I was recently "inside" my 81 F100 and had my foot near the fuel pump area. I was working to remove and later clean my wiper motor. Upon leaving the engine bay on one occasion my foot got a little hung up. I didn't think I had pulled anything too hard but now it seems that occasionally when I go to start my truck I hear a buzzing noise that I cannot completely track down. I believe it may be coming from the distributor. Does anyone have any suggestions what it may be or perhaps what to test to identify the problem?

Thanks

 

Do you have AC? Messing around up by the carb at all?

On the 82, 81 should be similar, there are a few things up in that area. A buzzing sounds like it would be a relay or solenoid....something energized

If you have AC, there should be a Throttle Position Solenoid located on the carb linkage. Energizes when the AC is on to give the engine a little RPM to compensate for the compressor load.

There is a relay by the distributor (choke relay) that goes the the carb choke heater. Check that.

There is a carb float bowl vent solenoid located by the carb.

These are some things that energize that may cause a buzzing in that area. Try turning your AC off, if you have it, and try again and see if it goes away.

Both the carb float bowl vent solenoid and choke heater relay can be disconnected for a short period of time to test. disconnect one at a time and see if any of those are the buzzing.

 

I removed the spark plug wire from the ignition coil to the distributor and the buzzing stopped but instead made a quick spark sound. Then I decided to put my screwdriver inside the plug wire and watch the arc on a ground. I saw the arc but what was surprising was that the sound I realized was the sound of electricity. When the arc was created it made the exact same buzzing sound.

I then began to think that I am not hearing or seeing any attempt by the starter (which I was confident was fine) and so I wiggled some wires near the starter solenoid and tapped the solenoid itself and the truck started.

It's gets me to wondering whether this buzzing sound is taking place while the truck is running and I simply cannot hear it with the engine running. I mean I know the distributor is supposed to create and arc at each spot it passes. Just don't know if there is possibly some other arcing going on.

Anyone know whether the ignition coil is supposed to receive power and create a spark even before the starter starts? If yes, I am wondering why I have never heard this sound in any other vehicle when it wouldn't start. My only thought is that perhaps my experience with vehicles not starting has been mostly due to poor batteries or poor connections and therefore not enough current was flowing to initiate such a spark and therefore the sound.

 

No your distributor should not be buzzing nor should your coil be putting out a spark when the engine is not running.
Sounds like you got something going on with either you module or distributor. Something is telling the ignition to fire when it shouldn't be

 

Maybe I should clarify. When the ignition is turned all the way to start the vehicle (not the "on" position) is when I get a spark. I was unsure whether the ignition coil got it's current from the alternator which would only create electricity when the engine is being rotated or whether there is a line from the battery (perhaps then through some sort of relay) and then to the coil. If the latter was the case then theoretically current could be created without the engine moving. Is that normal?

 

The ignition system does indeed get power in the start position. But it should not spark unless it it rotating. The armature trigger in the distributor is what sends the signal to the module to fire the coil...this only happens when the engine is rotating. This is the entire pupose of the distributor...to fire the ignition.

Although it does have power in "run" and in "start"

 

I've attached a couple pictures out of the EVTM that will help understand how the circuit works in start and run. I hope they are readable.

If you look at the diagram....you will see both the module and coil get power from different circuits in Run and start. Coil gets full 12V in start and goes through a resistor when in run.

The 3 wires that goes between the distributor and module powers the armature and carries the signal from the pick up in the distributor to the module and then fires the coil. Again....the coil should only fire and spark when the armature is rotaing from by the engine....although the module and coil have power to it.

 

this one may be a little easier to read

 

Thanks 82f100460,

The pictures and explanation are very helpful. I am hoping I understand this better.

Am I correct that in the run position the solenoid gets a reduced power load due to a resister, but in the start position it gets a full load. But, just because it is getting a full 12v doesn't mean it should be firing without the engine moving. The reason is that the coil should only fire when the pick-up sends the signal to the module which in turn sends the signal to the coil to fire. And the only way the pick up sends the signal to the module and on to the coil is when the engine is being turned over by the starter. Does that sound correct?

If so, then I guess the question is how the coil is getting the cue to fire even without the module telling it to. Could the module be stuck? Is there a better explanation like a short. I am trying to keep all this in perspective of how my foot slightly catching a wire or tube to the right of the engine could cause this.

 

Shark,

I'm confused by your use of solenoid, but you've got a pretty good idea of what's happening.

To clarify, you've determined the noise you've sometimes been hearing is an electrical arc, correct?

And:

• With the engine not running

• With the key in RUN

• The high-tension wire connected to the coil but disconnected from the distributor cap

You insert a screwdriver to the wire's boot and hold it to ground and see a strong, blue spark, correct?

That's bad.


The coil will receive full battery voltage with the key in START but around 7V in RUN.

My guess is you have frayed or seriously hacked-up wiring under your hood, and jostling 'em with your foot disturbed them enough to cause something to be sending power to the coil when it shouldn't be.

As Kevin explained, the pickup mechanism in the distributor instructs the module to fire the coil. The engine is normally rotating to fire the coil, either via the starter motor or from normal combustion.

What kind of engine is in this thing?

 

The term solenoid I was referring to the starter solenoid which in my case is mounted under the hood just behind the battery (not attached to the starter itself). I've seen the term starter relay used and not sure whether that is a more appropriate term or if they are interchangeable.

Thanks for your input, but a few things I may have not done a decent of job of explaining.

The buzzing sound is from the arc.

When this problem is happening the starter does not turn over the engine at all (not even a clicking sound).

I only hear the buzzing when the ignition is in the "start" position.

I remove the high tension wire from the distributor (center post). I put a screwdriver in the wire in order to arc to a ground. I have someone else turn the ignition to "start" and an arc form. I wouldn't call it a strong blue (probably because my coil is old) but none the less it does the job.

I was able to resolve the problem by playing with the small wire going to the starter solenoid and also by tapping the starter solenoid.

 

OK, cool.

You see, my 1986 DD sometimes makes a buzzing sound when I turn the key to START, but the starter motor engages at the same time.

It's been doing this for at least 10 years now and nothing has failed yet, although I sometimes think of swapping out the starter solenoid (aka relay, same thing) but the buzzing isn't repeatable on demand and happens mainly when it's cold out.

The arcing you could be hearing could be happening inside of your starter solenoid, see THIS THREAD for an explanation.

 

Thanks, I will take a look.