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.
Cheers, Ian.
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.
Take Care
Bob Burnyeat
So it’s been some time since this was discussed. Tach was non-functional most of summer and car is now away for winter and I’m working on it. I have the Tach out. Checked it for function on another car and nothing so am assuming it’s not a break in wires etc.
It has the Crane XR700 system with a Lucas sport coil and has been working as connected for a number of years. Not sure what’s wrong now but am ready to send it out to get looked at.
Any thoughts/advice?
Bob,
Several year ago studied the Tach and had help from Mark Olsen. Give www.Accutach.com
a look. There you will find a brief description, detailed schematic and testing procedures to evaluate your tach. Mark also sells a nice little test box which can be run from a laptop computer. Found my tach was accurate to 5,000 rpm; above 5,000 rpm the tach was inaccurate which could lead to over-revving the engine. Also, there is a Texas Instruments IC chip - MIC7 /C which, if it goes bad, is not available. TI also won’t release the working of the chip either!
Your tach is overr 40 years old so expect capacitors, etc. to fail. Best recourse, send it to a recommended tach repair shop and have the old Smiths circuitry replace. Money well spent IMHO.
There is also a ground within the tach that fails. That may be your culprit, especially given the intermittent operation.
Please post your investigation results.
Happy Trails,
Dick Wells
The generic replacement for the chip in the S3 Tach is SAK215. Identical circuit & pinout. Lots of info available for tach driver circuits using this chip on the net. Google SAK215 and you will see that they can be had for a few dollars.
HTH,
Steve D.
Thanks for all the info. This sort of electrical is beyond my abilities so have shipped it off to a fellow here in Canada that has fixed gauges for me before. I’ll be sure to ask what he finds and pass along.
Take Care
Bob Burnyeat
Hi Steve,
I appreciate the info on the SAK215 IC. Where did you obtain the original logic data for the Texas Instrument MIC7/C chip to compare with the SAK215? These old steam gauges aren’t getting any younger so your information is quite useful. Contacted Texas Instruments (UK) and they weren’t helpful at all reqgarding the MIC7/C chip. Proprietary information was their reply.
Again, thanks for the update. Electronics wasn’t my strong suite so info like yours is very helpful in maintaining 40+ year old steam gauges…
Best Regards and Happy Trails,
Dick W.
Ok all here’s an update on my tach issues.
In a nutshell it turned out to be the ballast resistor that was the culprit.
Had the tach checked by Oliver Bienz here in Canada and it was fine. Put it back in car expecting to see it run and I couldn’t get the car to fire!. Did a lot of testing checking etc and ultimately as per Oliver’s suggestion, bypassed the ballast resistor on the white/blue tach wire (so connected it directly to the –ve side of coil) and voila, problem solved. Car ran again and tach is working.
Can’t say what happened to the ballast resistor, but mine sits under the coil and came mounted at the front of the engine between the cylinder heads, so it gets pretty hot in there and that may have been the culprit. But after 46 years and 47k miles it’s not unreasonable to expect some failures.
Thanks again for all the advice.
Took the top crash roll off to get at the tach, so will recover that and reinstall. Just need to snow and cold to disappear now…
Bob Burnyeat, SAA (Retired), FRAIC, LEED AP
Home 306.477.2634
Cell 306.222.9317
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