johnwill Posted April 25, 2022 Author Report Share Posted April 25, 2022 On 4/16/2022 at 1:24 PM, RobH said: The wire needs to go on the terminal which connects from the coil to the distributor. That ought to be the "-" one on a negative-earth car but it is possible the coil could be connected the wrong way round. Rob, not sure if you’ve seen my additional message above, but I’d like to get your take on it. Plus, I have a little more info. Did the drive it day yesterday, and upon starting the car, and for the first 12-15 minutes of running, the tachometer bumps up to higher revs, then returns to the correct revs, then bumps up again, usually to a different number. However, after this time, it then starts to maintain the correct revs and doesn’t bump at all. When I switch off for awhile, it starts the same process again. First bumping, then settling to normal revs. I’ve read your piece on coils, but can’t spot anything that helps (although, I have little electrical knowledge, so probably lack of understanding doesn’t help ) I am assuming that this fluctuation is coming from the coil, can’t see why it could come from anything else. any ideas? (Will be talking to JDO later) John Quote Link to post Share on other sites
Lebro Posted April 25, 2022 Report Share Posted April 25, 2022 After initial starting up the battery will be being charged at quitr a high current, to replace the energy you used to start it. After a while the current will drop. I'm thinking interference of the rev counter circuits by the AC component of the nominal 12V supply. This will reduce as the current drops. You could try a capacitor over the +ve supply to the rev counter say 100 to 500 uF at 25V rating. Bob Quote Link to post Share on other sites
johnwill Posted April 25, 2022 Author Report Share Posted April 25, 2022 1 hour ago, Lebro said: After initial starting up the battery will be being charged at quitr a high current, to replace the energy you used to start it. After a while the current will drop. I'm thinking interference of the rev counter circuits by the AC component of the nominal 12V supply. This will reduce as the current drops. You could try a capacitor over the +ve supply to the rev counter say 100 to 500 uF at 25V rating. Bob Bob Would that be the constant or ignition +ve? Thanks John Quote Link to post Share on other sites
RobH Posted April 25, 2022 Report Share Posted April 25, 2022 As Bob says - the tacho may be sensitive to noise on its power supply line(s) and mis-interpret them as signal. Another thing is that the signal to the tacho is the switching pulse as the points open and close. That is not a very 'clean' waveform and it is possible that until things warm up there is excess ringing. causing the tacho to double-count. It might be useful to check the condition and setting of the points and to try a different condenser as that affects the ringing (as can the coil itself). Quote Link to post Share on other sites
ntc Posted April 25, 2022 Report Share Posted April 25, 2022 Did some 4s have a resistor wiring? Quote Link to post Share on other sites
johnwill Posted April 25, 2022 Author Report Share Posted April 25, 2022 I bought the distributor a few months ago from the “doc” so points and condenser brand new (ish). Will check them but expect them to be quality items at least. put my old coil back on, and same issue Quote Link to post Share on other sites
RobH Posted April 25, 2022 Report Share Posted April 25, 2022 If capacitor smoothing of the power lines doesn't cure it, you could try a ferrite choke on them as well. The choke should come before the capacitor, nearer the power source. That is just a bead or ring that you wrap the wire through, a couple of turns. They can be bought quite cheaply or salvaged from old computer cables or power leads which have a 'lump' in them like this; the lump being the ferrite ring usually in a plastic case which can be split apart. Quote Link to post Share on other sites
stillp Posted April 25, 2022 Report Share Posted April 25, 2022 I've been meaning to ask - why did you want to switch to an electronic tach? Pete Quote Link to post Share on other sites
johnwill Posted April 25, 2022 Author Report Share Posted April 25, 2022 12 minutes ago, stillp said: I've been meaning to ask - why did you want to switch to an electronic tach? Pete Been having issues with tachometer for a few years now, replaced cables and it helped a bit, but went again. Dropped a new cable in again, and no difference, so was going to have it repaired, but just decided to go electric which I hoped would solve all the issues in one hit. Quote Link to post Share on other sites
stillp Posted April 25, 2022 Report Share Posted April 25, 2022 37 minutes ago, johnwill said: Been having issues with tachometer for a few years now, replaced cables and it helped a bit, but went again. Dropped a new cable in again, and no difference, so was going to have it repaired, but just decided to go electric which I hoped would solve all the issues in one hit. Well, at least it's changed the issues! Good luck, hope you get it sorted. Pete Quote Link to post Share on other sites
stuart Posted April 26, 2022 Report Share Posted April 26, 2022 16 hours ago, ntc said: Did some 4s have a resistor wiring? No. Stuart. Quote Link to post Share on other sites
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