1985 F250 460/carb w/Duraspark. When i put a multimeter on the coil, i'm only getting a reading of about 4 volts. Battery reads good, as well as alternator. Engine stumbles a little at idle. How many volts should i be getting, and is the low voltage causing poor spark?

 

If I am not mistaken, you should be getting about 7 to 7.5 volts. Low voltage could be causing poor spark.

 

clean the coil to bracket and where the bracket bolts to the block.also what is the voltage between negitive bat to positive coil.this will tell you if you have bad ground.

 

4 volts is to low. I did not think it would still run at 4 volt???

Check it at idle, at the coil POS.
Did you add a ballast resistor down stream of the resistor wire.
You can just jumper battery POS to coil POS to see if it runs better. Are you using a round or square coil?

Jim

 

I put a new coil in it(round style), but didn't improve anything. The old one had Motorcraft stamped on it, so i figured that it was original. Anyway, here are the readings:

Battery: 12v
Battery/Running: 14.5
Coil Pos & Neg Running: 4
Coil Pos/Battery Neg Running: 10
Coil Neg/Battery Pos Running: 8
Coil Pos/Battery Neg Ign On: 6.5
Coil Neg/Battery Pos Ign On: 10.5
Coil Pos & Neg Ign On: 5
Battery Pos & Coil Neg Ign Off: 12

 

Battery: 12v Normal
Battery/Running: 14.5 Normal
Coil Pos & Neg Running: 4 see note 1 below
Coil Pos/Battery Neg Running: 10 Same as my truck
Coil Neg/Battery Pos Running: 8 Not really sure what normal would be
Coil Pos/Battery Neg Ign On: 6.5 see note 2 below
Coil Neg/Battery Pos Ign On: 10.5 see note 3 below
Coil Pos & Neg Ign On: 5 see note 4 below
Battery Pos & Coil Neg Ign Off: 12 normal



NOTE 1: I not sure this is a useable measurement as the coil magnetic fields are doing a lot of weird stuff at the same time while it is running.

NOTE 3: This shows a 1.5 volt drop in the ground path down stream of the coil Negative, this seems too high but I have never measured it. From the coil NEG, current flows into and out of the DS2 module, then to the ground screw under the Dist cap.

NOTE 4: resistor wire = 5.5v. coil = 5v. ground wiring = 1.5 v so all 3 add up to 12v total applied voltage.

NOTE 2: This means 5.5 VDC drop across the resistor wire and 6.5 VDC drop across the coil and ground path down stream of the coil NEG, 5.5 + 6.5 = 12 volts….. BUT it is my understanding is there should not be any current flow in the circuit if the distributor is not spinning around… just having one of the teeth of the reluctor lined up with the magnet under the cap should not trigger the DS2 module into providing a ground path for the coil. It will depend on the type of module you have if there is current flow... not a clear answer here.

Distributor Pickups

AUTO REPAIR HELP - DISTRIBUTORS



Jim