[aakrusen]

Edit: I updated the status of this problem in post #24

My car has an after market Tach, installed by the previous owner, so I have no idea if it works properly or is wired properly.

Problem = The needle on the tach jumps up about 500 RPM about every second or two, but the engine sounds normal, no idle issues.

What I know = The tach has 4 wires (Black, Green, White, and Red). The Black runs to ground. The green goes out to the negative terminal on the coil. The White and Red wires are put together and go to a switched power source. I'm assuming one of those last two wires is the power wire and the other would normally go to the light switch so the tach would light up when the car lights are turned on. In this case, the Tach lights up when the car is on.

When the car is idling, the needle mostly sits at what would be the idling RPM, but then it'll bounce up a few hundred RPM and fall back down. The sound of the car does not change, so one can tell the actual RPM is not bouncing like that.

What can cause this needle bouncing? A bad ground? A bad power source? A bad coil? A bad Tach?

 

[aakrusen]

Sounds like a defective tach. Does it have a brand name on it? Ckinkanese crap?

Brand is Equus, model is Pro Tach. It's black, mounted to the steering column. It looks exactly like this unit on Summit Racing.

Edit: it is a 6088 model, it says it on the front of the Tach.

 

[aakrusen]

Agreed that is sounds like a bad tach, but I would stick a volt meter at the same ground and power connections and see if you are getting some weird momentary voltage spike.

I grabbed my Voltmeter, put the ground lead of the voltmeter at the ground for the Tach (to make sure I had a good ground for the Tach) and I placed the positive lead at the terminal that I'm supplying a switched 12V source. The voltmeter read a constant voltage the whole time with no bumps, dips, or spikes in voltage. I then changed to a different ground (for fun) and I got that same voltage.

 

[aakrusen]

What ignition are you running? Pertronix Ignitors are notorious for signal voltage spikes. Perhaps you need a signal filter.

It's like a Pertronix, but it's some other brand. I have had a Pertronix on my last two Mustangs and the unit on this car sort of looks like a Pertronix, but it's not. There's no name on it.

 

[aakrusen]

We're going to need the brand/model.
And if everything is straight-forward...... has points, no box, no Pertronix, etc.

Petronix-like electronic ignition, no box, MSD Blaster 2 (#8203) coil. Tach is an Equus Pro Tach, #6088.

Would the primary resistance on that MSD coil have anything to do with this? I thought I read somewhere that you have to match that resistance of the coil when figuring out which unit you are upgrading to. Does that sound right?
This MSD Coil has a primary resistance of 0.7 ohms, and the my previous car had a 1.5 ohms unit.

 

[aakrusen]

I would first try to find correct install instructions for tach.

Verify all necessary settings for your engine on tach.

Temporarily wire tach with long pigtails in correct locations as per instructions.

This will remove any poor connections in vehicle that could possibly affect operation.

Even a loose connection can cause erratic operation on a gauge.

Looking at instructions online, the four wires on the back of this Tach are going to the places they need to go. With the exception of the wire that is supposed to light up the Tach when the car lights are on, this wire is connected to the same switched 12V source as the power wire for the Tach, so that should make the needle bounce.

 

[aakrusen]

I'm seeing other people having a similar issue (with different brand Tachs) put a 10k Ohm, 1/2 watt resistor in line on the wire that goes to the Coil. It looks like it's an "electrical buffer" type of setup

Have you tried the suggestion by Ateenandhisstang and change to a constant power source?

Even just for troubleshooting.
Worth a try.

Do you have a ground between engine and firewall?

No, partially because the instructions I found online say to hook it to a switched source, and partially because once the car is running there's no difference between a switched source and a constant source.

I have two grounds on the engine, one goes to the firewall and one goes to the passenger fender lining (near the battery).

 

[aakrusen]

Also, I put a voltmeter across the terminals on the coil and voltage seemed to bounce like the needle on the gauge. The voltage was around 2.5V, but it would bounce up to 5V and down to less than a Volt, sort on the same randomness that the needle was bouncing on the Tach. Does this matter?

 

[aakrusen]

Update.
Here's what I've tried the and the results for each:

• I did confirm I have the "pink" resistor wire that goes between the ignition switch and the wire that connects to the positive terminal of the coil, so no change.
• Changing the ground wire of the Tach to a new location, no change.
• Checked the voltage source for the Tach for bumps or spikes in voltage, it's smooth, no change.
• Adding a 10k ohm, 1/2 watt resistor in series on the green wire (the one that goes to the negative terminal on the coil) to see if it smoothes out the signal, no change.
• Replacing the coil with the 1.5 ohm resistance (vs the 0.7 ohm on the previous coil), no change.
• I was wrong before, I do have a Pertronix ignition inside the distributor, it appears to be fine (don't know how to test it other than trying a new unit), no change.
• I purchased another Tach (the same unit because it's cheap), hooked it directly to the battery terminals and negative terminal of the coil (just for this test, it sat in the engine bay during testing), still had the jumping Tach needle, no change.
• I then added the 10k ohm, 1/2 watt resistor to the green wire, of the new Tach, no change.
• Lastly, there does not appear to be any kind of audible difference in the motor idling when the Tach needle jumps up each time. Since the needle is jumping 300-500 rpms above idle, you'd think there'd be some kind of audible difference, even for a split second.

So, to summerize:
Two different Tachs
Two different power sources and grounds for the Tachs
Tried adding the Resistor on both Tachs
Two different Coils (one with 0.7 ohm and one with 1.5 ohm resistance)

At this point, the only thing I haven't tried is a new Electronic Ignition since I don't know how to test the Pertronix unit in the distributor.
Both of the Tachs are the same model, but one is brand new. I had this same Tach on my last two Mustangs with no issues (they were already on each car when I bought it). I'm not ruling out that these are crappy, cheap Tachs, but the odds would be low that both of them would be bad, especially when one is brand new.

So this keeps me going back to the Pertronix Ignition. Do you guys see anything else?

 

[aakrusen]

I would look at the engine to chassis ground. You want that connection to be really good.

Yep, two engine grounds, both are secure and a heavy gauge wire. One off the back of the motor to the firewall. The other off the front of the motor to the fender near the battery.

 

[aakrusen]

Do you have a points distributor to drop in and see if it makes a difference? Easy check of the Pertronix causing the problem?

Unfortunately I do not. While a set of points and a condenser are much cheaper than buying a new Pertronix unit to see if that's the issue, I'd holding out on that option in case someone else has a way to test this setup without needing to buy those parts.

 

[aakrusen]

It's the Pertronix. I tried to give you a way out in post #21........... just the same as the Equus instructions do.

Replacement Inductive Pickup for Tachometers

Replacement Inductive Pickup for Tachometers

www.iequus.com

BTW, you can test a Pertronix.......... if it's the original version. It's not going to tell you much other than it works or it doesn't-

View attachment 828672

Yes, I saw your post, checked out the link, but because I don't have a coil-on-plug design, I thought that it wouldn't work. But, I am likely wrong and it doesn't matter the coil/ignition system I have, maybe that inductive pickup is what I need.
Looking at that troubleshooting pic above, I am wondering if the Pertronix is faulty, like you mentioned. I checked the voltage at the coil like the pic shows and I was getting less than 5V. I never saw it reach 12V. I can go and check again and see if I get the same voltage.

 

[aakrusen]

It's the Pertronix. I tried to give you a way out in post #21........... just the same as the Equus instructions do.

Replacement Inductive Pickup for Tachometers

Replacement Inductive Pickup for Tachometers

www.iequus.com

BTW, you can test a Pertronix.......... if it's the original version. It's not going to tell you much other than it works or it doesn't-

View attachment 828672

OK, so I hooked up the wiring as shown in the above pic, and with the engine turning over, the highest voltage I could see was in the 9V range. With the key in the "on" position, and not starting the motor, the voltage was a steady 12V. But when I turn it over, it never gets up to 12V.