Erratic Tach...

[Carl83911]

Hello, I have an 83 SC.

Beginning yesterday, I noticed the Tach going nuts. Erratic. At idle the needle is at 2500rpms but listening to the motor the idle was normal at about 900.

As I go into gear the Tach jumps about to between 4000 to 5500 rpms while i know that the car should only be at between 2000 and 2500 rpms at the speed that I was driving and by listening to engine noise.

That being said, i am assuming that my alternator is beginning to go out of whack. but I just replaced the alternator about 4 years ago. it should last longer than that since the original lasted 120,457 miles. should it already be breaking down? () Another reason why i believe it may not be the alternator is that on idle the voltage is steady @ 13.5v +/- .3 volts and the generator warning light is not on or flashing as it did with the first replacement. At hwy speeds the tach seems to fall into the correct rpm readings. The car is running ok. It's the tach that's erratic.

I just had my cd unit replaced with the new permatune. couldn't afford the oem Bosch cd. wonder if that's affecting my tach.

Could it be that the brushes are worn down and need replacing? hmmmm. That would make my alternator light glow.

Any feedback on what is causing this gremlin is much appreciated.

Thanks for reading/responding in advance.

Carl

[Noah930]

Can you find a tach to borrow? Maybe it's the tach, itself.

[wwest]

Started with Permatune...??

Measure the voltage in the engine compartment.

[timmy2]

Check how the wiring for the tach signal is now connected with the permatune?

[jasonlee]

could be a sign that the alternator is giving up. Thats what happened to my 82 SC

[Carl83911]

Originally Posted by wwest

Started with Permatune...??

Measure the voltage in the engine compartment.

My first alternator replacement, i still had the bosch cd unit.

[mark waller]

Happened to me on my 83 SC right before the alternator went out.......

[timmy2]

Originally Posted by Carl83911

Hello, I have an 83 SC.

I just had my cd unit replaced with the new permatune. couldn't afford the oem Bosch cd. wonder if that's affecting my tach.

Carl

Sorry for the brief response earlier from my phone.
First, check your voltage while running, should be about 13.5-13.8, turned off, the battery voltage should be about12.5.
If this is correct, I would look into the tach signal wiring from your new permatune.

Good luck and report what you find.

[S2GART]

check your voltage with the engine at 1500 rpms. It shouldn't read anything higher than 14.5. I experienced the same periodic tach bounce, never got a battery light. It was a bad VR. Since I went to the trouble of removing the alternator (original) I took it to a local shop to get it checked out and they said it was perfect.

[1983Cabrio]

My 1983 SC - same thing - was the Voltage Regulator - $80 bucks, easy fix.

[Joe Bob]

Check for voltage spikes with digital volt meter. Likely the alternator is overcharging. Check the battery for overheating.

[Carl83911]

Thank you for your feedbacks.

I'm still observing my issue.

The voltage @ idle is between 13.5 and 13.8
@ 1500 rpms to about 14.2

When below 2500 rpms the tack will read between 2000 up to 4500 depending on speed.

Once my rpms reach true 2500 rpms engine speed, my tack will read normal until I slow back down to below 2500 then the tach resumes it's crazy read.

Below 2500:

? rpms = True rpms

4500 = 2500 rpms
2500-3000 = 800-1000 rpms

I'm leaning towards the bad VR. I believe that as long as my alternater reads constant voltage measurements and the alternator light indicator doesn't light up, my Alternator is still ok.

[buzdrum]

Id be happy to test it for free. If it is the tach i can also repair it!

Buz
seattlespeedometer.com

[thumbdoctor]

You may need a signal conditioner due to the permatune's reference signal not matching your tachometer's needs. Have you recently installed electronic ignition?. I had the same issue and installed a Tach Adapt from our host with great results.

[Joe Bob]

You need a way to read the voltage when it is bouncing. Never assume, it costs money. Could be something else, the gauge or the signal from the source.

[timmy2]

Take a picture and explain how you hooked up the tach signal from the permatune to the main harness wiring.
If it was a simple plug and play using the 6 pin connector there may be something wrong with the permatune signal.
Your voltage readings sound normal, if the VR was failing the voltage would be spiking or dropping off wildly from 11 to 18 volts.

[Carl83911]

Originally Posted by timmy2

Take a picture and explain how you hooked up the tach signal from the permatune to the main harness wiring.
If it was a simple plug and play using the 6 pin connector there may be something wrong with the permatune signal.
Your voltage readings sound normal, if the VR was failing the voltage would be spiking or dropping off wildly from 11 to 18 volts.

I tend to agree with you. After thinking it through, with my voltage being stable, i will then assume that thr vr is ok. My voltage readings seem normal.

The permatn is plug n play and have had it for about 4 months. The premature was installed by my mechanic from a reputable garage. He explained to me the differences between thr oem and the permtn. And that the prematune was spec'd for my 83sc.

I guess I may have to have it tested to see if it's malfunctioning.

[Carl83911]

Originally Posted by buzdrum

Id be happy to test it for free. If it is the tach i can also repair it!

Buz
seattlespeedometer.com

Thanks for the offer. I live in Sacramento.

[Carl83911]

Originally Posted by Joe Bob

You need a way to read the voltage when it is bouncing. Never assume, it costs money. Could be something else, the gauge or the signal from the source.

The voltage is stable.

[timmy2]

Maybe there is someone in your area that would be willing to loan you a good Bosch CDI box to try and see if the problem goes away.
If it does then the permatune is the cause.