LarrBeard
11-21-2015, 08:57 AM
OK, time to quit whining and figure out just why the starter won’t spin the engine over.
I spent a morning or so looking at information about 6-volt systems. Since I’ve had this truck all my life (well, 60 some odd years of it anyway), I figured I knew enough about 6-volt systems to figure this out. Well, even old coots can get better information and make more decisions if they sit down and read the information that the internet lays out for them.
It’s one thing to just say “6-volt systems take more current”, but when you get to reading you appreciate just how much current “more” really means. At no load, with 5.5 volts at the power stud on the starter, the motor should spin at 4300 RPM (that’s really fast) and it will draw 70 amps. Now, load up the starter with a new, tight, engine. If the starter stalls trying to turn the engine over, the starter specs say that it will be drawing 420 amps with 3-volts at the stud. That’s six times the no-load current, never mind that the voltage has dropped to 3 volts. Now, one bit of technical stuff here. Power wasted in cables, switch contacts, dirty battery terminals and rusty mating surfaces for cable terminals is proportional to the square of the current flowing. The no-load current is 70 amps, the stall current is 420 amps, a ratio of 6-times. Square that 6, and you see that there is 36 more times power wasted during a hard cranking cycle than just the motor spinning on the bench. This begins to make an impression about why 6-volt battery cables need to be hefty.
I’ll guess that I had been lucky with the old engine before the rebuild. It was well worn, so it didn’t take a lot of energy to spin it over. I had gotten away with chintzy 12-volt parts store battery cables from the old days. If you look at the battery cable pictures for original 6-volt Jeeps (internet again), you see that the cables were of heroic proportions. Specs I see for original cables are for #1 AWG cables, modern 12-volt cables are at best #2 AWG and some can be #3 or #4 with big insulation on them. A #2 AWG cable has about a third more resistance than a #1 AWG cable. In a 6-volt system where every tenth of a volt counts, you lose with #2 AWG cables. Don’t opt out for convenience, get the good cables!
OK, now back to the starter story. The starter has been back to the rebuild shop twice; once for a new armature and once for a check-up where the guy put in new bushings. At this point we’re betting that the basic starter was OK, so we pulled the switch. We had assumed that the switch was new (yeah, that word – *** U ME), but once we got it off the core - WHOA! It wasn’t new – it looked like it had been sandblasted to clean it, but the contact surfaces were badly eroded. Ok, no fooling around, add a new switch to the cable order. Cables and switch will arrive just in time for Thanksgiving break.
Now, let’s do some scheming and planning to get all the power to the starter motor. The way that things were set up, the negative cable of the battery goes to a boss on the engine just under the rear of the starter. That boss also has the starter support bracket on it. The back of the engine butts up to the bell housing cover plate. The starter mounts to the cover plate. As we looked at the current path from the engine block to the starter housing, the engine/cover plate interface is a big part of that path. How good is that as an electrical connection? Trust nothing. We decided to connect the negative battery cable to one of the mounting ears where the engine bolts to the bell housing. (Hmmm, we could have just gone to the starter flange…) Then, to ‘way over engineer it, we’re adding a #2 AWG jumper from that point to one of the bolts that ties the cover plate to the bell housing. That ties everything to the cover plate electrically as well as mechanically. We made one more pass to clean the area where the starter mounts up to the cover plate, as well as the flange on the starter as well. We even considered a jumper from the starter flange to a cover plate bolt, but we’ll pass on that for now. Oh yeah, we’re adding the braided strap across the motor mounts to the frame to make sure that lights and such see a good ground.
Now. If that doesn’t crank that engine…..
And, yeah – if we’d gone to a 12-volt system ….. but I wanted it original. I’m old and grouchy- humor me.
12/29 Update:
Here is a photo of the battery with the serious new cables. It spins over really nicely now. And, yes - the battery is a 100 amp-hour like the original. Batteries have gotten more size efficient in the last 70 years.
I spent a morning or so looking at information about 6-volt systems. Since I’ve had this truck all my life (well, 60 some odd years of it anyway), I figured I knew enough about 6-volt systems to figure this out. Well, even old coots can get better information and make more decisions if they sit down and read the information that the internet lays out for them.
It’s one thing to just say “6-volt systems take more current”, but when you get to reading you appreciate just how much current “more” really means. At no load, with 5.5 volts at the power stud on the starter, the motor should spin at 4300 RPM (that’s really fast) and it will draw 70 amps. Now, load up the starter with a new, tight, engine. If the starter stalls trying to turn the engine over, the starter specs say that it will be drawing 420 amps with 3-volts at the stud. That’s six times the no-load current, never mind that the voltage has dropped to 3 volts. Now, one bit of technical stuff here. Power wasted in cables, switch contacts, dirty battery terminals and rusty mating surfaces for cable terminals is proportional to the square of the current flowing. The no-load current is 70 amps, the stall current is 420 amps, a ratio of 6-times. Square that 6, and you see that there is 36 more times power wasted during a hard cranking cycle than just the motor spinning on the bench. This begins to make an impression about why 6-volt battery cables need to be hefty.
I’ll guess that I had been lucky with the old engine before the rebuild. It was well worn, so it didn’t take a lot of energy to spin it over. I had gotten away with chintzy 12-volt parts store battery cables from the old days. If you look at the battery cable pictures for original 6-volt Jeeps (internet again), you see that the cables were of heroic proportions. Specs I see for original cables are for #1 AWG cables, modern 12-volt cables are at best #2 AWG and some can be #3 or #4 with big insulation on them. A #2 AWG cable has about a third more resistance than a #1 AWG cable. In a 6-volt system where every tenth of a volt counts, you lose with #2 AWG cables. Don’t opt out for convenience, get the good cables!
OK, now back to the starter story. The starter has been back to the rebuild shop twice; once for a new armature and once for a check-up where the guy put in new bushings. At this point we’re betting that the basic starter was OK, so we pulled the switch. We had assumed that the switch was new (yeah, that word – *** U ME), but once we got it off the core - WHOA! It wasn’t new – it looked like it had been sandblasted to clean it, but the contact surfaces were badly eroded. Ok, no fooling around, add a new switch to the cable order. Cables and switch will arrive just in time for Thanksgiving break.
Now, let’s do some scheming and planning to get all the power to the starter motor. The way that things were set up, the negative cable of the battery goes to a boss on the engine just under the rear of the starter. That boss also has the starter support bracket on it. The back of the engine butts up to the bell housing cover plate. The starter mounts to the cover plate. As we looked at the current path from the engine block to the starter housing, the engine/cover plate interface is a big part of that path. How good is that as an electrical connection? Trust nothing. We decided to connect the negative battery cable to one of the mounting ears where the engine bolts to the bell housing. (Hmmm, we could have just gone to the starter flange…) Then, to ‘way over engineer it, we’re adding a #2 AWG jumper from that point to one of the bolts that ties the cover plate to the bell housing. That ties everything to the cover plate electrically as well as mechanically. We made one more pass to clean the area where the starter mounts up to the cover plate, as well as the flange on the starter as well. We even considered a jumper from the starter flange to a cover plate bolt, but we’ll pass on that for now. Oh yeah, we’re adding the braided strap across the motor mounts to the frame to make sure that lights and such see a good ground.
Now. If that doesn’t crank that engine…..
And, yeah – if we’d gone to a 12-volt system ….. but I wanted it original. I’m old and grouchy- humor me.
12/29 Update:
Here is a photo of the battery with the serious new cables. It spins over really nicely now. And, yes - the battery is a 100 amp-hour like the original. Batteries have gotten more size efficient in the last 70 years.