Cranking Voltage Drop Test Today

[Anteek29]

I'm running a stock 6V system in my '55 Fairlane. Out of curiosity I decided to measure the voltage present at my starter during cold cranking because the engine turns over real slow - even with a fairly new, full charged battery.
Under crank load the voltage at the starter motor was 3.5 volts.
Under crank load the voltage at the battery posts was 5.5 volts.
This tells me I'm losing 2 volts in the supply path...no wonder the engine barely turns over. Obviously, all the serial connections will be cleaned and tightened. Yes, the cables are the heavy gauge ones and I used an analog meter.

 

[dmsfrr]

How old are the existing cables?
Are all the cable terminals clean and tight? Inside the crimped ends too?
Clean connections on the (large) Ground cable from the block or head directly to the battery?

 

[Anteek29]

How old are the existing cables?
Are all the cable terminals clean and tight? Inside the crimped ends too?
Clean connections on the (large) Ground cable from the block or head directly to the battery?

Those are all the proper critical points that I will be addressing.

 

[Daves55Sedan]

5.5 volts at the battery is low to begin with. Your generator regulator is probably adjusted to cut out at around 7.4 volts when engine is running and battery fully charged.
Could be that the ground connection to the starter needs to be beefed up. I ran a heavy cable from an engine block bolt to one of the starter mounting bolts to improve ground.
Make sure that oil and grease are cleaned away from all electrical cable connections.
The negative (-) cables from batt to starter relay and from starter relay to starter terminal may look good on the outside, but if you disconnect the ring-tonque terminals of the cables and look closely where the cable is inserted into to terminal and see a greenish tarnish, that may be the problem point. Usually, cable replacement is the only way to fix this, unless you can snip off the old terminal, strip the cable insulation off enough to put on a new terminal. And would doing this make the cable too short? What about if the copper cable is tarnished under the cable jacket you stripped off too. At that time, I would throw them away and get new ones.
BTW, this cable jacket (thermoplastic insulation) can shrink after a while and begin to corrode (tarnish) after a while.
I also have the stock 6-volt system in my '55 Ford car. Upon first seeing the insulation shrink away from the terminals on my cables, I got out a household caulking gun with clear silicone caulk and caulked around the area that was affected by the shrinkage to SEAL between the terminal and insulation. That has kept corrosion off the cables for decades. Mine still starts like new.

 

[Anteek29]

Thanks, Dave. When I bought the car several years ago an 8 volt battery had been installed so obviously, the prior owner band-aided a starting deficiency. Sure, 8 volts kick starts a car BUT light bulbs and radio tubes burn out faster.

 

[Daves55Sedan]

Automotive electical components can operate normally between a range of plus or minus 10% of specified voltage of the components (if not more, say 15%).
Actual voltage at individual components will fluctuate depending upon battery charge and generator output and cutoff by the regulator contacts.
I'm still using many original 6-volt lightbulbs in my '55, including headlamps, although the headlamps are noticably dimmer than they were 50 years ago, but that is not due to varying voltage, it is mostly due to age of lamp.
The effect that a lamp sees during full battery charge is full brightness, but if the voltage dips to below the specified voltage, the lamp gets dimmer.
Normally fluctuating voltages within the 10 - 15% range will not affect lamp life very much.