I posted last week with problems pulling any power out of a 472 hemi. Anyway after hours of trouble shooting, It may be weak spark.
I installed a complete new under hood harness from Year one. The car is a 70 but I ordered an electronic ignition harness and it came with a dual ballast. The wiring was straight forward,but now with the key in the run position, The coil has 2.5 volts. I cant remember the exact number but I think the voltage should be up around 8 or 9. Am I correct? Any suggestions or Insight to the problem?
Thanks Glenn

ya, should have 7-9v with it running. check the voltage on both sides of the ballast resistor. your coil might be dragging it down.

This is with it running right?

disconnect the coil and see if you have 12v there in run

I think my brain is firing on 3 volts. I will fire it up and try it.... Large Coffee please

There's 5 volts on the pos side of the coil and 3 volts on the neg side... suggestions?

By the way, it's a dual ballast

Bump for the morning crowd

btw, why do you have a dual ballast resistor? How many pins does your ECU have.

The reason i am using a dual ballast? The old harness was trash so I ordered a new engine harness from year one for a 70 cuda with an incorporated electronic ign conversion (so I wouldnt have to cut up the new harness and convert it myself) and apparently the electronic conversions are dual ballast like the 73 up cars.
In hindsight I should have ordered an original harness and modified it my self

Cucu, the ECU is the MP chrome box, will this matter with the dual ballast?

well it could but probably not in your situation. the dual ballast resistor does not have the same values on both resistors. one resistor is for the coil and the other is for the ECU. but the harness is keyed so the connectors to the ballast can only go on one way. As long as you didnt cut the little pegs off the connectors, those pegs will fit into the holes or slot in the dueal ballast making sure they are connected correctly.

The chrome ECU is a 4 pin so the wire from the other side of the ballast get used.

The harness you bought was designed to work with both the 4 & 5 pin ECU. If you continue to have problems, give Tom at M&H Electric Fabricators, Inc a call. He built the harness and is very knowledgeable.
BTW: He's one of the good guys.

Cucu, The pegs are still there and the ballast resistor connectors will only connect one way.
I don't know if this matters but If I disconnect the Blue coil pos wire from the coil and check it with the ignition in the run position, A voltage test to the disconnected wire indicates 10.5 volts. Then reconnecting the wire to the positive side of the coil with the car running, with a volt meter red lead to coil pos and volmeter black lead to ground, volt tester reads around 5 volts.

that tells me you are losing 2v somewhere. Without the coil connected, there is no load onthe ballast. no load means no current means no voltage drop across the ballast- thats why you measure 10v with the coil disconnected. It should be the same on the other side of the ballast (same leg, just on the other side of the resistor) with the coil connected and disconnected.

At the same time, measure the system voltage at the battery, and at the batt terminal on the alt and at the stud on the starter relay. make all measurements with the ground lead on the neg battery post. then make them again with the neg lead on a chassis ground somewhere.

Sooner or later, we'll find out where those 2 v went

Cucu.... I appreciate your help man. I am heading home and will do a bunch of testing . Will talk to you tomorrow
Glenn

So, I borrowed a digital voltmeter from work and did a few checks. There is a 1 volt drop at the ballast from the battery and from the dicharge side of the ballast to the coil there is no voltage drop...
After that I removed the dual ballast. I had 2 single ballast, so I installed the M/P ballast and plugged the in electrical connectors so the unnecessary auxillary circuit was isolated. The result was 7.2 volts at the coil at idle. Then I tried the single ballast that came with the Msd coil and got 9 volts. Anyway, now I have a strong blue spark at all 8 plugs.
Thanks Again

remember this- corrosion is your enemy. it will cause power drops. Steel wool and sandpaper will help to clean them up.

sounds like youre getting it licked. soon, you'll be an expert. the voltmeter is your freind

Thanks again Cucu... 1st- 2 dead spark plugs, 2nd -vac advance in the wrong port, 3rd- low voltage at the coil.... now i can turn my attention to tuning the engine