Occasional starting difficulty with Digital HP CD box in my D200

[MopaR&D]

I have a 1972 D200 that I installed a Pertronix Digital HP ignition system on along with Summit billet (mag pickup) distributor. I have an issue where the voltage to the wire that tells the box to turn "on" sometimes drops below the minimum voltage during cranking. Oftentimes if I crank the engine, release the key (to RUN), then crank again the ignition won't fire and I have to turn the key to OFF, wait a second or two then try again and it'll fire up.

The truck originally had points ignition; I removed the ballast resistor and used a 3-way male spade connector to link the terminals from the 2 sides of the ballast together (start and run) together along with the "on" wire for the ignition box so that it gets an "on" signal whether the key is in START or RUN position. I find that when cranking, the voltage there sometimes drops just below 9V which is the minimum the ignition box needs to see at the "on" wire.

The rest of the ignition box wiring is per the instructions; main power leads directly to battery terminals and trigger wire leads to mag pickup in distributor, plus tach signal. Once running it has no issues (stalling out etc.) and if I go to restart the truck when it's still warm I usually don't have this issue. I searched and saw in another thread that the factory electronic ignition has a 0.5-ohm resistor going to the ignition module so that it sees enough voltage during cranking, might that be my solution? If so where would I put the resistor?

 

[Sterling]

I don’t think that you want a resistor in the circuit, that would lower the voltage. If you only have 9 volts at the terminal, I would think that you have high resistance in the circuit. Could be the connections, or the ignition switch .

 

[MopaR&D]

This past weekend I redid the engine-to-frame ground, also cleaned and added some conductive paste (copper anti-seize) to the starter-to-bellhousing bolts thinking that wasn't grounded well. The starter turns over a bit faster but the voltage drop when cranking still seems to be pretty high. I also bypassed the bulkhead connections for the ammeter and alternator charge wires a couple weeks ago, that improved some other things (brighter lights, amp gauge reads more steady) but didn't fix the issue.

I'm going to check the rest of the connections in the starting circuit to see if there's something funky going on. I'm likely just going to put a relay in and have a wire coming straight from the battery + to feed the ignition box "on" wire and also the sensing wire for the voltage regulator. I have a feeling the new ignition switch I put in the truck a few years back wasn't good quality and has some resistance in it.

 

[Woodys_Cuda]

I agree with Sterling, no resistor. My MSD ignition box requires a power wire directly to the battery and resistor bypass. I think yours would require the same.

 

[TT5.9mag]

How old is the battery?

 

[MopaR&D]

I agree with Sterling, no resistor. My MSD ignition box requires a power wire directly to the battery and resistor bypass. I think yours would require the same.

The power wires are connected directly to the battery. My issue is with the wire that tells the module to turn on, supposed to be connected to 12V ignition on source.

How old is the battery?

About 4 years, it's a Duracell from Batteries Plus so I'd be surprised if it's already going bad but the truck has always been parked outside and until last Spring sat through multiple Colorado winters and summers. I'll check the voltage after it's been sitting a few days.

 

[TT5.9mag]

Have the battery load tested.

 

[MopaR&D]

I was planning to have the battery load tested but I went out today after the truck had been sitting a couple days and the voltage was at 12.6 at the terminals. Then while cranking over the voltage at the battery terminals only dropped to 11.3 which is 2.5 more than the ignition box needs at the ON signal wire. I'm going to start looking in the dash for wiring issues, clearly there's something going on if the voltage is only 9 during cranking in the ignition RUN circuit but over 11 at the battery itself.

I've had batteries go bad before and you can tell by how the starter cranks slower. Last one in my Duster got low enough that it wouldn't trigger the Mallory CD box to turn on and it was obvious because the starter cranked much slower than normal.

 

[Mattax]

I was planning to have the battery load tested but I went out today after the truck had been sitting a couple days and the voltage was at 12.6 at the terminals. Then while cranking over the voltage at the battery terminals only dropped to 11.3 which is 2.5 more than the ignition box needs at the ON signal wire. I'm going to start looking in the dash for wiring issues, clearly there's something going on if the voltage is only 9 during cranking in the ignition RUN circuit but over 11 at the battery itself.

Do a voltage drop test. If there is a difference during starting along the circuit the meter will show it.

I haven't had any reason to look at the RTR, but the regular CD box has a seperate direct power source - the run wire is only signal to the relay inside the box. Whereas with a regular points or Chrysler ECU, ignition run is power when running, and ignition start is power when starting.

I searched and saw in another thread that the factory electronic ignition has a 0.5-ohm resistor going to the ignition module so that it sees enough voltage during cranking, might that be my solution? If so where would I put the resistor?

No.
The factory resistors worked like this.
During start - power goes to the ECU through the 1/2 resistor.
During run - the power was controlled by a combination of direct (14 Volts) and going through the 5 Ohm resistor.

 

[MopaR&D]

I think I figured it out... I started thinking about grounding and after looking around I didn't see any ground straps going from the cab to the frame. I assume the factory relied on the cab mount bolts for electrical grounding. Well mine have some good surface rust being 50+ years old so I figured adding a ground couldn't hurt. I added a wire going from the engine to the firewall and the voltage to the ON wire for the ignition box went up a decent amount, it's now about 9.8V during cranking. Still seems to be some voltage drop through the ignition switch but I haven't had any starting issues since.