weird issue with factory tach

[sigtauenus]

I started the car today to back it out and sweep the garage, and left it idling for about 5-10 minutes or so.


When I went to pull it back in, the tach was no longer reading. The factory tach circuit in this car is in series with the ignition, ie, if I have a bad connection in the tach circuit, the ignition won't work, ie, the engine won't run.


When I rev'd the engine, the tach would spin up rapidly to what looked accurate in the 2500-3500 range, but when I let it back down to idle, it would hang around 700 or so then drop to nothing.


Anybody ever see anything like that?

 

[Midlife]

I can't help you with your issue, but I do want to correct something you said. A series tach MAY cause the ignition to fail if the tach is bad, but not always. In series tachometers first use a transformer to pick up the changes in voltage due to the coil firing, but there still is a direct path to the coil should the tach-side of the transformer fails allowing the car to run. It is only when the ignition-side of the transformer fails that the car cannot run.

 

[sigtauenus]

That's good to know, didn't know that.

Regardless, I just had the tach serviced by the Tachman, so I'm doubting its the tach. Will check the connections on the coil and behind the cluster.

 

[sigtauenus]

So the tach is reading current flow to the coil, correct? Does that mean that if I have my electric choke hooked up to the coil that I am throwing off that reading? If so that would explain the reading on the tach changing after the engine warmed up. I wasn't tracking it but it was about the same time. I know its not supposed to be hooked up that way, but it was quick and easy and only meant to be temporary. I'll run the feed to a switched 12V connection under the dash and see if that changes anything.

 

[AzPete]

Power the choke off of the Sta post on the back of the alternator.

 

[sigtauenus]

OK, will do first thing in the morning. Have off tomorrow, well, taking a day off, need to take a final exam for my PME class. Will post back if moving the choke wire changes anything with the tach.

 

[Midlife]

That is probably the cause. My tach reads about 200 rpm lower with a Petronix installed rather than points.

 

[sigtauenus]

test test test

 

[sigtauenus]

Here's a quick video. As you can see, it works, then inexplicably goes to zero. Then it works again. Choke power is on the alternator STA post. I haven't figured out any correlation between the tach dropping to zero and anything else I may be doing with the car.

68 Mustang factory tachometer intermittent operation

 

[tarafied1]

I have installed a factory tach in my car (it wasn't an original tach car). I have the same problem. In fact I don't think it is very accurate at any rpm but I haven't verified it. this is how I wired mine since I couldn't find a schematic I made my own.

 

[sigtauenus]

That's how mine is wired except I bypassed the pink wire because I need a full 12V to the coil. I have a jumper that goes from the tach directly to the + side of the coil. I just sent the tach out to be repair and it was calibrated against a Sunpro, and I have pics of it at 1000, 3000, and 8000. At 3000 it is dead on, at 1000 it is 100 low, and at 8000 it is about 200 high.

I ran this car for many years with the tach working just fine day in and day out. Not sure what the issue is. Its just annoying, I want everything to work. Period.

 

[sigtauenus]

Here's those calibration pics


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[tarafied1]

I should send mine out too. Was it expensive?

 

[sigtauenus]

http://www.tachman.com/

I had him do all my instruments, and don't recall the specific charges for the tach. I want to say it was on the order of $100 or so.

 

[tarafied1]

thanks for the link and info, now I haven't helped you a bit!!! :char

 

[sigtauenus]

No worries. I'm concerned though about you having the same problem. What kind of ignition system are you using? I have a pertronix II with the matching flame thrower II coil.

 

[tarafied1]

I have an Accel Duraspark II "Blueprint" distributor, a Duraspark II box and MSD Balster II coil.

 

[sigtauenus]

Hmmm. Weird. There has to be a reason for it to be intermittent.