When I hit Brakes Tach RPM Increases 300 rpm?

[Mike Z]

1968 Charger Electronic ignition.
Please Help. Every time I hit the brake pedal The Tach (Autometer Not Stock) Increases 300 RPM. The Engine Is Not Speeding up Idle at all just the tach going up.
Car has power brakes but I'm not running the booster off of engine Vacuum Using electric pump.
I did notice the radio has just shut off and powered back on intermit too but not at the same time (aftermarket Radio). Tach is grounded on column bolt and good tight and clean.
Thanks in advance
Mike Z.

 

[gtxno1]

1968 Charger Electronic ignition.
Please Help. Every time I hit the brake pedal The Tach (Autometer Not Stock) Increases 300 RPM. The Engine Is Not Speeding up Idle at all just the tach going up.
Car has power brakes but I'm not running the booster off of engine Vacuum Using electric pump.
I did notice the radio has just shut off and powered back on intermit too but not at the same time (aftermarket Radio). Tach is grounded on column bolt and good tight and clean.
Thanks in advance
Mike Z.

when did it start happening can you draw out a small wiring schematic where all hook ups are going

- - - Updated - - -

Did you tap anything into the brake lamp circuit at all?

 

[Mike Z]

Thanks,
I will look. I just swapped out the voltage regulator no difference. Issue is current as of today. I changed nothing. I will gladly go check the brake lamp circuit.
Mike Z.

 

[440+6]

This is interesting because my tach increases about 300 rpm when I let off the gas pedal, say I'm cruising in fourth gear 60 or 70 mph, let off the gas and rpms go up for a few seconds, but obviously engine speed stays the same or rather decreasing with speed.

Wondering where this goes.

 

[Mike Z]

Nothing is wired into the brake lamp circuit at all. I just switched the tach circuit to multiple different power curcuits off the fuse panel. None had an affect. I did notice any kind of load meaning headlights, Brake vacuum pump, turn signals ect do effect the rpm on the tach.
I Love Old Mopars. Thanks,
Mike Z

 

[440roadrunner]

I would start with two things......

Take a good look at or maybe re--ground the tach ground wire

Second, "rig" a voltmeter onto the tach power wire and see what it's doing with brake light application.

 

[Hemi Rebel]

Check grounds. Tail light, and body grounds.

 

[bm02tj]

99% to grounding trouble with feed back
Start cleaning all grounds and if need be add bonding straps

 

[Mike Z]

thanks guys,
I found only 11 volts off and when running at battery terminals. Getting a new alternator today. Hopefully i get enough time to install it tonight.
Mike Z.

 

[Mike Z]

New Alternator on. Much better 13.90 at idle with no load. 13.00 with lights, vacuum pump, radio all on.
the car has 1 wire connector for the brush. Should I ground the other brush lead to the case? Will the voltage improve even more?
Thanks,
Mike Z.

 

[440roadrunner]

That voltage isn't worth a damn with loads on unless this was a "dead slow" idle.

The alternator should "keep up" with heater lights, whatever on, and with engine at "low to medium cruise" RPM it should maintain a MINimum of 13.5. your idle / no load voltage at 13.9 is OK

Read the MAD article if you have not. Even if you don't do "this mod" the article shows the weak points in these girls

Bulkhead connector is most likely the top of the list

http://www.madelectrical.com/electricaltech/amp-gauges.shtml

So what RPM did you read this loaded condition?

Other troubles may be the alternator is not actually capable of much output, IE has a bad diode or other trouble.

So far as "grounding the terminal" can't answer without a photo. It would not be charging if the second brush was not grounded. You might want to REMOVE that terminal blade so you don't accidently connect the field wire to it.

 

[cwhubb]

get in touch with nacho, he knows more about mopar wiring and charging systems than anybody

 

[440roadrunner]

get in touch with nacho, he knows more about mopar wiring and charging systems than anybody

There's a few of us might dispute that claim. He seems to have a fixation on using stock ammeter circuits, and even "Ma" herself finally realized that was a poor idea.

 

[cwhubb]

come on now, give the devil his due,Nacho knows his stuff.

 

[Yatzee]

The newer Autometer tachs are sensitive to spikes or changes in voltage on the power side. I had the same effect with mine and the tech said to try feeding from a different circuit. Think of it this way, if you have a volt gauge you will notice a slight bouncing of the gauge needle when you activate the turn signals. Caused by the load of all those old inefficient incandescent bulbs. I had to use a relay with power directly from the battery and the coil of the relay activated by an accessory feed from the ignition switch. I believe that newer tachs are highly accurate but because their electronic their operation is directly affected by voltage variances. The instructions that came with my tach explicitly said to install a 3 amp fuse on the power side to the tach. Don't know what tach you purchased but older Autometer did not display these oddities. Best to call Autometer in the event that you have a faulty unit because they will take the problem seriously and offer what ever solutions are necessary.