Voltage drop at ballast resistor

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Paladin06

Paladin06

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Ok, I’m checking my electronic ignition system on my 72 Duster, 340. When checking voltage at the battery I got 12.43 volts. When I turn the ignition switch on I get 10.55 volts at input wire (disconnected from resistor) to the ballast resistor. With the wires connected to the resistor and the ignition switch on I get 4.75 volts at the positive side of the coil.

Your input on these numbers appreciated.
 
What do you get at the battery with ignition on and with ignition off?
What loads are on when the ignition is on? (radio, Electronic ignition, etc)

My recollection.... (I'm old so take it for what you will) is that the coil really wants to run at ~6V. when the car is starting you bypass the ballast resister and provide 12V to the coil for a short amount of time. after starting the coil voltage goes through the ballast resister and you end up with about 6V. If you are starting at 10.5 and subtract 6V you get 4.5V.

Seems about right might be a weak battery at this point.

does it run?
 
Dana67Dart said:
What do you get at the battery with ignition on and with ignition off?
What loads are on when the ignition is on? (radio, Electronic ignition, etc)

My recollection.... (I'm old so take it for what you will) is that the coil really wants to run at ~6V. when the car is starting you bypass the ballast resister and provide 12V to the coil for a short amount of time. after starting the coil voltage goes through the ballast resister and you end up with about 6V. If you are starting at 10.5 and subtract 6V you get 4.5V.

Seems about right might be a weak battery at this point.

does it run?
Click to expand...
There no other systems on while making this check. Radio is off. The car does have electronic ignition. The car does run but can be difficult to start at times once warm.
 
What is the ammeter showing?
A drop in voltage makes me suspect current is flowing. The questions are wheres it going, and where is it loosing power (resistance points).
Normally the voltage drop test is done with some equipment on or a 10 amps across a carbon pile resistor. In this case the only draws are the ECU and the coil, maybe the alternator field too but shouldn't be much.
So there shouldn't be any more than .1 V drop from battery to the run wire connection on the ballast. Check for drop at each accessible point along the way: solenoid relay, bulkhead in, bulkhead out,.
 
Mattax said:
What is the ammeter showing?
A drop in voltage makes me suspect current is flowing. The questions are wheres it going, and where is it loosing power (resistance points).
Normally the voltage drop test is done with some equipment on or a 10 amps across a carbon pile resistor. In this case the only draws are the ECU and the coil, maybe the alternator field too but shouldn't be much.
So there shouldn't be any more than .1 V drop from battery to the run wire connection on the ballast. Check for drop at each accessible point along the way: solenoid relay, bulkhead in, bulkhead out,.
Click to expand...

Thank you but can I get specific locations on the relay and bulkhead? The amp meter fluctuations have occurred for some time now.
 
I've clean them and put them back together and they worked fine and I've never had to replace one.
 
6493035-wiring-diagram-full-valiant-duster-1973-dodge-dart-sport-schematic-1024x745.jpg

First you need this. It's 1973 but should be the same. Then you need a good grasp of how the current and ground flows from the battery to the coil.
There's two systems, ign 1 (run) and ign 2 (start). Is your ECU well grounded?
 
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Paladin06 said:
Yes, ECU is grounded and thx.
Click to expand...
Good. 4.5 volts in run at the coil seems low. For sure. Start there and troubleshoot back, you have high resistance somewhere. Mine ended up being at the ECU connector. Was wallered out and sloppy after 45 years.
 
Paladin06 said:
The amp meter fluctuations have occurred for some time now.
Click to expand...
What do you mean fluctuations?
Lets take a minute and briefly go over Amps and Volts.
Volts are potential energy. Like water in a big holding tank on a roof, even when the electricty is not flowing, it has potential to flow. Same idea with a battery.
Amps are a measurement of flow. If a tap at the bottom of that water tank is opened, the flow coming out into a hose can be described in gallons per minute. If we punched a big hole in the tank, so much would come out it would knock down anyone trying to stop it from reaching the ground. Note it was the flow from 14' above the ground that knocked the guy down, not the height of the water sitting in the tank.
The ammeter shows how flow is making its way from the battery to the main junction, or how much current is flowing to the battery to charge it.

While starting the engine, the ammeter should show discharging to power the ignition, and the starter relay.
As soon as the engine is running, the ammeter should show charging of the battery. As the battery charges, the meter will show less and less current until the battery is fully recharged.

Paladin06 said:
Thank you but can I get specific locations on the relay and bulkhead?
Click to expand...

Pretty easy. On the Battery Positive post there are two cables. The big one leads directly to the starter. The smaller one, 10 or 12 gage, probably red, goes to a stud on the starter relay. Joining it there is main wire to the bulkhead connector - follow that to the bulkhead (in that wire will be a smaller gage wire called a fusible link).
The diagrams here will give you a general idea of what to look for.
Understanding Charging Systems with Ammeter
A '72 shop manual will get you the exact wire colors and cavities of the bulkhead terminals.
 
Last edited:
Paladin06 said:
Ok, I’m checking my electronic ignition system on my 72 Duster, 340. When checking voltage at the battery I got 12.43 volts. When I turn the ignition switch on I get 10.55 volts at input wire (disconnected from resistor) to the ballast resistor. With the wires connected to the resistor and the ignition switch on I get 4.75 volts at the positive side of the coil.

Your input on these numbers appreciated.
Click to expand...

THAT IS ONE HELL OF A LOT OF DROP

Do you have stock/ factory wiring?

You have diagrams/ shop manual?

"Review" the MAD article AND FOR THE DAMN NAYSAYERS whether YOU LIKE THE BYPASS OR NOT there are some VERY GOOD POINTS in that article whether "Danny boy" thinks "they are Chevy heads " OR NOT!!!!

Catalog

Now, on that page is THIS diagram:

amp-ga18.jpg


And the IMPORTANT THING about that diagram is that it is a very good simplified diagram of the MAIN POWER DISTRIBUTION in your car

Here is the "functional path" of current (backwards from electron flow AGAIN INFORMATION FOR THE NAYSAYERS) and what causes voltage drop, in no particular "importance"

From the battery, starter relay "big stud" (junction point) we have the FUSIBLE LINK. This and the TERMINAL in the BULKHEAD CONNECTOR is your first point of interest for voltage drop.

Onward we go to the AMMETER TERMINALS. This is your second point of possible drop, including the wire terminal ends, problems at the studs, and problems right inside the ammeter itself.

"Out" the ammeter (black wire), we go to the WELDED SPLICE

It is rare but these can, have and do FAIL

From the splice branches out and feeds several points.....depending on year/ make model

A big wire (hot UNFUSED) feeds the hot accessory buss in the fuse panel

A big UNFUSED wire feeds power to the ignition switch

Unfused power feeds to the LIGHT SWITCH

If that splice fails, IT IS NOT LOGICAL that all the branches will fail. One or two wires could break off the splice, and the rest might be OK. Do not assume!!

Then we go (from the splice) to the IGNITION SWITCH CONNECTOR, and THROUGH THE SWITCH and BACK OUT THE SWITCH CONNECTOR

All three of the above points are yet 3 more possibilities.

Out the switch on "ignition run" (IGN1) usually blue. THROUGH THE BULKHEAD CONNECTOR yet again.

ALL of the above listed points are possibilities for voltage drop

YOU NEED TO CHECK THEM UNDER LOAD. Connect every thing up "normal" get your meter clipped into the points you can reach easily "first" and then turn the key on to "run" long enough to take a reading. (Wiggle the key, too!!)

SOME THINGS TO START.................

If you look at that diagram, the black wire to the alternator is "downstream" from the ammeter and welded splice. So, with the engine not running, START by reading right at the alternator stud, then turn the key to "run" and see if it drops. Post both readings.

IF THAT READING DROPS it tells you that the drop is either the "red wire" bulkhead connector terminal, the ammeter, or the welded splice.

If that reading is OK, move to the ignition switch.

Take readings at the wire feeding TO the switch, and the IGN RUN feeding OUT of the switch.

If both those are OK, then the next suspect is the "ignition run" wire going through the bulkhead connector back out into the engine bay
 
Paladin06 said:
to the ballast resistor. With the wires connected to the resistor and the ignition switch on I get 4.75 volts at the positive side of the coil.
Click to expand...
'72 should be a dual ballast resistor. What is on your car?
The 5 ohm resistor has a wire going to the ECU - it will show a big voltage drop.
The 0.5 ohm resistor should have the wire going to the coil - it should not show nearly as much voltage drop.
One possibility is these are connected to the wrong resistors.
Another is that something in the coil or the ECU circuit is a drawing a bunch of current.

The drop to 10 volts from 12.5 even with the resistor's feed side (probably blue?, or blue with trace?) is something else. Power is going somewhere and its not the ECU or coil. See if disconnecting the alternator's field ground (green wire at the alternator) changes that reading. If not, try the field positive (probably blue?).
 
What voltage do you get at the battery with ignition OFF and what voltage do you get at the battery with the ignition ON?

The reason is to determine if there is a large draw some where, or if the battery is weak. If the voltage at the battery with ignition on or off doesn't change much than there is excessive resistance some where between the battery and the rest of the car causing you to loose volts. if it does change a lot than either the battery is weak or there is a large restive load somewhere.

Easy... bring car to parts store and have them check the charging system.
Easy... have parts store check the battery.
Easy... what is the voltage drop while cranking (ignition disabled so car wont start) what is battery voltage with ignition off, what is voltage while cranking

More complicated... wiring - bad connections, bad relays, alternator issue.

If you have an amp meter you can check the amperage draw of the various devices that are energized when the ignition is in the run position (engine not running), and see of they are excessive.
 
Mattax said:
'72 should be a dual ballast resistor. What is on your car?
The 5 ohm resistor has a wire going to the ECU - it will show a big voltage drop.
The 0.5 ohm resistor should have the wire going to the coil - it should not show nearly as much voltage drop.
One possibility is these are connected to the wrong resistors.
Another is that something in the coil or the ECU circuit is a drawing a bunch of current.

The drop to 10 volts from 12.5 even with the resistor's feed side (probably blue?, or blue with trace?) is something else. Power is going somewhere and its not the ECU or coil. See if disconnecting the alternator's field ground (green wire at the alternator) changes that reading. If not, try the field positive (probably blue?).
Click to expand...

When you say dual resistor do you mean 4 pole? My car has the 2 pole resistor.
 
67Dart273 said:
THAT IS ONE HELL OF A LOT OF DROP

Do you have stock/ factory wiring?

You have diagrams/ shop manual?

"Review" the MAD article AND FOR THE DAMN NAYSAYERS whether YOU LIKE THE BYPASS OR NOT there are some VERY GOOD POINTS in that article whether "Danny boy" thinks "they are Chevy heads " OR NOT!!!!

Catalog

Now, on that page is THIS diagram:

View attachment 1715161489

And the IMPORTANT THING about that diagram is that it is a very good simplified diagram of the MAIN POWER DISTRIBUTION in your car

Here is the "functional path" of current (backwards from electron flow AGAIN INFORMATION FOR THE NAYSAYERS) and what causes voltage drop, in no particular "importance"

From the battery, starter relay "big stud" (junction point) we have the FUSIBLE LINK. This and the TERMINAL in the BULKHEAD CONNECTOR is your first point of interest for voltage drop.

Onward we go to the AMMETER TERMINALS. This is your second point of possible drop, including the wire terminal ends, problems at the studs, and problems right inside the ammeter itself.

"Out" the ammeter (black wire), we go to the WELDED SPLICE

It is rare but these can, have and do FAIL

From the splice branches out and feeds several points.....depending on year/ make model

A big wire (hot UNFUSED) feeds the hot accessory buss in the fuse panel

A big UNFUSED wire feeds power to the ignition switch

Unfused power feeds to the LIGHT SWITCH

If that splice fails, IT IS NOT LOGICAL that all the branches will fail. One or two wires could break off the splice, and the rest might be OK. Do not assume!!

Then we go (from the splice) to the IGNITION SWITCH CONNECTOR, and THROUGH THE SWITCH and BACK OUT THE SWITCH CONNECTOR

All three of the above points are yet 3 more possibilities.

Out the switch on "ignition run" (IGN1) usually blue. THROUGH THE BULKHEAD CONNECTOR yet again.

ALL of the above listed points are possibilities for voltage drop

YOU NEED TO CHECK THEM UNDER LOAD. Connect every thing up "normal" get your meter clipped into the points you can reach easily "first" and then turn the key on to "run" long enough to take a reading. (Wiggle the key, too!!)

SOME THINGS TO START.................

If you look at that diagram, the black wire to the alternator is "downstream" from the ammeter and welded splice. So, with the engine not running, START by reading right at the alternator stud, then turn the key to "run" and see if it drops. Post both readings.

IF THAT READING DROPS it tells you that the drop is either the "red wire" bulkhead connector terminal, the ammeter, or the welded splice.

If that reading is OK, move to the ignition switch.

Take readings at the wire feeding TO the switch, and the IGN RUN feeding OUT of the switch.

If both those are OK, then the next suspect is the "ignition run" wire going through the bulkhead connector back out into the engine bay
Click to expand...
Hello I'm back,

OK from your note below:

SOME THINGS TO START.................

If you look at that diagram, the black wire to the alternator is "downstream" from the ammeter and welded splice. So, with the engine not running, START by reading right at the alternator stud, then turn the key to "run" and see if it drops. Post both readings.

IF THAT READING DROPS it tells you that the drop is either the "red wire" bulkhead connector terminal, the ammeter, or the welded splice.

With the switch off I read 12.67 volts at the block wire on the alternator. When I turn the switch on I can 11.47 volts.
 
That is a HELL OF a lot of drop in that circuit. Review the mad article (for the simplified diagram) and a good complete wiring diagram, follow along:

Catalog

amp-ga18.jpg


Above is a simplified diagram of the factory wiring, here is what you are measuring and the possible drop points. What follows is the FUNCTIONAL path, not electron flow

From the battery, cable, to the starter relay stud which also acts as a junction point, through the FUSIBLE LINK. That is likely not the problem here

On the red wire through the BULKHEAD CONNECTOR terminal. This is a likely suspect

On to the AMMETER both WIRE END TERMINALS and the AMMETER itself. All three of these as a group are a possibility.

Out the AMMETER on the black ammeter wire, to the WELDED SPLICE. These fail rarely but can, do and have.

On out the BULKHEAD CONNECTOR on the black wire to the alternator. While the BLACK wire terminal in the bulkhead connector can be a problem, you are not stressing it with current flow in your test. You are merely using the black wire from the alternator all the way to the welded splice as a test lead extension for your meter

JUMP BACK to the load bath. From the WELDED SPLICE the wire feeding power TO THE IGNITION SWITCH CONNECTOR, the SWITCH CONNECTOR, through the SWITCH, and back out the SWITCH CONNECTOR on the IGN1 (run) wire.

That group right there is suspect, the 2 terminals in the connector and the switch itself

Next, after leaving the SWITCH CONNECTOR, the IGN1 (run) goes back out the BULKHEAD CONNECTOR again. This is yet ANOTHER point that can cause problems

What to do from here?

Easy in theory, more difficult physically. Access points I mentioned that are easiest to get to first. Example, the ignition switch. Back probe the bulkhead connector terminals. "Wiggle" the switch/ switch connector, and bulkhead connector and see if the reading gets worse/ better.

If you can reach the ammeter with your hand, disconnect the battery for safety, reach up in there and clip a alligator to either of the ammeter terminals, make certain it's not shorting, and then hookup your meter there, and to the battery, hook battery back up, and see what that reads.

What you are TRYING to do is "go down the loss road" but since you cannot drive down it easily, you have to skip around to "get there."
 
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