rpm if I lift off at any speed. Then it settles to it's normal erractic reading.

Any ideas peeps?

[crimsonchim]

the dodger said:

After changing my plug leads to Magnecor and new heatshield adaptors, and replacing the dizzy cap, rotor arm and coil my tacho has started playing up. It's got very erratic and doesn't show a steady reading, getting worse up the rev range. Stranger still is that when travelling around 70mph (tacho dancing between 2200 - 2600) and I lift off the tacho increases to about 3000 - 3200 momentarily. It shows this increase of about 500rpm if I lift off at any speed. Then it settles to it's normal erractic reading. Any ideas peeps?

Is it running OK?
Sounds like one of the leads/adaptors isn't quite on right.

Did you keep the old coil/dizzy cap/rotor arm?
You may want to change them back one at a time and see if it gets better, but check all the plug leads to start with.

Cheers,

Andy

[the dodger]

Yes it's running OK although it sound "different" at low rpm (misfiring? - I can't tell) - it sounds OK when accellerating. I'll check the connections but I'm pretty sure they are all on OK. The dizzy cap, arm and coil are almost new and unlikely to cause this fault.

How about the ignition module? I think this is where the tacho gets it signal Anybody know about these? - or does it come from the ECU which has been pulled out a couple of times (but not disconnected)?

[Incorrigible]

LT coil leads would be the first thing to check, unlikely to be a coincidence, but a lot of them do do that sir

[shpub]

The Tacho gets its signal from the LT side of the coil. ANything that can cause electrical interference can upset it and that can be ignition (inc the module), bad earth, duff alternator fuse and so on. It could also be a duff tacho.

Steve
www.tvrbooks.co.uk

[the dodger]

Thanks Ben/Steve. I didn't get a chance to look today as it wasn't "playing with the car weather". First job - check/clean LT connections to the coil. It still seems very strange to me that the revs rise when I take my foot off though......but I suppose a poor contact may cause this?

[nc3]

Does the coil require a supression capacitor? Is this connected? If not then the taco could be receiving too much electrical 'noise' making it difficult for it to work out the correct RPM signal. An indication might also be found with increased noise on the radio (try AM as well as FM)