8N Generator Acting Erratically

[WayneMo]

My 8N Generator is suddenly acting erratically with my digital voltmeter reading jumping around showing voltages from 0 to 19 volts with the engine running. I do not see any wires loose on the generator and the regulator is hidden below the tool box so I cannot see it. The voltmeter just flickers showing readings of everything between 0 and 19 volts or so at the battery terminals.
What could cause the erratic voltages jumping around so? Could it be something broken inside the generator or the regulator? I guess I could disconnect the regulator field wire and just ground the generator field manually to see if it settles down. I just do not understand what could cause such jumping and what could cause the high voltage readings!

 

[teddy52food]

It's your meter. Get an old fashioned one with a needle & try before you goof something up.

 

[Royse]

Digital voltmeters do not like the electrical noise generated by
these old systems. Use an analog meter.

An unloaded generator will produce way more than 12V.

 

[Bruce (VA)]

As others are saying.....see tip # 47.
75 Tips

 

[WayneMo]

I have used these digital meters with the engine running and measured steady voltage at the battery terminals before with success. I tried 2 separate digital meters and both are erratic now. I will see if I have an analog meter tomorrow.
I agree that an unloaded generator will produce higher voltages but do not know what would cause the unloading. Also a grounded field would cause higher voltages would it not?

 

[Bob]

Disconnecting the "F"ield will get you ZERO output.

 

[Tim PloughNman Daley]

RadioSnack sells a cheap analog Multi-meter for around 15-20 bucks. You can do simple motoring test on the generator without removing it from tractor and you can also take it, the VR to your trusty local starter/alternator shop and they will bench test them under load usually at no charge. You don't specify what model year your 8N is and a few other specifics but here is what the early 8N back dash wiring looks like. Later 8Ns used a different genny. The VR is the original AUTO-LITE style but yours maybe a newer aftermarket type. The VR is not hidden 'below the toolbox' -see pix below. The toolbox is off the right from the VR. Get a copy of the I&T F-04 Manual with all the valuable info in it including wiring drawings and exploded views. We don't know where you are measuring voltage from, why you are trying to take measurements, nor what the rest of the system is set up for.

Tim Daley(MI)

 

[HCooke]

Wayne, I agree with the others. Try an analog meter. If you still get erratic voltage reading, time to generator and VR to Central Auto Electric.

 

[HFJ]

i'll add that my cheapo digital meter (free from harbor freight with other purchase, so u KNOW it has to be good :roll: ) doesn't like modern vehicles that are running either. i tried to use it to diagnose misbehaving trailer lights on a running SUV and it insisted i had minus 19 volts and change at the fixture - even when i reversed the leads (which were right to begin with.)

because of the coincidence with 19 volts, now i wonder if wayne has a free harbor freight meter too

 

[souNdguy]

It has to do with the sample rate ( speed ) of the DVM, and at what instant it is catching the generator and regulator. Remember the regulator is actually an opening and closing set of contacts, meaning field current and thus charge current is actually pulsating from low to high, and thus voltages are swinging. the battery averages out the net charge.

Full field the generator by grounding the field terminal either at the regulator or the generator.. no need to remove any wires. this will make max field current, and max possible charge current ( based on rpm ).. so set rpm at mid throttle.

This should give you a non pulsating reading.

PS.. some very cheap meters won't like the emi / rfi interference of the points ignition as well.,, so it could be a compound problem of poor sample rate and poor noise rejection of the meter circuit.

 

[WayneMo]

Well, an analog meter did solve my problem and everything is normal now! Thanks to all and especially your explanation of the system. I swear in the past I have used digital meters to adjust regulator voltages, etc with no problems. You are right that shorting the field to ground does not require removing wires and will put the generator in steady output.
Hope others here learned something too! Good to have the help this board gives!

 

[souNdguy]

The digital meter you used before may have been of higher quality, better noise rejection, better sample rate.

Glad you got it fixed.