5 volts across coil and ballast

Posted By: urdustd

5 volts across coil and ballast - 02/16/11 03:12 AM

I've been searching throughout the electrical and fuel systems on my small block trying to figure out why it won't idle with out rattling (sounds like occasional dieseling). Well I finally pulled out the multimeter and I only have 5 volts across the coil and ballast resistor. I figured I would have 12.8V minus the drop across the ballast, so maybe 10 or so volts.

The car is a 74 Duster with dual field alternator, electronic voltage regulator, MP Orange box, single ballast and a MP electronic distributor and a Blaster 2 coil. Prior to my new findings above, I swapped coils, ballast, cap, wires, ECU and regapped and check resistance across the pickup coil. None of these changes made any difference to how it ran.

Does anyone have any idea why the voltage is so low at the coil? I visually checked the wiring, but everything looks pretty good as the engine harness was replaced 8-10 years ago.

Posted By: RapidRobert

Re: 5 volts across coil and ballast - 02/16/11 04:32 AM

Quote:

to figure out why it won't idle with out rattling (sounds like occasional dieseling).

More info, what it's doing at idle. What year, how long doing it, what recent mods, esp any preceding it acting up

Posted By: urdustd

Re: 5 volts across coil and ballast - 02/16/11 05:16 AM

Ran great than I pulled engine to fix leak in oil pan, changed converter, steering box and headers. Put engine back in and have had this idle issue ever since. Seems fine above 2000 rpm, below it seems to spark knock. Initially intermittently as in every few seconds, as it warms up seems to be almost constant, like a dieseling engine. Above 2000 rpm seems just fine.
74 Duster 360 - 10:1 compression 360, Hughes 2430 cam, air gap, Edelbrock 750 carb. It took me a few months to get all the work done (long story), so I pumped old fuel from tank (probably a gallon or so left) and refilled with 50/50 mix of 100 and 91 octane unleaded.
Making me nuts....

Posted By: RapidRobert

Re: 5 volts across coil and ballast - 02/16/11 05:28 AM

it's not a spark knock, I think I've heard it b 4 it's like a hiccup where it misses and a shudder at the same time for a split second ??. Is that it or something else? Trying to get a handle on what it's doing

Posted By: urdustd

Re: 5 volts across coil and ballast - 02/16/11 05:41 AM

That's what it initially did, but much worse now. Sure seems like spark knock to me. Not sure how else to describe it. It really sounds like a 70's GM car dieseling.

Posted By: RapidRobert

Re: 5 volts across coil and ballast - 02/16/11 05:53 AM

jump full 12V to coil positive pri terminal to bypass the ballast for 5 seconds to elim/confirm that. Gotta hit the hay it's 11PM here, I'll sleep on it.

Posted By: urdustd

Re: 5 volts across coil and ballast - 02/16/11 06:03 AM

I'll give it a shot tomorrow...late here too.

I'll see if I got a video from when I had the hiccup. Right after reassembly. Probably should video what it's doing now too. A picture/video is worth hundreds of typed words.

Video Link

Thanks

Posted By: Yancy Derringer

Re: 5 volts across coil and ballast - 02/16/11 06:09 AM

Quote:

I only have 5 volts across the coil and ballast resistor. .........................
Does anyone have any idea why the voltage is so low at the coil?

You need to measure the voltage

ground probe on the battery negative

pos probe on the "top" of the ballast --coming from the switch.

Voltage RUNNING then compared across battery should be within .2--.3v that's THENTHS of volts.

If not, you have problems with the bulkhead connector going INTO the dash, or the connector on the ignition switch or the switch itself, or the bulkhead connection coming OUT of the dash. All these things add up to "voltage drop."

If you have not modified the ammeter or bypassed it, there's a couple MORE places for drop to occur.

As someone said, make up a clip lead, and hotwire direct from the battery to the switch side of the ballast and see it the problem disappears.

Would not be a bad time to make sure charging voltage is within limits.

Posted By: Sinitro

Re: 5 volts across coil and ballast - 02/16/11 09:01 AM

Quote:

I've been searching throughout the electrical and fuel systems on my small block trying to figure out why it won't idle with out rattling (sounds like occasional dieseling). Well I finally pulled out the multimeter and I only have 5 volts across the coil and ballast resistor. I figured I would have 12.8V minus the drop across the ballast, so maybe 10 or so volts.

The car is a 74 Duster with dual field alternator, electronic voltage regulator, MP Orange box, single ballast and a MP electronic distributor and a Blaster 2 coil. Prior to my new findings above, I swapped coils, ballast, cap, wires, ECU and regapped and check resistance across the pickup coil. None of these changes made any difference to how it ran.

Does anyone have any idea why the voltage is so low at the coil? I visually checked the wiring, but everything looks pretty good as the engine harness was replaced 8-10 years ago.

What is the resistance of the ballast resistor?,
The original was 1.25 Ohms but with the Blaster II coil you should be using a 0.25 Ohm..
Big difference in coil voltage between these.

Just my $0.02...

Posted By: TooMany62s

Re: 5 volts across coil and ballast - 02/16/11 12:14 PM

Try measuring the voltage with the positive lead to the coil disconnected. I'm not sure but I don't think you'll get an accurate reading with the coil in the circuit. Make sure #5 and #7 plug wires are not reversed.

Posted By: Mr.Yuck

Re: 5 volts across coil and ballast - 02/16/11 01:23 PM

you need a new ballast. My chager did the same thing as soon as I dropped it in gear it would flutter and die. Swap it out and you'll be ok.

Posted By: urdustd

Re: 5 volts across coil and ballast - 02/16/11 02:37 PM

I've tried 3 different ballast resistors, two Wells Ampco (4-wire) and 1 MSD (2 wire, low resistance) and they all run the same. I didn't check the difference in voltage at the coil as there really didn't appear to be needed as it still knocked. The voltage readings I got were with the MSD ballast.

I need to check the charging voltage to rule out the alternator and voltage regulator. I will also disconnect and clean all the bulkhead and other connectors in the harness to be sure I don't have a massive voltage drop somewhere in it.

I will also disconnect the coil and check voltage, but I vaguely remember checking years ago and it was ~11V, but who knows for sure as I'm getting old and can't remember schmidt, if you know what I mean!
I appreciate everyone's input so don't be shy if you have an idea.
Added YouTube link to video, listen to beginning (idle) of video for the knock YouTube

Posted By: Mr.Yuck

Re: 5 volts across coil and ballast - 02/16/11 03:01 PM

start at the battery and work backwards. The drop in volts is the cause.

Posted By: can.al

Re: 5 volts across coil and ballast - 02/16/11 03:41 PM

..had about 3 volts across my coil when running.
cleaned every elec connection with scotchbrite and dressed with dielectric grease,including the bulkhead
voltage across coil jumped to about 8
changed the plugs and added a bit more initial timing...prob solved.

Posted By: YO7_A66

Re: 5 volts across coil and ballast - 02/16/11 03:48 PM

I had 4.5 volts at the coil (when I was running a ballast setup) which resulted in a poor and inconsistant idle. So I also cleaned all of the connectors and replaced the ones that had too much of the original wire showing before the terminal. Then I too cleaned the bulkhead fitting and then had 8+ volts.

Since then, I now have a voltage guage to read the coil voltage and one that reads the battery voltage. This way I can reference both voltages from inside of the car while driving when needed. They are both hidden inside of the glove box.

Posted By: roe

Re: 5 volts across coil and ballast - 02/16/11 04:36 PM

what is the best way to disconnect all the connections at the bulkhead without damaging the connectors? I may need to pull mine and clean/grease rhem with a good dielectric

roe

Posted By: Mr.Yuck

Re: 5 volts across coil and ballast - 02/16/11 04:46 PM

Quote:

what is the best way to disconnect all the connections at the bulkhead without damaging the connectors? I may need to pull mine and clean/grease rhem with a good dielectric

roe

outside (engine side) is pretty easy just be careful with the tabs as they get brittle w/ age. Under the dash can be a PITB, just take your time and be care not to yank on anything and they should all come out. I took a small flat head screw driver and folded a small piece of sand paper to it..I ran it in and out of every contact point, made sure everything was nice and shiney before I sprayed it and pluged it all back together.