Voltage drop question

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Uncle Bob

Uncle Bob

Shiny paint causes stress.
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I've got a voltage drop problem and am looking for ideas.

I'm using a 4-pin ECU and a 2-connector ballast.

With the key in the RUN position, I measure about 5 volts at the coil or the ballast resistor, with it in START I get about 8 volts.

When I unplug the ECU, according to my notes I get 12 volts in either START or RUN, which doesn't entirely make sense why it wouldn't be less in RUN but maybe I measured from the wrong terminal. Either way, there is a huge voltage drop with the ECU connected, and it goes away if I disconnect it.

I double-checked all the ECU connections. It takes 12v off the blue wire at the ballast resistor, as it should. The ECU harness is new.

I replaced the ECU and ensured it is grounded. No change, still a big drop with a different ECU plugged in.

The car has been in the process of getting a new engine for a while now, but AFAIK it was doing this before I took it apart with the exception that I didn't check it with the ECU unplugged. It did start and run though.

I'm hoping to get the voltage up since it's time for first fire-up and I want to be sure everything is as right as it can be before trying that.

Any suggestions on what could be wrong or things I should check are appreciated.
 
What are you experiencing? What are the symptoms? What made you check voltage drop in the first place?
Is your ballast resistor backwards?
 
ECU draws alot of amps in the run position engine off. 4 amps usually running 2.5 ampsish. Prob what you are seeing.
 
I had a hard starting / no starting issue I went through couple months back. If I unplugged voltage regulator car would start immediately. Reason is Voltage Regulator was sending 5 amps or so to the alternator field circuit and voltage regulator is on same circuit as coil. So when I unplugged the voltage regulator that reduced the number of amps that circuit had to flow freeing up the amps going to the coil.

To resolve I had to clean the thick Red and Black wire terminals in the bulkhead (pulled out of bulkhead used small wire brush), cleaned terminals at ignition switch, and new ignition switch. I went from 8 to 11 volts at the coil when cranking (Start position) as a result.

One thing to note, I had zero ohms through all of my circuits when tested with a multi meter. However, ohm testing does not test circuits under load (amps flowing). So what I learned is you really have to do voltage drop tests to know if circuits can flow the amps for the given load.

Ohms Law.png
 
Does make it easier to understand. Any time.
hey a load light will usually tell you if a circuit can handle amps. Easy to make one. I use a headlight
 
Last edited:
340sFastback said:
I stole that from your post. When people steal your stuff that means it was good. So thanks for posting that up.
Click to expand...
Every time I post stuff I get yelled at? Does that mean it’s good or bad.
Sorry - just adding humor to the puzzle.
 
Syleng1 said:
What are you experiencing? What are the symptoms? What made you check voltage drop in the first place?
Is your ballast resistor backwards?
Click to expand...
Getting ready for first start on a new engine after repainting the engine compartment and reworking the harness from a BB to SB configuration. Checked voltage as part of making sure it is right. Basically I expected 12 volts, got 7-8.

I did not know you could install a ballast backward. It's screwed to the firewall with the screw above the terminals, blue wires on the terminal toward the center of the car.

The voltage is A-OK with the ECU disconnected.

I think this explains it maybe:

halifaxhops said:
ECU draws alot of amps in the run position engine off. 4 amps usually running 2.5 ampsish. Prob what you are seeing.
Click to expand...
That makes sense, thanks. When I noticed low voltage before, I would've been doing it with the engine off too.
 
Your voltage readings sound normal to me. Without some theory in electrical circuitry, it can be difficult to understand the numbers you are getting. Just sayin...
 
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