Low Voltage to the coil

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Paladin06

Paladin06

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OK, I've been experiencing a number of ignition problems with my 72 Plymouth Duster 340. The latest problem is I only read 3.5 volts at the input to the coil. The ignition unit, ballast resistor, ignition switch and coil all have been replaced.

Please HELP!!
 
That may be normal, depending.

You need to find out if the coil/ ballast COMBINATION you have is correct, that is, a heavier, hi - ratio coil will draw MORE current, and pull supply voltage DOWN through the ballast

The reason these are called "ballasts" is because they do change resistance with heat. It's "how they work."

A better test is to check the "key side" of the ballast. Better yet is to measure "harness voltage drop" directly

To do this, turn the key to "run" with engine OFF. Hook one meter probe to battery POS, and the other to the key side of the ballast (opposite from the coil side)

You are hoping for a VERY low reading the lower the better

Anything MORE than .3v (three tenths of a volt) means you need to look for a problem

Your top suspects are the bulkhead connector, the ignition switch connector, and the switch itself

The circuit path involved here is battery -- fuse link -- bulkhead connector -- ammeter circuit -- ignition switch connector -- through the switch -- back out the connector (dark blue, IGN run) -- back out the bulkhead connector -- to the "key" side of the ballast

When making this test, if you have breaker points ignition, make sure the points are closed.
 
Hello,

My 72 does not have points. It has the pointless distributor. As stated we have already replaced the ignition switch, so I would assume we can eliminate that as a possible failure.

What started this whole problem was the car started fine the first start of the day, after that, if you let the car sit for 5 minutes or more, it took 6 to 10 seconds of cracking to fire up. If you ran it for a while and did an immediate shut down and restart it would fire right up.

Please advise.
 
What kind of ignition you running? If you are running a Mopar ECU you do not normally want to bypass the resistor on the street

It seems to be a mixed bag (depending on coil type) for those like me running a GM HEI module. I checked mine for heat carefully over some hot day's running, and it seems to do fine.

Other systems, MSD, etc, vary. Follow their destructions.

AND if you are having a voltage drop problem (poor connection) bypassing the ballast wont' help
 
I run a Blaster 2 with resistor supplied with Blaster 2, orange box spark control on my 67, and it runs and starts very well.

I suspect a voltage drop in the ignition circuit as mentioned above often caused by poor connections in bulkhead connector, and ignition switch. Other reasons for low voltage at coil can be caused by electric choke being feed from ignition side of ballast resistor, and poor grounding of spark controller, voltage regulator, and alternator cases.

A ground loop picking up the various cases via a conductor attached by one of each device’s mounting screws leading back to negative battery terminal will provide proper ground, and eliminate voltage drop on the ground side of ignition circuit.

Best way to eliminate resistance at bulkhead connections is to remove and replace the brass connections. This is tedious one wire at a time work, but brings the bulkhead connector back to like new condition. Second best way to eliminate bulkhead resistance is to just clean, and tighten each existing brass connection.

Also clean and remake battery terminal connections, and ground attachment points at block, fire wall, and while you are at it headlight ground to sheet metal connections.

If equipped, powering electric choke from under hood source (battery alternator starter relay) with a relay triggered from ignition circuit will eliminate almost all voltage drop caused by the choke at the coil.

I had my choke directly powered from ignition circuit; it caused a one volt drop at coil. A relay system controlling choke corrected the voltage drop.
 
Latest development. We have determined that with the negative wire disconnected from the coil we have the correct voltage at the positive terminal of the coil. When we connect the negative wire the voltage drops to 3.5.

Now we have determined that I have the incorrect ballast resistor for the Blaster 2 coil. I need a .8 ballast resistor for this coil application. I can only find the .8 in the 2 pin configuration.

Can I change from a four pin ballast to a two pin configuration and if so how??

HELP...
 
IF you are running a newer 4 pin ECU, you can use a 2 pin resistor

Just trace the wiring. The wires at ONE end of the old 4 pin resistor will be jumpered together. SO that goes on one end of the new 2 pin resistor

On the other end of the resistor, (the end with the "U" shaped cutout in the ceramic) one wire feeds ONLY and DIRECTLY to one pin of the ECU. This is the un-used wire of the 4 pin ECU


If your ECU only has 4 physical pins, you are home free. IF it has 5, you'll have to use an ohmeter to see if the "5th" pin is internally hooked to anything

From "MyMopar:"

http://www.mymopar.com/index.php?pid=31

4 pin ECU, 2 pin resistor:

Notice the "blank" in the connector pattern of the ECU

Ignition_System_4pin.jpg


5 pin ECU/ 4 pin resistor

In this diagram, the wire off the LOWER LEFT of the resistor is the part that is eliminated with a 4 pin ECU/ 2 pin resistor

Ignition_System_5pin.jpg
 
Please excuse my ignorance but, I have a new 4 pin ignition module and new 2 pin ballast. I want to replace the 5 pin ignition module and four pin ballast. Can you diagram the conversion?

Thanks so much for your help.

Greg
 
I thought I did. Look at the above two diagrams.

Notice that on the bottom diagram THE TOP END of the resistor is jumpered across. One of these connectors will hook to the new 2 prong connector. So if there's a dual connector there, you can either leave it, or cut right at the connector and replace the one end as necessary

The only other thing you do (for appearance) is remove the ONE wire shown in the bottom diagram, that goes from the BOTTOM LEFT of the resistor, down to the ECU connector.

You don't HAVE to do this---it's only for appearance. You can actually just hook the new 2 pin ballast as indicated to the "right side" of the ballast in the bottom diagram. The one wire going to the "5th" ECU pin will simply not connect to anything.
 
For others reading, if upgrading from points to electronic ignition, better to use the GM HEI module and eliminate the ballast. These questions with the old Mopar electronic ignition come up regularly. It is an outdated system and confusing to install and maintain. If you have an SB or RB, look at the "read to run distributor" (search ebay).
 
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