I should leave well-enough alone

[archigator]

Okay, although my Elan was running like a scalded cat, I couldn't leave well-enough alone. My original Smiths RVI tach had a fried internal loop (don't ask...) and the needle did not move, but it worked well enough to provide the proper circuit to fire the engine.

While re-wiring my electric window circuits, I thought, "What better time to change-out my old RVI tach with a new RVC tach that I had purchased months ago." I replaced the tach with the new RVC tach using the instructions (see below) that came with it. I ended up with a green wire that was attached to my former RVI tach, now dangling uselessly under my dash... that location having now been usurped by a ground (black) wire.

I turn the key... the ignition light comes on... the engine turns over... and a little curl of white smoke emanates from the crash pad grill! In a panic, I pull the battery disconnect and then inspect the wires behind the dash... no fried wires evident. I re-attach the battery circuit... but the car won't start. Did I fry the tach? HELP!

Gary
'71 Sprint FHC
Miami, Florida

[archigator]

Wow. Going on 180 downloads of the photos and no suggestions... that's not good.

Gary
'71 Sprint FHC
Miami, Florida

[RogerFrench]

It's a bit hard to see, but it seems that the green wire and the brown/yellow wire in your RVI photo are for the ignition warning light and have nothing to do with the Tach wiring per se. That green wire should come live when the ignition is switched on. By substituting a ground (earth) wire you have grounded output from your alternator. That's not good. The red wire I take to be the instrument light, there's a blue wire - high beam? I can see the white in and out wire but cannot make out the other. There should be a ground (black) connected to the terminal in the middle of the casing that seems to have nothing on it.

What did you do with the white wires from your RVI? Typically you can plug them together, as they feed the coil.

How did you arrange power to the RVC tach? Or sensor from the coil? If my suppositions are right, and your green wire is still working as it should, I'd suggest you put it back where it should be, and use it also to power the tach. Run a separate sensor wire from negative side of coil to tach, if you haven't already done so. Run a ground wire to the RVC tach casing. Then try again. If it doesn't work take it to someone who knows what they're doing.

[collins_dan]

I would put the old tach back in to see if the problem is within the new tach or the wiring. From the wiring diagram, the green wire looks to be a power source, so not using it shouldn't be a problem. Did you notice if the new tach moved when you started the car? Dan

[archigator]

it seems that the green wire and the brown/yellow wire in your RVI photo are for the ignition warning light and have nothing to do with the Tach wiring per se.

The red wire I take to be the instrument light, there's a blue wire - high beam?

That's correct.
The posted photograph shows my original RVI tach, wired correctly. The car would start in this configuration, although the RVI tach internals were slightly fried and the tach needle did not move. (That's why I decided to replace it.)

When the RVC tach is installed with the configuration shown in the photo (the original RVI configuration), I blow a fuse in the fuse box, and the engine does not start. When I change the configuration as instructed to do in the sketch that came with the RVC tach (ie swap-out the green (power) wire with a ground (black) wire... the engine does not start, but I do not blow a fuse. I have an e-mail into the RVC manufacturer for his input too, but no answer so far. In every configuration, the tach does not flinch when I turn-over the engine by turning the key.

UPDATE: I just disassembled the RVC tach, and everything internally appears normal... no melted wires, scorched case internals, etc. I assume the RVC tach is still operational.

Thank you; I very much appreciate your input!

Gary
'71Sprint FHC
Miami, Florida

[collins_dan]

I did some google searching and your diagram sounds right. Sensing is from the negative side of the coil with RVC tachs. It makes me wonder though. If the sensing was coming from the positive side of the coil via the ignition switch with the RVI set up, maybe the ignition switch needs to be rewired differently. It would seem like it is getting input from both the positive and negative sides of the coil. Good luck. Wish I could be more help. Dan

[Frank Howard]

Gary,

The ignition light has two wires. A brown/yellow one to the alternator and a white one to the ignition switch. I don't know why your white wire is green and that may be adding to the confusion. The green wire (and if you had only one green wire, you couldn't connect the wrong one) is supposed to power the RVC tach just as it powered the RVI tach. I don't agree with the instructions provided with your RVC tach. I believe the green wire (not the green one that goes from the ignition switch to the ignition light which should be white) should be connected where is says "To Ignition switch". The RVC tach should be grounded with the same black wire that grounded the RVI tach. The two white wires that used to form a loop inside the old RVI tach should be connected together and here is the most important part. An additional wire (pick a new color, mine is orange) should go from the new RVC tach where is says "To negative side of coil" to you got it.......the negative side of the coil. Let us all know if this solves your problem and if it does, you now owe me two beers.

[archigator]

Frank, I was hoping to hear from you, since you resolved my electrical/tach issue last time!
So the two white wires (one goes to the ignition switch, and the other to the negative side of the coil should now be joined together, independent of the tach altogether.) Another wire of a color of my own choosing should run from the tach to the negative side of the coil, my green wire now attaches where it says "to ignition" and the ground wire goes to the third terminal as shown on the mftr's. drawing... I think. (My original RVI tach was grounded at the two threaded stems.)

I'll try that solution, and let you know what happens.

I'll gladly buy you two beers, regardless!

Gary
'71 Sprint FHC
Miami, Florida

[memini55]

Gary,
With no tach connected and no change to any other wiring if you plug the two white wires together the motor should crank and start. The green wire that was connected to the old tach is a 12 volt power wire feed from the fuse box and would supply power when the ignition switch is turned on via the fuse box.
So I would trace the white wire which goes out to the coil and see if that is connected to the negitive side or positive side of the coil. On your new tach it shows it connected to the negative side. The other white wire should be coming from the ignition swtch as switched power and I would take a volt meter and measure that to ground, +12-14 volts depending on the battery condition.
In your picture I see a black wire floating with an eyelet connector which would be a normal ground that was more than likely tightened under one of the thumb screws that secured the tach in the dash. If you have an ohm meter I would measure that ground to the negative of the battery. Is thats the ground wire for the new tach and was it connected to the ground tab or spade?
Good luck
Mark

[RogerFrench]

Thank you Frank, that is pretty much what I was trying to say too, though the white wire should not connect to the ignition warning light, that is the job of the green wire and replacing that with a ground is heading for trouble. It's green because that is the right colour according to every wiring standard this old Englishman knows. I'm away from home so can't verify the Elan wiring diagram but I'll bet those 2 beers I'm right!
Gary, you seem to have it understood right, though if your car is negative earth, as is implied by its year and the diagram of your RVC tach, then the white wire, now plugged together as one wire, should not touch the tach but go from ignition switch to the positive side of the coil, and the extra sensor wire from the negative side of the coil back to the tach.

There should have been 2 green connections around the old tach, one to the warning light, the other to the tach itself. Your old tach didn't work and without asking why not, I can safely say it wouldn't without a power supply! Perhaps you removed it when it fried?

[archigator]

"Okay, the pellet with the poison's in the vessle with the pestle, the flagond with the dragon has the brew that is true... uh, I mean the chalice from the palace has the pestle with the dragon... no that's not right..." (with apologies to Danny Kay)
http://www.youtube.com/watch?v=CetQrxFp4XI
Thanks everyone, I think I'm getting there...

1. (Yes, my car is negative earth.)
2. The wiring to the ignition light stays wired as-is.
3. The two white wires that formed the sensing loop are now to be joined, independent of the tach altogether... however the white wire that went to the negative side of the coil previously, now should run to the positive pole of the coil. (please confirm that one)
4. A totally new wire should run from the negative terminal of the coil to the coil input terminal of the tach, as noted in the sketch.
5. The green power wire should run to the "to switched ignition" input terminal of the tach, as noted in the sketch.
6. A black ground wire should run to the "ground" terminal of the tach, as noted in the sketch.

Correct? If so, I'll get to it in the next few days and report back.

(I fried my RVI tach after working on it, not securing it back fully into the dash, taking my Elan for a quick test spin, the tach came free of the dash, wires touched ground that shouldn't have, the whole thing shorted out, smoke filled the glass on the gauge, the driver let out a few choice expletives, and the car died... there, I've said it!)

Gary
'71 Sprint FHC
Miami, Florida

[collins_dan]

My bad. I thought the white wire went to the positive side of the coil, but looking at the wiring diagram, I looks like it goes to the negative side. I can't look at my car to confirm, but you should be able to on your car, which is really the one that matters. I think it is as simple as Frank stated. Connect the white wires together, connect the green that was hanging loose to the connection to the ignition switch, run a new wire to the negative side of the coil, use the black ground wire the same as with the rvi tach by connecting it under the thumb nut that holds the tach tight to the dash. Start it up and get some joy (hopefully). Dan

[RogerFrench]

Gary - Yes, but it occurs to me I should make one thing clear. Your car is negative earth, so the negative side of the coil should also have the wire that goes to the distributor connected to it. That's where the RVC tach picks up the signal from.

[archigator]

Your car is negative earth, so the negative side of the coil should also have the wire that goes to the distributor connected to it.

Thanks, yes there is a wire from the negative terminal of the coil to the distributor.

I believe I am also now to have a wire from the neg. terminal of the coil to the tach, and another (third), white, wire from the ignition switch to the negative side of the coil (aka: the two white wires that formerly made the loop in the tach.) A total of 3 wires frrom the neghative side of the coil. Correct?

Thanks!

Gary
'71 Sprint FHC
Miami, Florida

[Frank Howard]

Gary,

No, no, no. The two white wires that formerly made the loop in the tach are connected together to make one wire. That wire goes to the positive side of the coil. That's where it was when it left the factory and it should stay there regardless as to which tach you are using. To be absolutely correct, on your car, the white wire actually makes a couple of stops before it gets to the positive side of the coil. First, it goes to the anti-theft switch in the glove box. Then it goes to the ballast resistor near the coil. Finally, it goes to the positive side of the coil. It is possible the the P.O. removed the anti-theft switch and/or the ballast resistor.

Dan. I appreciate your help, but you may want to take another look at the wiring diagram. It clearly shows that the white wire goes to the positive side of the coil, not the negative side. I think that may have been what got Gary a little confused.