OK, let me preface this by saying I have read through all the threads on the subject and still I’m scratching my head.
Let me explain, I did a full mechanical rebuild on a 1970 S2 OTS and rebuilt the distributor using standard P&C’s using Advanced Distributor. There were new electrical engine harnesses run for the engine and front part of the car. There is a new coil that has been tested to 1.5 ohms.
The tack does register but never gets above 1200 rpms and is obviously reading incorrectly. The tachometer worked perfectly before the mechanical work began but that was roughly five years ago.
I’m trying to eliminate wiring issues as a potential cause of this Tach problem. FYI - I tested the continuity of the white and black wire from the negative side of the coil to the distributor and it’s fine.
Please take a look at the following pics and tell me if you see something wildly wrong. Thanks!
It’s a little hard to tell the difference in your photos of white-blue versus white-grey but in general, the wire leading to the tach would be connected to the (-) terminal of your coil. I only see one wire attached to the negative terminal.
Correct Harvey - only the blk/WHT wire to Neg on the coil. See below.
There is a 2 wire harness with Blk/WHT and WHT wires. The WHT wire to the ballast and the Blk/WHT to the NEG side of the coil.
The Blu/WHT wire runs to the POS side of the coil. I believe the other end of that blu/WHT runs to the 33231 relay (though I don’t know where that relay is or I’d test continuity)
here are a couple links you may find helpful.
I suspect your 70 model has the one transistor version.
If you can solder you can swap the capacitor very easily yourself.
The capacitor would be my guess, sounds like your wiring is correct.
I agree that you have it wired per the wiring diagram you have posted. But that wiring diagram makes no sense to me. I just googled “which side of the coil is the tach connected to” and the generic answer is the negative side. Maybe Jaguar did it different but to me, the tach is just going to see 12 volts, reduced by the ballast resistance. You got me!
I got you Harvey! That schematic and the wiring were widely discussed and that map was sited as being correct in a good many threads… Believe me, I read them all! Thanks for the help!
I’m going to guess there is enough of a voltage variation on the + side of the coil, that it provides a suitable signal to the tach that is converted to frequency and thus rpm. I’ve just never seen it done that way. Interesting!
Most tachometers work on voltage pulses from the point side of the coil. Not on Jaguar. The series II tachometer works on the current pulses on the hot side of the coil. The tachometer has an internal current transformer that converts the pulses to a voltage to drive the movement. The more pulses/second the more signal to drive the needle.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
13
Nuts. That would mean, of course, that the coil must be precisely to spec or the tach won’t read right.
Not so, It’s the frequency of the current pulses that matters, not the current amplitude. I calibrated mine on my electronics bench. I found it to be insensitive to the current amplitude as long as you had enough to trip it.
And lets be honest, measuring the current is likely better than the point voltage, less detuning of the tank circuit with a stray wire running up to the dash, and much less generation of noise. Having several hundred volt spikes running around the car makes for lots of EMI. Much easier to keep ignition noise out of those old AM radios this way.
Update: Just left Palo Alto Speedometer. Walked in and the owner asked if I wanted the tach checked out after explaining the issue. He walked in back and came back 3 mins later to tell me it’s fine. Perfect! Oh wait. Now I’m back to the wiring.
He mentioned that he had to swap the white wires to get it to read correctly. Which leads me to believe the mechanic probably swapped the white wires in the tach plug when the new harness went in.
So I swapped the position of the white wires in the Tach 3-prong plug and the tach is perfect! Luckily Palo Alto Speedo mentioned having to swap the wires in passing. Otherwise I would have been hunting forever
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
18
Yes but there are no corresponding rings on the harness side to indicate a match. There was a small yellow ring on one of the white wires which I didn’t understand because the color didn’t match anything on the tach side plug so just disregarded it… Not sure how they got switched but they did. I swapped the polarity at the tach plug to match the harness side and it worked immediately! Note :took me a few minutes to figure out how to release the wires from the plugs (very small jewelers screwdriver) but once I figured that out it was simple.
Tach works fine now. Palo Alto was super helpful.
For reference, the Tach read extremely low but worked . Wouldn’t get much over 1200 rpms but again was operating to some degree.
Side note: While I had the tach out t decided to clean it up. I cut the gasket off the bezel and took the glass off to clean. It was terribly dirty. I did not realize the tach bezel just twisted off. No bending of prongs or anything. Good design. I waxed and then sealed the inside glass so it wouldn’t fog and reassembled. Which brings me to a question. How do you replace that soft rubber gasket? Do they sell replacements?
Tach and several other gauges were not working in my '68 FHC when I bought it in 2000. I sent them all to Palo Alto Speedometer back then and they are all still working fine. I have never had any problems with the Tach working with Pertronix either.
Thanks for the help!