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Tach wiring problems

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Jedi Trainee
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So the car is returned from XKmotorsports with intermittent starter problems, with good help from Dave at British Starters I get the wiring correct and feeling smug I take it out for a run and see that the rev counter is not working so I go home and start to look into it, Since owning l985 I had noticed that where the two wires come up out of the loom along the firewall (just under rear SU), the two wires that go to the little generator back of cam for RPM signaled by voltage through those wires, were covered in elec. tape all the way to the spades onto the generator. However sometime years earliler a connector just before wire went into loom had come apart. Now the tach worked just fine so I let it be.
Now the tach was not working, I reconnected the open connecter and tach took to sitting at 2000 revs and moving in a herky-jerky manner when engine revved. I put a volt meter on it and that wire was carrying 12 V. With generator wires disconnected , the 12 V was still coming up the wire when voltmeter other side to ground.
Bought car from Uk car enthusiast, had auto airconditioning shop, on test drive smoke out of dash. Bought on condition he fixed it which he did. Wanted to see what might be amiss on back of tach, pulled it forward from dash and could see probable remains of dusky smoke but the electrical tape wire rap continued on the two wires all the way to the connectors on tach. Running up firewall when tape removed correct color to schematic, could not tell under dash.
I guess erratic tach caused by 12 volts. On same line of thought maybe somewhere along time someone did something to wiring that inject 12 volts into system.. Under dash each of the two wires single as along firewall.
The weird part is that tach had always worked with that one wire having a disconnected terminal along firewall untill things got moved around in gearboz-engine removal. The schematic in Haynes book has separate box for tach simply showing the connections from generator to tach and back. No other wiring involved in counting revs.

Anything I am concluding wrong and suggestions for further trouble shooting. Sure tach once worked since remember doing calculation to see if it could really go 150 mph. no way---kerblammo

As always thanks and gratitude, Jay
 
You mentioned owning the car since 1985 but what year and model is the car? Can you post a clear picture of the tach showing the text on its face?
 
two wire generator on an XK should have no voltage at all unless running and meter set to AC.
’67 Etype Rdstr.
http://xkedata.com/pdf/42wiring.pdf
Yours is last year for Type 1. Look on the .pdf, over on the right, about 1/3 of the way down, you will see a horizontal rectangular box, tach and tach generator.
No power involved.
 
Thanks Doug, it is a 67 Etype, series 1, right near end of series 1 production. Just switche from windows7 yo 10 and need a little more work to transfer photo from phone to email. yes I have Haynes manual and am aware it is a closed circuit with no voltage inputs, part of the big question mark about what is going on. Best, Jay
 
Three possibilities. Bad tach circuit (or capacitor dried out anyway). Bad transmitter. Broken, intermittent wiring.
That the needle moves at all seems to indicate the transmitter works. How well is anybodies guess at this distance.
I have a MK2, same transmitter, had the oillite bearing move, and the dog disconnect from the cam. Maybe if it's dragging you could get odd readings on the needle, but my guess is inside the tach gauge.
There are writeups on that...and apparently the caps do dry out and fail over time.

"I put a volt meter on it and that wire was carrying 12 V. With generator wires disconnected , the 12 V was still coming up the wire when voltmeter other side to ground."

That makes no sense. You cannot have 12V on the wires hen disconnected from transmitter. The transmitter generates the signal. You've got something cross-plugged somewhere to give 1V on those wires.
 
Let me get the chain of events right.
You've owned this since 1985.
Bought it from a UK car enthusiast A/C guy.
Smoke came out of the dash.
Wires for tach under bonnet covered in tape as far as the eye can see.
You connected tach generator.
You sent the car to XKs for a trans job which was non-satisfactory.
The car wouldn't start.
You had Dave at British Starters help.
You're sure the tach worked as you calculated 150MPH.
You now have 2K RPM steady with jerking.
You pulled the tach, the taped wires previously noted "as far as the eye can see" now terminate at the tach.
You have 12V on the wires from the tach to the transmitter.
There is evidence of smoke damage on back of tach.

First off, unplug wires from back of tach, see if power is on the wires at any place with them unplugged at both ends.
If not, the power is somehow feeding from inside the tach.

If power, you have a serious melt and short in wiring.

You CAN just run two new wires of sufficient length from transmitter to tach, but at some point you are going to have to open the loom and inspect/repair from back of dash to under bonnet.


Your UK car enthusiast/AC guy fixed your "smoke" issue with tape.
 
Guess I better take a writing refresher course, much of your understanding is close but no cigar. Just got back from hardware store with bag of wire and terminals, new generator arriving tomorrow, all to do exactly as you suggested, route a new tach-generator loop. thanks for the time and effort, jay
 
There is no power in those wires. If melted and taped, somebody got power to them. may have damaged the tach. make sure nothing is cross wired on the back of the tach. Like, dash lights.
 
will do, as objectionable as taping is,the fellow who did it, from where the wires exit the loom at that sort of loom elbpw underback SU they are taped all the way to the tack very neatly; disturbinbly there is other taping in the area, but how the 12V got into the wire may have to remain a mystery but I will examine k and make sure insulated from an inadvertent ground. Appreciate all the thought you have given this, look forward to arrival of new generator puttin "clean" wiring loop in.
 
Like I said at one point, you may be cutting the loom apart at least as far as the tach wires are loomed, to see if anything is melted together. Unless, the tach wires are not loomed up with the main loom.
Then you need to find out how power got there.
Hopefully a mis-plug to power at the engine end, but somebody accidently connecting 12V at the tach end is more likely.
Make sure nobody Scotch-Locked something to that harness somewhere.

You problems started with smoke out of the dash, fixed with a neat tape job, at least cosmetically.....but something is still wrong with the readings you're getting.

I think the 2K is probably base-line 12V and the erratic is the transmitter trying to overcome it.
 
I don't think the transmitter is bad...well, let me re-phrase that. There is absolutely no way the transmitter can generate a steady-state 12V on the line.
If you've got the money, why not.....but save the old one on your "spares" shelf.
 
Yahoo, new generator, new wire circuit mimics PDF and Haynes manual and all is well.The "lug" from the generator that mates with the female part on the end of the cam housing as I remembered from my rebuild with whittled generator brushes, if spun by hand would move the DC voltmeter as did the brand new one fresh out of the box from Barrett about $200. XKsmess from high torque starting motor wiring and now tach generator all sorted out and car runs fine. The lug from the XKs rebuilt generator takes great effort to turn, never could spin. Oh one last thing, first drive around block when delivered, the oil pressure gauge was pegged, for some reason XKs added enough oil to extend way up check-rod and added synthetic to my plain old oil. As always thanks for all the help.
 
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