I just got my '73 S3 out of winter storage and the tach has suddenly decided to act up. Erratic readings from 0 to actual on and off and at times not at all. I installed a Crane Fireball SR7000 Ignition and Lucas Sport coil many years ago and it’s all been working fine since then. Can’t see or find any obvious loose connections etc. Car seems to run well. Any ideas what might be causing this?
Check that all the alternator cables are making good contact on their spade connectors.
Had the same problem - a bit of fiddling fixed it.
Thanks Ian - bit slow responding as I’ve been busy. Did as you suggested and no better. Actually now Tach doesn’t work at all. Am off on a road trip tomorrow, without the tach! Will dig deeper when I return.
Assuming the configuration of the V12 tach is similar to the 6, there are capacitors in the unit (three, in the case of the 6 cylinder tach) that may go bad and produce the issues you describe.
The tach on my Series 3 has quit after installing a Pertronix ignitor. Could someone confirm if the tach wire is connected to the positive pole of the coil?
After tracing the tach wire from the firewall, the wire is not white with a spiral blue stripe, it is a white wire with a plastic collar when it enters or leaves a plugin junction. This wire is connected to the Negative pole of the coil. My car is an early 71 S3 so yours might actually have a spiral blue stripe on a white wire! I installed a 100 ohm resister in line as per a suggestion by Terry’s Jags but accuracy of tach yet to be determined.
Ok - update on my S3 and Tach problems. My Tach wire connects to the -ve terminal of the coil, but my upgrade is to Crane not Pertronix. I just got back from a 3000 mile road trip in the Jag (ABFM in Vancouver, hence the response delay). Tach started the trip jumping up and down or not working, then about 100miles in, started working and continued to do so for the whole trip! Guess the 6 month winter hibernation gummed things up and it just needed to run. Now if I can just figure out the source of that new bumping noise somewhere around the rear end…
bumping or thumping? Could be a few things but do check out your four halfshaft universals. If one of those babies lets go at speed you’re in for a bad time.
Got under there this morning - looks like it’s the rear transmission mounting “bushings” not much left and what’s there is pretty mushy. Thankfully nothing worse.