Tachometer Issue

[Gary AAR]

I have an issue with the tachometer not working correctly in the rallye dash of my 70 Cuda.  I read through a lot of the older threads about tach issues, but couldn't find an issue similar to mine.  I finished the build of the car last Fall, and have about 300 miles on it since it was completed.  The tachometer never did work correctly.  I seems to read fairly accurate when the engine is first started and idling, but when the engine rpm is increased, the tach never reads more than 1500-1700 rpm.  Sometimes the needle will get stuck around this 1500-1700 rpm range, even though the engine is running faster or slower than this.  When the needle does get stuck, it returns to zero rpm when the engine is shut off.  The following are details of what I have:
1. The tach circuit board was replaced with the Real Time Engineering circuit board that has the built-in calibrator.
2. Dark Blue wire provides 12 volts to the tach.
3. Gray wire from tach is connected to the minus side of the coil.
4. I am running a Pertronix distributor (electronic ignition) and a Pertronix coil.
5. Resistor wire in the Ballast Resistor was replaced by a heavy gauge wire so that the coil always sees 12 volts per Pertronix instructions.
6. Followed Real Time Engineering instructions to calibrate the tach prior to installation.  The tach calibrated fine at both the 1000 and 4000 rpm test points.

With all of the above done, the tach has the issue of getting stuck in the 1500-1700 rpm range.  I have since removed the entire dash from the car to hopefully fix this issue.  Once the dash was out, I removed the tach and went through the entire calibration sequence again.  Just as it did initially, the tach calibrated just fine at the 1000 and 4000 rpm test points.  I reached out to Real Time Engineering, and they suggested looking for something that might be mechanically limiting the tach needle movement (such as a wire routed incorrectly).  I didn't see anything that could cause an issue.  I hate the thought of putting the tach and dash all back together only to find I still have a problem.  Hoping the experts on this site may have seen something similar, or would have some suggestions on things to check. 
Thanks for the help.

[MoparCarGuy]

Can you post a picture of the back of your Rallye instrument cluster showing the wiring to the tachometer?
Is the tachometer isolated from the factory Instrument Voltage Regulator (IVR) which supplies 5V to the other gauges?
You mentioned the Dark Blue wire is supplying 12V to the RTE tachometer. A picture may help us determine what may be affecting the tach "sweep".
I am not sure if the Pertronix has a tach output that should be used instead of the Coil (Neg).

The attached article discusses "If your pointer had seemed sticky, hanging up at times, you might consider loosening - very slightly - the jewel-bearing adjustment (arrow).
The captioned photo below should help.

[Gary AAR]

Included is a picture of the back of the instrument cluster to show the tach wire connectors.

Since my first post, I have searched the Pertronix website for tech support.  They have a FAQ section that included a topic on tachometer issues.  The following is a copy of their comments on how to potentially deal with a tach issue:
"If the cars tachometer is reading to a higher RPM then engine is running. Or the
tachometer works ok, to a certain RPM then stops or starts reading incorrectly.
We recommend installing a resister in line with the tachometer wire and coil
negative. This helps knock down the voltage spike/fire line on coil negative to
look more like a set of points. Allowing the tachometer to read the Ignitors signal
correctly.
Depending on the tachometer will depend on the resister needed. We have found
that a 10K ½ Watt resister fixes most problems and suggest starting with this
resister. If you find it doesn't fix your problem you might need to use a larger
value. Like a 15K ½ watt resister or 20K ½ watt resister.
In some cases the resister might not work, so you can try a capacitor. We
recommend using a .01 Micorfarad 1000VDC capacitor or you can come down to a
500 VDC."

Anybody ever heard of or tried the above suggested solution?

Thanks for the help.

[MoparCarGuy]

Your wiring is connected correctly with Dark Blue to the silver tach stud and the Gray with tracer from the coil NEG to the copper-plated stud. I assume the RTE tach conversion card was clear on having the internal Green wire to the Copper stud and the internal Yellow wire to the Silver stud. The reference photo from the Mopar Action article is below.

I find it strange that the RTE tach conversion will not take the normal coil NEG as a tach signal and work properly. I am running the Holley Hyperspark ignition box with the Hyperspark coil feeding my original factory Rallye tach and an MSD Tach Adapter.  https://www.summitracing.com/parts/msd-8920  Everything worked fine.  The MSD Tach Adapter is far more expensive than the inline resistor or capacitor fix but, if you value your time, go with the tach adapter.

Either way, your RTE tach card should be far more accurate than the factory tach's ever were. Let us know what route you take and what solves the issue.

[dodj]

 Need to order a board but the website hasn't changed in what seems like 20 years. Is RTE still a functioning business?
When I see a page like that I wonder if it is just orphaned and the owner retired years ago?

[Gary AAR]

I've been in recent E-mail contact with RTE as I try to resolve my tach issue, and they have been quick to respond.  So from what I can tell they are still in business.

[dodj]

I've been in recent E-mail contact with RTE as I try to resolve my tach issue, and they have been quick to respond.  So from what I can tell they are still in business.

Thanks much Gary 

[Jay Bee]

Need to order a board

Premium Dash Decals (aka Mr. Heaterbox) sell their version of a tach board too. One for points ignition (link) and one for MSD ignition. Just sayin' in case you didn't know. Sorry, I don't mean to highjack this thread.
https://premiumdashdecals.com/shop/dodge/dart/dart-demon/68-74-a-b-and-e-body-points-electronic-ignition/

[dodj]

Need to order a board

Premium Dash Decals (aka Mr. Heaterbox) sell their version of a tach board too. One for points ignition (link) and one for MSD ignition. Just sayin' in case you didn't know. Sorry, I don't mean to highjack this thread.
https://premiumdashdecals.com/shop/dodge/dart/dart-demon/68-74-a-b-and-e-body-points-electronic-ignition/

Well I have an MSD, but never heard of premium dash decals. Good reputation? Been around a while?

[Jay Bee]

They've been around for ages, an off-shoot of Mr. Heaterbox in Harrowsmith, Ontario. Don't ask Murray about their reputation, he wasn't treated very well by Mr. Heaterbox. I know a guy that was very happy with the restoration they did on his A body tach. Like everyone else, it's a toss up. An old Ohio bud I used to hang with bought one of their boards, I didn't hear any complaints from him about it.
https://premiumdashdecals.com/shop/dodge/dart/dart-demon/68-74-a-b-and-e-body-msd-ignition/

https://www.youtube.com/watch?v=c00gzYNcLyA

Sorry again Gary AAR for the highjack. This'll be my last post here unless I have something constructive to add to your situation.

[dodj]

Included is a picture of the back of the instrument cluster to show the tach wire connectors.

Since my first post, I have searched the Pertronix website for tech support.  They have a FAQ section that included a topic on tachometer issues.  The following is a copy of their comments on how to potentially deal with a tach issue:
"If the cars tachometer is reading to a higher RPM then engine is running. Or the
tachometer works ok, to a certain RPM then stops or starts reading incorrectly.
We recommend installing a resister in line with the tachometer wire and coil
negative. This helps knock down the voltage spike/fire line on coil negative to
look more like a set of points. Allowing the tachometer to read the Ignitors signal
correctly.
Depending on the tachometer will depend on the resister needed. We have found
that a 10K ½ Watt resister fixes most problems and suggest starting with this
resister. If you find it doesn't fix your problem you might need to use a larger
value. Like a 15K ½ watt resister or 20K ½ watt resister.
In some cases the resister might not work, so you can try a capacitor. We
recommend using a .01 Micorfarad 1000VDC capacitor or you can come down to a
500 VDC."

Anybody ever heard of or tried the above suggested solution?

Thanks for the help.

This sounds very odd to me. A resistor or capacitor to different things to a waveform. I'd ask which works for a current type tach. You don't want to bo going out and buying a bunch of discrete components. Having a random resistor or capacitor taped to the harness isn't ideal either.

[MoparCarGuy]

Yes, RTE is still operating. I just bought their Instrument Voltage Regulator (IVR3).
Got it three days after the order.