Tach is going south

[69gmachine 15]

Well the good news is that I was able to go for a test drive and the turn signals worked great as did the speedometer. The bad news is that now the tach isn't working right.

 

Has anyone with a tach observed it as it failed? Mine now only reads up to 1100 rpm, no matter how much I rev the engine. I can't trust any of the readings from it now (i.e idle), but my biggest concern is if it's about to fail open causing my ignition to fail. I really don't want to end up on the side of the road again.

[PaleRider 12]

there is a way to bypass the tach see attached - so car does not die if tach goes out

 

http://1970mgr.org/_TachBypass.htm

[danno 128]

Yes, there is a 2 wire connector on the tach. It connects to a 2 wire connector behind the tach that goes to the +12 volts and the ignition coil. It the tach fails, put a shorting wire between the 2 connections on the connector that the tach attaches to. This will allow the motor to run.

 

The tach in these cars is really just a current meter. It measures the amount of amps going to the coil. The faster the engine is going, the more amps it needs. The odds are your problem is mechanical with the tach. That is a very delicate mechanism that allows it to work. If you want to take it apart very carefully, you might see why it is not working. If it turns out to be an electrical problem, I might be able to help you out with that.

[69gmachine 15]

Thanks for the replies. My main concern was should I take the dash apart again before driving any distance. If this is a mechanical problem and doesn't indicate failure (open circuit) is imminent, I'll live with it for the time being.

 

The year is halfway over and I just now got my car on the road again. I'll wait until November to tear it apart again. Right now I just want to drive it. After converting a Cougar XR7 harness to work in a Mustang and adding several new circuits, I'm intimately familiar with the under dash harness and what goes where LOL!

 

 

Thanks again!

[69gmachine 15]

I went to the Tachman website and he indicates there should be 1.5 ohms when reading into the tach and that the black wire should never be cut because it is a solid tungsten wire that cannot be repaired.

 

I don't remember for certain but I know one of the wires was cut and spliced together, but I've been driving on it for years with no problem.

 

He also states that if a ballast resistor (or resistor wire) is not used ahead of the coil to drop it down below 10V that it will overheat the tach circuit and cause it to burn up, plus the engine will not run right. I have been running a sneak circuit (full 12v) to the coil for years also and the tach has worked just fine. If it was affecting it the lower resistance should make it read high not low.

 

I made a jumper that ties into the brown 262 (hot only at start) wire under the dash so the output from the tach goes in parallel to the pink resistor wire and to the regular brown wire that then goes to the coil. I think to be safe I should probably pull the dash back off and bypass the tach so I can drive without worry.

 

I will take some measurements with my voltmeter to see just exactly what I get.

Edited June 28, 2012 by 69gmachine
spelling

[danno 128]

That is interesting. The sneak circuit will work, but the coil will run just a little hotter, you will go through points a bit quicker (if you have them), and your spark will be stronger. If you have less resistance in the coil wire, you have more current and voltage at the coil. More current in the coil means that your tach will read high by a very rpms, but I do not understand why it would burn up the tach. Maybe for some other tachs in other cars it will, but in this car it will not. That is why it has worked for you for 2 years.

 

Take some voltage measurements and see for yourself what the differences are.

[69gmachine 15]

I'm using a duraspark ignition and blaster II coil. I'll take some measurements but I expect it to be darn close to 12V. It's almost certainly higher when the engine is running since the alternator is putting out 14.2V and it's just the voltage drop of the wiring through the harness.

 

I'm so tired of working on it I will proabably take a chance for the nearterm and just bring some tools and a jumper with me in case it decides to burn out on me.

[LoneWolf2U 136]

Sorry but your having a tick,tick,tick failure headed your way.

The factory indash tach is different from the aftermarket add on tach's.

Your jumper tie into the brown wire will be backfeeding the tach signal and will cause failure in time. The engine will quit leaving you stuck till you bypass the tach lead behind the inst panel.

[69gmachine 15]

It can't backfeed the tach, all it can do is provide a lower path of resistance from the tach to the coil. There is no source of power coming from the coil.

[69gmachine 15]

I measured 11.16V at the coil. That's lower than I expected and only marginally above what the tachman considers safe. Given the number of years I've been driving like this I don't think it's a problem.

 

Now that the short has been resolved in the main under dash harness, it starts easily, and with the distributor set up right it idles better than it ever has. I'm going to leave it alone for now. It's crazy hot now and without working AC an early morning drive is about all I care for.

[danno 128]

Yes, 11.16 volts is a bit high, but you should be alright with a duraspark coil. If you want to drive the car until fixing your tach, you might bypass it just to be safe.

[1969 Mach1 14]

69gmachine,

 

My OEM tach in my 69 Mach 1 acted as yours is shortly before it completely failed. However, mine was connected to my MSD through a Tach adapter so it didn't affect the engine. I ultimately sold the IP on EBay and reinstalled the original non-tach IP as the movement or isolation transformer typically fail and not available new. If you really like your factory tach I would check into Red Line Gauge Works in Southern California as they can retrofit it with new internals. I would not recommend repairing it with 40+ year old used parts. This is just my opinion with these factory tachs.

[69gmachine 15]

69gmachine,

 

My OEM tach in my 69 Mach 1 acted as yours is shortly before it completely failed. However, mine was connected to my MSD through a Tach adapter so it didn't affect the engine. I ultimately sold the IP on EBay and reinstalled the original non-tach IP as the movement or isolation transformer typically fail and not available new. If you really like your factory tach I would check into Red Line Gauge Works in Southern California as they can retrofit it with new internals. I would not recommend repairing it with 40+ year old used parts. This is just my opinion with these factory tachs.

 

Thank you 1969 Mach1!!

 

That's exactly what I was looking for; someone with 1st hand experience observing their tach fail and how it behaved right before it failed.

 

It's about 200 degrees in my garage now, so working on the car is out of the question. Such a shame because it's running better than it has in a very long time.

 

Going back to a non tach cluster would mean replacing the engine bay harness, alternator harness and under dash harness. It would also render my oil pressure gauge in the console useless and make all the hours I spent converting a Cougar under dash harness into a Mustang harness wasted. I'm not ready to pull the pin on that grenade. For the short term I'll bypass it. Long term I guess converting it is the best option.

 

Thanks to all who offered help.

[nymustang1969 22]

I had Red Line Gauge Works convert my tach about 2 years ago. I think it was about $125 dollars. Turn around time was about 2 weeks. Works great and don't have to worry about it dieing out on the road.

[1969 Mach1 14]

69gmachine,

 

Mine acted like yours and after talking with the Tachman, and Red Line Gauge Works, I believe the movement was sticking. I understand sticky movements are very common and unforunately nothing can be done for it. If you ever open up your tach you will see the isolation transformer attached to a small circuit board. It has two sets of windings, the winding with the larger diameter wire is in series with your ignition coil and the smaller diameter windings drive the movement. If the larger diameter wire windings in the isolation transformer go open your ignition system will stop working.

 

If you want to keep the factory tach I would do what nymustang1969 did. I tinkered with my tach for hours replacing the resistors and capacitors on the circuit board. I even found another used isolation transformer and replaced that. All with no success.

 

Like your facing, when I went back to a non tach cluster I had to pull the under hood and under dash harnesses and undo all the changes I had made for the factory tach. Fortunately I had notes of the wiring changes, but it still took a few weekends.

 

Good Luck

Edited July 1, 2012 by 1969 Mach1

[69gmachine 15]

By 9:00PM last night the temperature dropped enough that I pulled the dash before heading to bed. This morning I made a little jumper and put the dash back in before heading to work.

 

I'll drive this way until November when I put it away for the winter to send out the tach for conversion and resume with more upgrades. Next winter I'll keep them to a minimum to make sure I don't miss the start of the driving season in late February/early March.

 

I also replaced the temperature sending unit. Works perfect now; just a hair to the right of center after warming up! I'm pretty sure I had a switch for an idiot light instead of a sending unit for a guage. Anyway, easy fix and huge peace of mind knowing I can keep accurate track of the temperature.

 

I found out that my new JVC radio is dead... don't know what i did to kill it, but that explains why the remote didn't work.

[bnickel 10,004]

larry, this guy http://www.rccinnovations.com/index.php?show=menu-tach-all can retrofit your factory tach from a 2 wire unit to a 3 wire unit just like any aftermarket tach, and quite a bit more accurate as well. I know that the Tachman (or someone, one of the old time tach repair guys) recommends not to convert to a 3 wire unit but the cars reliabilty with the three wire tach is just so much better with the 3 wire tach to me it's absoultely a no-brainer.

 

my old 70 cougar gave me fits with an intermittent ignition isure for about 6 months, it would just quit running all of a sudden and usually in the middle of nowhere between home, school or work which were anywhere from 30 miles to almost 60 miles apart from each other, about 120 mile round trip every day from home to school, to work and back home and i delivered pizzas at work with it too.

 

finally figured out it was the tach going open intermittently for some reason, i actually had a spare that came with the car when i bought it and that finally fixed the problem but it cost me a lot of time and money before i finally figured it out since i'd replaced every single piece of the ignition system, the entire starting and charging systems including the battery and ignition switch, solenoid and voltage regulator and a good chunk of wiring as well before the "AH-HA" moment hit me. the funny thing is that the tach never acted funny even once, obviously it would quit working when the car died (or rather the other eay around) but that seemed like a perfectly normal thing for a tach to do when the engine quit running

[miketyler 15]

I am putting a tach panel in my car and had Midlife do the harness conversion. Whats the downside to the 3wire conversion? Cost?

[1969 Mach1 14]

miketyler,

 

I think the conversion is the way to go. From what I hear the internals from a 2-1/16" VDO tach are installed. The only down side is you absolutely cannot convert the tach back to originial after the conversion. The pointer and other parts get modified that I think cannot be reversed.

[bnickel 10,004]

another benefit of the 3 wire conversion is that you can use the tach with any aftermarket ignition without having to use one of those tach adapters and it's a much more accurate tach than the factory piece. i believe it also reacts quicker than a factory piece too.

[69gmachine 15]

larry, this guy http://www.rccinnovations.com/index.php?show=menu-tach-all can retrofit your factory tach from a 2 wire unit to a 3 wire unit just like any aftermarket tach, and quite a bit more accurate as well. I know that the Tachman (or someone, one of the old time tach repair guys) recommends not to convert to a 3 wire unit but the cars reliabilty with the three wire tach is just so much better with the 3 wire tach to me it's absoultely a no-brainer.

 

 

thanks Brian.

 

I agree it's a no brainer, but it will mean rewiring the car to bring a wire in from the neg side of the coil. I plan to use a 69 rev limiter repair harness that has the factory looking boot terminal for the coil with two wire leads coming from it. One will go to the Duraspark box, the other will go in the Shelby underhood harness into the cockpit to the tach.

 

I made a jumper for it before I started driving around because I don't want to risk getting stranded. RCC is in Texas, so the shippiing will be a tad cheaper, other than that they are just about the same as the one in California wrt price and turn around time.

 

It's too darn hot to work on it right now.

Edited July 6, 2012 by 69gmachine
spelling

[miketyler 15]

Glad to hear you have a workaroud while the temps are creeping up. I plan to upgrade to Pertronix at some point. Any issues with factory tach and Pertronix III?

[69gmachine 15]

The only issue I know of would be how much current your coil is pulling. If you don't bypass the pink resistor wire there should be no issue. Even bypassing it I didn't have a problem for years, and I don't know that the increased current had anything to do with my problem. It may just be that it's 40+ years old and they're starting to fail.

[bnickel 10,004]

the pertronix 2 and 3 both require a full 12 volts but they have a relay setup that allows you to use the resistor wire as a trigger thereby eliminating any worries as for as current draw. the problem with the factory tach, however, has always been that if it fails you're stuck unless you pull the dash apart and bypass it or do what Larry did and install a jumper wire so that if it does fail you won't have to pull the dash apart on the side of the road.

 

Larry I completely understand about not wanting to work on the car right now...hell i don't even want to drive the mustang right now much less work on it or the cougar.....hell i don't even want to replace the ac blower switch on my DD town car because it's so hot, luckily the switch still works on everything but the high blower position so i do still have ac

[69gmachine 15]

If the resistor wire is only being used as a trigger for a relay, the factory tach won't have a signal. It will just point to wherever the static current level to the relay equates to an RPM.