Several years ago I installed a Pertronix electronic ignition module in my Lucas distributor for my 68 E Type OTS. Installation went well, engine started and performed well. Still runs great! However, you probably know where this is headed. Yep, my OEM Smiths tachometer displayed incorrect — about 2X scale. I chose to address it another day.
That day has come. I’m checking in to see if there is any new info available related to making mods to remedy the problem. Previously, there seemed to be all kinds of opinions out there — but few if any yielded any conclusive results.
NOTE: I’ve installed a temporary VDO Tach where the clocked was. (No mods to the panel for the purists out there). This tach provides an amazingly accurate measurement of RPM as compared to my dwell/tach analyzer. I’ll retain it until I can get the Smiths calibrated.
Any thoughts would be appreciated. FYI: I tried the 10 ohm resistor on the coil side Tach sensor. Engine wouldn’t start.
Only identification on the tach that might indicate model is a White sticker on back with “058” on it. Also screen printed on the face, bottom edge is “RV 4612 / 00”
Jim check that you have a white feed wire coming from your
igniton switch to your tacho then onto your coil +ve.This is your
ignition feedThen check if the white wire at the tacho is external
or internal.If external on the back of the tacho it should have
two loops.Remove one of these loops keeping the same direction .If
the white wire goes internal open the tacho and remove one of the
loops around the pick up. Once again observe the direction.Also
Bypass the ballast resistor mounted beside the coil .
Yes. The White wire loops twice through the sensor. The sensor is external. Both ends of the wire enter a jack near the ignition switch. One of the wires has a small Black band on it near the jack. I’ll assume this identifies the wire as either ignition or coil.
I’m also pretty certain this tach and wiring has never been tampered with.
Your wiring appears to be a little different to my 69 but it appears to be doing the same job.You have nothing to lose by removing one loop from the sensor ,you can always reloop it…If your tacho is out of the dash when testing be sure the body is earthed JOHN
I have the same issue after installing a 123 electronic dizzy, the RPM shows about double the engine revs. I tried to reduce one loop at the tacho, but it did not change anything.
John, thanks for your suggestion. I removed the loop. The wire now passes straight through the sensor, secured in the clip with an additional short segment (.5") of wire. This action reduced the displayed RPM somewhat, but not nearly enough for the fine tune adjustment to have any significant effect.
Jim, did you try connecting a capacitor from the + lead of the coil to ground? I experimented with 3 different values, 0.47uF, 0.68uF and 1.0uF caps @ 600V. The 0.68uF to 1.0uF values worked best and settled the tach down so I left the highest value in place. The 1.0uF.
I replaced the points in my Series 3 V12 with a Pertronix unit. Adding a 1K OHM resistor in line with the negative lead to the coil will result in a normal reading of your tach. Regards, Larry
I’d like to point out that the OP has a 6cyl engine. Not being that familiar with the V12 ignition, so it’s likely your experience might not apply to him. Although the 1K to the negative lead of your coil sounds a bit odd to me. Are you saying you have the black lead (neg) of the Pertronix going through a 1K resistor to the negative lead of the ignition coil?
Hi Tony, That is correct. The tach receives its signal from the negative pole of the coil. The resistor is necessary to correct the signal. I believe the Pertronix unit is similar for both units but, according to Terry’s Jags, the V12 requires the resistor, actually the value I mentioned should actually read 10K OHMs to have the tach read correctly, poor typing on my part. I do notice the price for the Pertronix unit is almost 50% higher than the six cylinder model so there probably is a difference. I called Pertronix directly and they were very helpful so that is probably the best bet if problems are encountered.
I have the same question as Tony, i.e., does the resistor go inline between the Pertronix unit and the coil? Or, between the coil and tach? Also, what was the wattage of the 10K resistor you used?
