Hey guys ran into an issue tonight. Ran my big tank out if fuel and air locked the system. Reprimed the fuel filter (whoo e-pump) and Bled what I could out of the injectors but only got halfway before the starter began to really poop out. With my new IP this is usually a quick process!

OK so my buddy came to save me, got her running and whipped out the multimeter.

Passenger batt: 14.1v engine running.
Driver's batt: 12.4v engine running.

Engine off Ohm test:
Ground post to ground post on batts: 0.0 ohms.
Positive post to positive post on batts: 52.5 ohms.

Does this mean my cable running between the two batteries needs fixing/replacing or can it also mean a bad battery? The batts are matched costco ones it just seems like the cable to me with the resistance numbers.

Thanks guys I'm sick today so hoping I can get a answer while resting.

 

Dielectric grease the battery posts on both positive and negative. Maybe clean the posts and clamps before using the grease. You have probably a buildup of oxidation thats creating resistance from second battery to main battery. If the cables are clamped onto the terminals that connect to the posts, you may need to clean the contacts then also put a thin coating of dielectric grease on it.

I had a weak start but after cleaning the contacts the 7.3 starts much quicker and better with both glow plugs going and cranking simultaneously.

 

Roger that. I'll have to get a better Ohm reading too, instead of post to post ill keep the leads on the cable itself and that will eliminate the connections and possible resistance there.

Started up great and got to work so at least I can worry about it over the weekend.

 

Seems like one or both clamps on the ends of the main positive cable are not getting a good connection to battery terminal clamps. Especially if this is a 'Do-It-Yourself' battery cable, in which case I highly recommend using tin-plated brass cable ends (very high corrosion resistance - marine use) that are soldered on using solder/flux pre-made slugs or regular solder and flux. The problem is finding one suitable for the Passenger side that can receive 3 cables although I did find some when I was looking into making my own cable assembly.
I ended up buying factory made one at O'Reilly's (Schucks, Kragen) for about $85.00 two years ago. It is a very well made high quality cable in case anyone wants to know. (I haven't installed it yet)


To test the main positive cable for good connection to the (+) battery clamps, with the engine running test the voltage across the terminals of the Passenger side battery.
In the first test you did, the charging voltage was 14.1V which is good. I think it's supposed to be 13.8V to 14.3V if I remember correctly.

THEN, while engine is still running, test the voltage from the (-)Negative post on the Passenger side battery to the (+)Positive cable CLAMP (not the post) on the Driver's Side.

It should read the same charging voltage as you got across the Passenger side battery.

Since you already know that the ground cables/connections are good (0 ohms between the negative battery posts) then you can also measure the charging voltage on the drivers side by testing from the (+)Positive battery CLAMP to good ground on the engine or brackets etc. The meter should show the same charging voltage as the passenger side test.

Engine Off: A resistance test (Ohms) between the two (+)Positive CLAMPS might show you something but I wouldn't go by that since I think that just a few strands in the cable with good continuity to the clamps would give you a good reading (0 ohms)

If this is an intermittent problem then a load test through the cable would be the best way to test it but I don't know at the moment exactly how to do that.

The only two probably things that I can think of (aside from what eagleye suggested) are: 1) Bad Ground Connection on the Driver's Side Ground Cable and 2) Bad Connection of the Main (+) Positive Cable to the (+)Positive Battery Clamps.

 

I was going after the obvious connections but the Ground is very important and those bolts to frames can corrode/oxidize/rust over time. Another good place to add a light coating of dielectric grease after the contacts to ground have been cleaned and polished. When Possible I like to lightly sand copper contacts as they usually oxidize over time and the sanding gives you fresh copper without oxidation just like plumber soldering copper pipe.

Soldering is always best copper connections to copper cable or brass. Don't try to solder lead contacts or the lead contacts will melt before the solder does. When you can't solder a crimp connection is best, but fill it with dielectric grease before placing cable into the crimp contact and crimping. This helps keep moisture out minimizing corrosion of the crimp contact. This is helpful with the heavy gauge battery cables.

The PO told me I would need new battery cables. The starter cable needed a patch. I used copper crimp connectors and a battery bolt to reconnect the short cable down to the starter cable. I rubbed any corrosion off the battery terminals and then applied a very light coating of dielectric grease. After doing this to my 7.3 both batteries are charging better and even. Start with the glowplugs and starter is super powerful and engine responds quickly to start.

 

If you have high resistance between the two positives, that takes the ground out of the picture (or at least puts them low on the suspect list). The obvious way to determine if it's the positive cross-cable and not the driver's side battery itself is to disconnect it from the driver's side battery and measure again.

 

Thanks for all the replies, guys! I did narrow it down to simple terminal corrosion. Haven't gotten after it yet but soon.

I should have said I switched to marine terminals, though it was just re-using the stock cables. Some are soldered, some crimped as I've had the solder melt out during hard starting once when trying to purge air from my fuel system two years ago.

I've cleaned the grounds and terminals and every thing very well in the past, seems like I'm prone to terminal corrosion.