I have an autometer tach that has the probe that goes on the alternator, and here is my problem, It started with my tach just jumping around a little at idle now it is progressively getting worse. The more juice I draw from the alternator the more calm my tach is, for instance: at night with the blower on high speed and the lights on and on high beam it is relatively still, but with nothing in the truck on and drawing it is all over the place. Or for instance if I turn on my grid heater (which is on a toggle switch) it draws so much that my tach works perfect. So I am leaning more towards my alternator or voltage regulator than the tach since the tach is only 3 months old, and the voltage regulator is no more that a year old. Anyone have this problem or have an idea?


Thanks
Chris

 

Try moving the pickup on the alternator to a better spot.

Edwin

 

I have the same issue with mine. I also noticed that if the radio is on a weak AM station, there will be a slight screetch that syncs perfectly with the jumping tach. I have tried switching to another stator wire on the tach with only temporary relief. If I can frind what is causing this, I will post what I find. Seems to me that in theory, there might be a problem with the alternator's rotor becoming de-energized momentarily when loads are low. When the regulator senses a modest load, more energy would be directed to the rotor & that would smooth out the tach. I think checking the voltage at the rotor wires of the alternator might reveal the pulses. If not, again in theory, maybe the slip ring/brush assembly inside the alternator might have problems. Hope this helps. Again, mine has the same problems & I have not done any major diagnostics yet.

 

The armature is energized and rotates to produce current in the windings and some of that rotating field escapes the alternator to generate a small current in the pickup coil. The Tach converts the frequency to an RPM reading.

The armature current is modulated to regulate the voltage output of the alternator but the tach isn't dependent on the amplitude of the field just it's frequency. As long as a minimum field is picked up the tach will work. If you have picked a bad place where the field is weak then move it around to a better place and it should fix the problem.

Edwin

 

My aftermarket tac gets its signal for the crank! ...?? never heard of it form the alt. but then again i have never buy a tac for a diesol, mine came w/truck!!

 

Yea my tach probe is on the alternator, I will try moving it around to a different spot and see if that helps any.


Thanks
Chris

 

Are you sure your alt. is working right? If your alt. or reg, is acting up it can cause your tach to act up. I work in a shop rebuilding alternators,and have had customers that had the same problem,and rebuilding the alternator seemed to fix it. Regulators acting up can cause it also.

 

wow... you can get tachs that hook to the alternator? thats cool! I read somewhere that the ealy 1g's dont even have a crank pickup... so no tach. This is good to know! Any idea where i could get one of these tachs and the price?

 

Autometer makes the Z series which has a pickup for the alternator It clamps on with a big hose clamp. I don't have the number right now but somebody on here will chime in I'm sure. If not I'll go out tomorrow and get the number off mine.

Edwin

 

The alternator tach I use is made by Teleflex and cost $69 at Boater's World. Even with the jumping, I still find it useful. If I turn on the headlights, the jumping stops. When I tow my boat, which is the primary use of this truck, the headlights are on anyways.

 

The Autometer Diesel Tach is #2888. It's sensor probe is particular about placement. Right along the middle at the 8 o'clock position is the sweet spot. You'll need to calibrate it via a tiny slotted screw accessible from the back. I used an optical tach to calibrate mine.