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Wiring problems Tachometer Mustang -73


nursieee
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Hi have my first mustang and im trying to fix it, got some problems with Tachometer for a start.

Tachometer doesn't work and never did. I did check the wiring so i think i got that right. When i run the engine, I measure about 12 v in to the tachometer and 12 v out and still it don't work? Is that how it should be? According to the wiring diagram there should be some kind of resistor between the firewall and the tachometer. I cannot find, it what is it, how does it look?

Is it the tachometer that is broke or could it be something else that's wrong?

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I believe you are troubleshooting it correctly. There's two possibilities, either the tachometer has failed or the previous owner bypassed the tachometer and resistor wire.

 

The resistor is a wire under the dash, and is between the tachometer and the dash. It is pink and has something like "Do Not Cut" written on it.

 

If the tachometer has failed your best option may be to have Rocketman, a forum advertiser and supporter, rebuild and modify your tachometer so it works like a modern three-wire tachometer: https://www.rccinnovations.com/

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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I believe you are troubleshooting it correctly. There's two possibilities, either the tachometer has failed or the previous owner bypassed the tachometer and resistor wire.

 

The resistor is a wire under the dash, and is between the tachometer and the dash. It is pink and has something like "Do Not Cut" written on it.

 

If the tachometer has failed your best option may be to have Rocketman, a forum advertiser and supporter, rebuild and modify your tachometer so it works like a modern three-wire tachometer:  https://www.rccinnovations.com/

Ok i´ll look for that cable or is it possible to connect a new resistor and if so which Ohm should it be? and also dont know if this tells anything, but the lights in the gauge panel doesnt work either?

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If the engine runs and the tachometer doesn't work, either the tachometer is bad or has been bypassed. If the resistor wire is bad the engine won't run, unless it has been bypassed.

 

By the way, please go to the introduction section of the forum and introduce yourself and your car. A few pictures of your car would be nice, too.

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“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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If the engine runs and the tachometer doesn't work, either the tachometer is bad or has been bypassed. If the resistor wire is bad the engine won't run, unless it has been bypassed.

 

By the way, please go to the introduction section of the forum and introduce yourself and your car. A few pictures of your car would be nice, too.

Now there is some pics of the Mustang. 

My idea is to put a resistor between the ignition coil and the tachometer on a new cable, just to see if the tachometer works. Im not sure how many ohms it should be. I did read 1,5 ohm anyone who can confirm that?

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I have the same issue, can't get the tach to work and I know nothing about wiring, lol. I tried three different tachs and my car does have MSD ignition and I got their tach adapter and still does not work. It is a car that did not come with tach but I changed the harness from the headlights to the fuse box and all under dash wiring. Everything else works, key buzzer, seat belts and all but no tach. 

Which a tach is a pretty useless thing unless you are drag racing and use it to stage. You change gears when it quits pulling no matter what RPM. It just looks cool, lol.

Will watch you post to see if I can learn something about this. I have 6 mustangs with factory tach and one parts car with. But only two of them running now.

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When a man is in the woods and talks and no women are there is he still wrong??:P

David

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The resistor wire should be about 1.5 ohms. If it is bypassed the tach will read high until the tach dies (too much current = shortened life span). If you add more resistance the tach will read low.

The tach can fail on the electrical or mechanical side. The movement (mechanical) is normally pretty solid, but in'71 Ford switched from Bendix to Faria as a tach supplier. You can guess why... "lowest bidder".

The Faria tach movements have stacked magnets that were epoxied together and tend to dry out and fall apart. They also used plastic internals that tend to dry out and crack or break. To put it nicely Faria products were (and still are) junk.

The electrical side is the most common failure though. Depending on what wears out and fails the car may or may not run with a dead tach.

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Bob

 

Tachs, Voltmeters, Headlight kits, Wiper delays and more at

http://www.rccinnovations.com/Images/smlogo.jpg

Rocketman's Classic Cougar (and Mustang) Innovations, LLC

 

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The resistor wire should be about 1.5 ohms.  If it is bypassed the tach will read high until the tach dies (too much current = shortened life span). If you add more resistance the tach will read low.

The tach can fail on the electrical or mechanical side. The movement (mechanical) is normally pretty solid, but in'71 Ford switched from Bendix to Faria as a tach supplier. You can guess why... "lowest bidder".

The Faria tach movements have stacked magnets that were epoxied together and tend to dry out and fall apart. They also used plastic internals that tend to dry out and crack or break. To put it nicely Faria products were (and still are) junk.

The electrical side is the most common failure though. Depending on what wears out and fails the car may or may not run with a dead tach.

Ok thanks, i uncovered the harness between the firewall and the tachometer and didnt find any pink cable anywhere. So im hoping a resistor of 1,5 ohm will help.

By the way i noticed that the lights in tach house, gas gauge, or speed gauge work i.e the whole gauge house dont work and i checked them they are not broke if that is of any importance to the tach.

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Just a guess on my part, but I believe that your tachometer and underhood wiring problems are related. My guess is that the fusible link burned up and someone connected the main power feed directly to the battery side of the solenoid without finding and repairing the short circuit. This burned up the underhood wiring and any other wiring that was involved in the short circuit. This led to the wiring "repair" under the hood and the 40amp circuit breaker and hard telling what other wiring had to be replaced.

 

Whether or not the wire that powers the coil has a resistor in it will not make the tachometer function. As Bob (Rocketman) noted, not having it will lead to early failure of the tachometer.

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“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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Just a guess on my part, but I believe that your tachometer and underhood wiring problems are related. My guess is that the fusible link burned up and someone connected the main power feed directly to the battery side of the solenoid without finding and repairing the short circuit. This burned up the underhood wiring and any other wiring that was involved in the short circuit. This led to the wiring "repair" under the hood and the 40amp circuit breaker and hard telling what other wiring had to be replaced.

 

Whether or not the wire that powers the coil has a resistor in it will not make the tachometer function. As Bob (Rocketman) noted, not having it will lead to early failure of the tachometer.

ok so you mean that because they ran the tach without the resistorwire the tach died?

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Either that or the power wire to the coil was involved in the short circuit and burned up the resistor wire and the tachometer.

 

Did it look like the wiring harness under the dash had been messed with? Maybe taped up with regular black tape and not the original kind?

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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Yes one of the wires (red-green) getting towards the tach has been cut and spliced, its been cut aprox 4 inch from the plug that connects to the tach. BTW wonder if its there where the pink cable should be?

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The 'pink' resistor wire should be between the tach and the firewall.

Here's a general overview of the tach wiring:

 

Ignition switch 'on' -> tach -> resistor wire -> ignition coil (+ side).

 

I don't have the specifics on a '73 as far as what colors or where each wire is, hopefully someone else can chip in with that info.

Bob

 

Tachs, Voltmeters, Headlight kits, Wiper delays and more at

http://www.rccinnovations.com/Images/smlogo.jpg

Rocketman's Classic Cougar (and Mustang) Innovations, LLC

 

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The red/light green wire is the wire that runs from the ignition switch to the tachometer, then the pink resistor wire runs from the tachometer to the firewall, then it's back to the red/light green wire from the firewall to the coil.

 

Can you tell where the spliced in wire goes to? Was the red/light green wire cut, or just have another wire spliced/added to it? The tachometer and resistor wire both may have been bypassed.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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From the green "glove" there is an all red cable that goes to male pin on the part that goes in to the tach.

And the other one that goes from saftybox to the female part is red and green.

I.e the right one on the pic is the cutted one and the left one is the red-green one

 

Impossible to get a 1,5 Ohm resistor so i got a bag of mixed ones. Did a combo of 10 resistors and got it down to 2 Ohm (not the right value and i dont know if that is a good idea/allowed or if i still need to try it with a single resistor that is 1,5 Ohm. However the car did start) and the tach is still acting dead.

 

 

20191210-181235.jpg

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  • 2 weeks later...

Got the resistor today 1,5 ohm 0.5 watt, attached it, turned on the key and smoke came from the resistor directly. So i guess i need more than 0.5 watt.

 

The electrician didn't think/guessed that the tach had burned just because someone took away the resistor.

 

Also screwed apart the tach. Couldn't see any obvious errors.

 

Ill be back when i get the new resistor. Any tips of which watt i need would be helpful so i don't have to buy every resistor from 1-20 watt or 50???

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You need a ballast resistor made for automobiles, an electrician should have been able to figure out what the current draw is across a resistor, based on the ohms of the primary side of the coil. You probably need around 80 watts to have a little safety factor.

https://www.summitracing.com/parts/sum-g5218?rrec=true

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“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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Here’s is the page from the Scott Drake reprint of ‘73 wiring showing the tach. I hope it helps. No, I’m not qualified to answer any questions - lol. Tach is located at B-11

 

 

 

3705532-E-6-E24-4-BA2-BF4-C-973-C1-CC21-A6-B.jpg

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Thanks for the wiring. I got almost the same, only, my doesnt have the small pics that is around yours. 1 question anyhow, I cant read the letters on the small pics. Does it say somewhere any values/numbers for resistors or the tach on the small pics under the wiring diagram?

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Thanks for the wiring. I got almost the same, only, my doesnt have the small pics that is around yours. 1 question anyhow, I cant read the letters on the small pics. Does it say somewhere any values/numbers for resistors or the tach on the small pics under the wiring diagram?

 

The only info on the tach is at the bottom on the left of the column of boxes on the right. I took a close-up c-383 and it doesn’t say anything about the resistor, although it’s plainly shown in the diagram.

 

The repro of this diagram is not the greatest.

 

F8-C82720-1592-4-B00-9-B3-F-3-AAD95-F5782-C.jpg

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if you're referring to the resistance through the tachometer, between the two connection points it will be very low, probably less than 1 ohm. The circuit that runs through the tachometer just runs through a set of windings in a transformer that actually works similar to the ignition coil. As the ignition coil is grounded (points close) to load the primary windings the same thing happens to the primary winding in the transformer in the tachometer. When the ground is released (points open) the field that has been generated collapses in both the ignition coil and the tachometer transformer, the ignition coil generating a high voltage pulse and the tachometer transformer generating a low voltage pulse. This low voltage pulse is what the tachometer circuitry converts into an analog signal (voltage) that causes the tachometer needle to move.

 

If checking the continuity across the two tachometer connections yields a reading of around 1 ohm the primary winding in the coil is OK, if it shows no reading the primary winding or connections to it have failed.

 

If the primary windings in the transformer have failed the there will also be no voltage passing through the tachometer to the ignition coil and the engine may start when the ignition key is turned to start, but as soon as it returns to the Run position the engine will die.

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“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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  • 3 weeks later...

An update of the tach. and of course some ?????

 

I took apart the tach and mesured the resistors and the electrical stuff in it.

 

What happened was when i measured the resistor with my multimeter, which i did set at ohm, the tach needle went to the top. I also put 3 1,5 volt batterys  together and took some wires and put it in to the contact to feed the tach with volts and the orange pin moved about 1-2 mm.

 

My conclusion here is that the tach works. There must be some other problem than in the tach, i.e the wiring or the ignition coil?, right ?

 

So for now in waiting for some parts including the tach circuit board from the USA so there will take some weeks before I can install the tach and try my new resistor which is at 100 watts and 1,5 ohm to se if that one will get the tach to work properly.20200106-192836.jpg

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All that you verified is that the windings in the tachometer needle are intact and moves the needle. You did not verify whether or not the circuitry between the input transformer and the needle are functioning. They have to take the pulses generated by the coil (actually the points) and convert them into a steady voltage that then moves the needle. The faster the pulse, the more voltage generated and the higher the needle moves. There are likely a couple of capacitors, an inductor (choke) or two and some other components, like resistor in the circuitry. I haven't seen the inside of one of the original tachometers, so just trying to visualize what components would be required. Old capacitors are the most likely component to fail, although a voltage spike could also take out the inductors. It's also possible that the secondary side of the transformer is bad.

 

Did you ever measure the resistance between the two wires that connect to the tachometer? If the primary side of the transformer is still good that should measure around 1 ohm.

 

I'm still not sure that you need to install another resistor. Did you ever measure the resistance (ohms) between the tachometer and the ignition coil? It should measure around 1.5 ohms if there is a resistor someplace in the circuit.

 

Edit: if you do install another resistor it will get hot, so make sure it is at a location with ventilation and not likely to come into contact with other wires. The normal place for a ballast resistor is at the ignition coil.

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“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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Today i measured every single component on the picture i dont know the name of them but got some resistance in all of them (Ohm).

So that should mean that they are properly working?!

Got the new voltage regulator and the new circuitboard today, ill be back when i got it all together and we will see if the tach will work after that if not :@ ....

 

20200115-095401.jpg

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