Nashville TJ's Build - Continued
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Wheelin' with Victor a few weeks ago I had a problem with the rig. With a completely full tank of fuel, sitting way off-camber to the passenger side on an obstacle, Vic tells me I'm leaking fuel. Climbed under the rig and could clearly see it coming from the passenger side vent on top of the tank. My guess at the time was that that plastic vent was hitting the bed, and had been damaged.
Now I'm a stickler about clearance, and when I set up that tank I made sure there was enough clearance on both of those vents (there are two), so I was a bit perplexed.
After leveling out and burning off a bit of fuel, it was not a problem for the rest of the trip.
Victor has had this problem with his vents in the past, and was able to give me the part number. It was available on Amazon, so ordering was easy. The replacement? not so much as I have to drop the tank.
So yesterday I took the time to pull the tank and replace the busted vent.
First up - drain the tank. Easily done with a hose attached via the dash-6 AN fitting on the fuel line...
... and a jumper on the fuel pump relay:
Empty fuel tank in about five minutes:
Here is the offending vent. It's tight, but even looking at it here there seems to be enough clearance. I just realized that there is a clue as to what happened in this pic, but I had not seen it at this point.
Now I'm a stickler about clearance, and when I set up that tank I made sure there was enough clearance on both of those vents (there are two), so I was a bit perplexed.
After leveling out and burning off a bit of fuel, it was not a problem for the rest of the trip.
Victor has had this problem with his vents in the past, and was able to give me the part number. It was available on Amazon, so ordering was easy. The replacement? not so much as I have to drop the tank.
So yesterday I took the time to pull the tank and replace the busted vent.
First up - drain the tank. Easily done with a hose attached via the dash-6 AN fitting on the fuel line...
... and a jumper on the fuel pump relay:
Empty fuel tank in about five minutes:
Here is the offending vent. It's tight, but even looking at it here there seems to be enough clearance. I just realized that there is a clue as to what happened in this pic, but I had not seen it at this point.
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So as I'm getting ready to pull the tank, I see this...
...and this...
...and instantly knew what happened. Those two holes are for the 5/16" stainless bolts which attached the front passenger side tank skid mount to the frame. They had not come loose - they were both sheared off. A piece of one was still stuck in the hole.
That allowed the tank to be pushed up on that side and drive the vent into the bed support. Mystery solved.
When you play hard, you break hard. Clearly those 5/16" bolts where not up to the task. When I put it all back together, I'll go with two 7/16" or 1/2" stainless bolts on each side. That should be enough.
...and this...
...and instantly knew what happened. Those two holes are for the 5/16" stainless bolts which attached the front passenger side tank skid mount to the frame. They had not come loose - they were both sheared off. A piece of one was still stuck in the hole.
That allowed the tank to be pushed up on that side and drive the vent into the bed support. Mystery solved.
When you play hard, you break hard. Clearly those 5/16" bolts where not up to the task. When I put it all back together, I'll go with two 7/16" or 1/2" stainless bolts on each side. That should be enough.
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Anyway, after I pulled the tank it was easy to see the damage to the vent. Here you can see the broken piece when compared to the new unit:
And at that point, the actual fix was easy.
Those bolts are countersunk allen heads - which are needed to clear the coilovers - so I had to order a few. Should be here on Tuesday, and I can stick it all back together.
And at that point, the actual fix was easy.
Those bolts are countersunk allen heads - which are needed to clear the coilovers - so I had to order a few. Should be here on Tuesday, and I can stick it all back together.
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Corrosion and strength are both important in this spot. I generally run grade 8 everywhere I can, but in this case I needed the countersunk allen heads to clear the coilovers, and I could not find those in grade 8 (didn't spend a lot of time looking, though). I run a lot of stainless on the rig, but generally in non-structural stuff.
Regular stainless is a bit softer, but you can get better alloy's which can approach the strength of grade 8 - but expensive and tough to find. The bolts I ordered from McMaster-Carr are 18-8. Not the crazy strong stuff, and the reason I jumped up to the 7/16" or 1/2" (ordered both). Hopefully that will be strong enough, but we will see.
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Jeff - are you sure the problem wasn't that the bolt loosened, and then broke? One problem with flat-head cap screws is that if the countersink you're installing it into is slightly off-axis, you get a concentrated load. Those concentrated loads result in crazy-high localized stress that will cold-work the bracket, making the hole slightly larger, which allows slight movement, and then that movement causes impact damage that opens the hole up even larger. And then it's a self-perpetuating loosening until you lose all clamp load. A few good hits once that happens, and the screw breaks.
Moving to a larger screw usually won't solve that type of failure, unless you go to a grossly larger screw that can handle the impact loads, but that's not a good solution because you don't want things moving, right? I'd recommend putting those screws on your pre-ride check list, and paint some hash marks on them to see if they are coming loose. If you keep tightening them, they'll eventually "seat in" to a point where there's no more cold-working and they'll stay tight (usually). Don't assume that the larger screws will alleviate the problem. My money is on the loosening being the root cause.
Also, the reason you didn't find flat-head cap screws in Grade 8 is because that standard, called the SAE J429 standard, doesn't cover cap screws. Cap screws (socket-head, button-head, and flat-head, primarily) are a different beast altogether and are covered under the ASME B18.3 standard. The tensile strength for standard alloy steel flat-head cap screws up to 1/2" diameter is 145,000 psi, which is almost as high as the SAE J429 standard of 150,000 psi, so they are practically of equal strength. The problem with standard alloy steel cap screws (of any head type) is that they are very hard to find in anything other than black oxide finish, which has horrible corrosion problems (they'll rust after the first time they get wet). The alloys used for cap screws is susceptible to hydrogen embrittlement (hydrogen atoms getting into the atomic structure of the steel during the plating process), so you can't find them anywhere with zinc plating. As you pointed out, there are other specialized alloys out there, but they are expensive.
Anyway, keep up the awesome work, my friend!
Moving to a larger screw usually won't solve that type of failure, unless you go to a grossly larger screw that can handle the impact loads, but that's not a good solution because you don't want things moving, right? I'd recommend putting those screws on your pre-ride check list, and paint some hash marks on them to see if they are coming loose. If you keep tightening them, they'll eventually "seat in" to a point where there's no more cold-working and they'll stay tight (usually). Don't assume that the larger screws will alleviate the problem. My money is on the loosening being the root cause.
Also, the reason you didn't find flat-head cap screws in Grade 8 is because that standard, called the SAE J429 standard, doesn't cover cap screws. Cap screws (socket-head, button-head, and flat-head, primarily) are a different beast altogether and are covered under the ASME B18.3 standard. The tensile strength for standard alloy steel flat-head cap screws up to 1/2" diameter is 145,000 psi, which is almost as high as the SAE J429 standard of 150,000 psi, so they are practically of equal strength. The problem with standard alloy steel cap screws (of any head type) is that they are very hard to find in anything other than black oxide finish, which has horrible corrosion problems (they'll rust after the first time they get wet). The alloys used for cap screws is susceptible to hydrogen embrittlement (hydrogen atoms getting into the atomic structure of the steel during the plating process), so you can't find them anywhere with zinc plating. As you pointed out, there are other specialized alloys out there, but they are expensive.
Anyway, keep up the awesome work, my friend!
Great info, Scott. Thanks.
I considered that they came loose first, and that certainly could have happened. I used a split washer as well as blue lock-tite. The other side was still completely tight. Those were 82 degree bolts, and I use an 82 degree countersink. Also, it’s only been a year or two since I last had the tank out, so I know they were tight at least that recently - so maybe 1 or 2 thousand miles at the most.
I’ve had this configuration for 15 years or so, and have had that skid out on many, many occasions. Never once when I removed it were those bolts not completely tight.
Truthfully, in retrospect those 5/16” bolts were no where near beefy enough to deal with how hard I wheel - and that certainly includes dropping the weight of this 6400 pound beast down hard on that skid - which I do frequently and with abandon.
I actually considered welding a lip on the frame above the bracket to take the load, but decided to go overlill on the bolts first. As I said - we will see. Hell, this one lasted 15 years…
gasiorv
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So I had the same thing happen to my front mounting bolts which resulted in the same issue. On mine, it was from the tank taking hits. I upgraded the bolts and then it eventually cracked the flange and ripped the bolt out of the flange. I had to weld on new front mounts which picked up the cross-member (not the frame) on my TJ. Eventually I just relocated the tank.
That's a good argument for sticking with the same bolts. 15 years isn't bad. Hell, you can change them once a decade to make it easier to remember.
I finished up the reinstall of the tank today. I wound up going with 7/16" stainless bolts for the forward tank skid mounts. Since what broke were 5/16" stainless bolts, these should be a good bit stronger. Strong enough? Only time (and wheelin' with Crazy Vic 
) will tell.
There is about 1/4" clearance above the vent fitting which got crushed, and now that the tank is no longer moving that should be good as well...
) will tell.There is about 1/4" clearance above the vent fitting which got crushed, and now that the tank is no longer moving that should be good as well...
And now to finish up another project. $750 later....
Jeff,
They are talking about ECGS 14 bolt axles and towards @ 5:48 of the video ECGS is talking about the shaved 14 bolt and how the shaved gear is for buggies since it doesn't oil as well or shed heat as well. Video linked to the picture.
They are talking about ECGS 14 bolt axles and towards @ 5:48 of the video ECGS is talking about the shaved 14 bolt and how the shaved gear is for buggies since it doesn't oil as well or shed heat as well. Video linked to the picture.
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That makes more sense. If you shave your rear "for buddies," well....
Well, a bit of wasted effort today... 
There is a thread on Optima Batteries where the discussion turned to increasing the charging voltage when running an AGM battery. I decided to look into it a bit as I've had a history of burning through AGM batteries. The thought has been that the stock charging table on the TJ, designed for standard flooded lead acid batteries, does not allow enough voltage to properly charge an AGM battery. This is the post where the discussion started.
https://wranglertjforum.com/threads/thoughts-on-optima-batteries.18376/post-1799055
Rather than clog up that thread even more, I'll post my effort here. Here is how it started. If you've seen this over on that thread already, just skip to my next post.
If you have not seen it, here is the detail:
OK, this is fascinating. Doing a little research on this, Google's AI gave me this information on charging voltages for AGM batteries at a given temperature:
________________________________________________________________________________________________________________________________________
Automotive AGMs are charged by the powertrain control module (PCM), which varies the voltage based on the battery's temperature
. This process, known as temperature compensation, prevents overcharging in warm conditions and undercharging in cold conditions, both of which can shorten battery lifespan.
How temperature compensation works
Lead-acid batteries, including AGMs, have a negative temperature coefficient. This means that for a given charging voltage:
The reference point for adjustment is typically 25°C (77°F). The standard compensation rate for a 12-volt battery is a reduction of -24 mV/°C (
-0.024 V/∘Cnegative 0.024 V / raised to the composed with power C
−0.024 V/∘C
) or about -13.3 mV/°F (
-0.0133 V/∘Fnegative 0.0133 V / raised to the composed with power F
−0.0133 V/∘F
).
PCM charging chart for a 12V AGM battery
The PCM uses temperature compensation to adjust the charging voltage throughout the multi-stage charging process, particularly during the absorption and float stages. The following chart provides approximate target voltages based on a typical 12V AGM absorption voltage of 14.4V at 25°C (77°F).
________________________________________________________________________________________________________________________________________
According to this info, for these PCM charging levels 77 degrees F is the reference point - meaning the point at which the target voltage is raised or lowered according to temperature. The values indicated are certainly more aggressive than the standard values in the PCM table on my Hemi.
From this information, I developed the following estimated voltage levels by temperature in the PCM table. The voltage for 32 degrees and 86 degrees come directly from the AI chart. The other 3 voltage levels I interpolated to get the charted line for the proposed voltages flat. The result is approximately a .5 volt increase at every temperature.
I have no idea yet if the PCM will accept these exact proposed voltages, but I may give it a try.
Does this sound reasonable? My question would be: Is the "Absorbtion Voltage" referenced in the AI information the proper target?
There is a thread on Optima Batteries where the discussion turned to increasing the charging voltage when running an AGM battery. I decided to look into it a bit as I've had a history of burning through AGM batteries. The thought has been that the stock charging table on the TJ, designed for standard flooded lead acid batteries, does not allow enough voltage to properly charge an AGM battery. This is the post where the discussion started.
https://wranglertjforum.com/threads/thoughts-on-optima-batteries.18376/post-1799055
Rather than clog up that thread even more, I'll post my effort here. Here is how it started. If you've seen this over on that thread already, just skip to my next post.
If you have not seen it, here is the detail:
OK, this is fascinating. Doing a little research on this, Google's AI gave me this information on charging voltages for AGM batteries at a given temperature:
________________________________________________________________________________________________________________________________________
Automotive AGMs are charged by the powertrain control module (PCM), which varies the voltage based on the battery's temperature
. This process, known as temperature compensation, prevents overcharging in warm conditions and undercharging in cold conditions, both of which can shorten battery lifespan.
How temperature compensation works
Lead-acid batteries, including AGMs, have a negative temperature coefficient. This means that for a given charging voltage:
- In cold temperatures, the internal resistance increases, so the voltage must be increased to ensure a full charge.
- In hot temperatures, the internal resistance decreases, so the voltage must be lowered to prevent overcharging.
The reference point for adjustment is typically 25°C (77°F). The standard compensation rate for a 12-volt battery is a reduction of -24 mV/°C (
-0.024 V/∘Cnegative 0.024 V / raised to the composed with power C
−0.024 V/∘C
) or about -13.3 mV/°F (
-0.0133 V/∘Fnegative 0.0133 V / raised to the composed with power F
−0.0133 V/∘F
).
PCM charging chart for a 12V AGM battery
The PCM uses temperature compensation to adjust the charging voltage throughout the multi-stage charging process, particularly during the absorption and float stages. The following chart provides approximate target voltages based on a typical 12V AGM absorption voltage of 14.4V at 25°C (77°F).
| Temperature | Voltage adjustment for 12V system | Target absorption voltage |
|---|---|---|
| -20°C (-4°F) | +1.08V | 15.48V |
| -10°C (14°F) | +0.72V | 15.12V |
| 0°C (32°F) | +0.48V | 14.88V |
| 10°C (50°F) | +0.36V | 14.76V |
| 20°C (68°F) | +0.12V | 14.52V |
| 25°C (77°F) | 0.0V | 14.4V |
| 30°C (86°F) | -0.12V | 14.28V |
| 40°C (104°F) | -0.36V | 14.04V |
| 50°C (122°F) | -0.60V | 13.80V |
According to this info, for these PCM charging levels 77 degrees F is the reference point - meaning the point at which the target voltage is raised or lowered according to temperature. The values indicated are certainly more aggressive than the standard values in the PCM table on my Hemi.
From this information, I developed the following estimated voltage levels by temperature in the PCM table. The voltage for 32 degrees and 86 degrees come directly from the AI chart. The other 3 voltage levels I interpolated to get the charted line for the proposed voltages flat. The result is approximately a .5 volt increase at every temperature.
I have no idea yet if the PCM will accept these exact proposed voltages, but I may give it a try.
Does this sound reasonable? My question would be: Is the "Absorbtion Voltage" referenced in the AI information the proper target?
So I thought it would be a fun project for an afternoon. I broke out my HP Tuners setup, and went to work:
When I updated the charging table, I had to modify my actual values just a tiny bit for the PCM to accept. Here are the actual charging voltages that the PCM would accept:
I thought I would be smart and actually test the results. So, prior to making the change, I fired up the HP Tuners data logger and ran a log. Voltage is tracked under "Control Module Voltage:"
Then I made the changes to the table, and ran another log. Feeling pretty good about my technical abilities at this point I exported both the before and after log data and dumped the detail into a spread sheet. I then averaged the before and after voltages, and here they are:
Stock Charging Table: 14.089 volts
New Charging Table: 14.097 volts
So yeah, my change did in fact increase the voltage: by 8 thousandths of a volt....
Well, crap. It took me just a few seconds to realize what was going on. I run a Premier Power Welder on the rig, and guess what - it has its own 14-volt regulator.
Nothing like wasting an afternoon.....
When I updated the charging table, I had to modify my actual values just a tiny bit for the PCM to accept. Here are the actual charging voltages that the PCM would accept:
I thought I would be smart and actually test the results. So, prior to making the change, I fired up the HP Tuners data logger and ran a log. Voltage is tracked under "Control Module Voltage:"
Then I made the changes to the table, and ran another log. Feeling pretty good about my technical abilities at this point I exported both the before and after log data and dumped the detail into a spread sheet. I then averaged the before and after voltages, and here they are:
Stock Charging Table: 14.089 volts
New Charging Table: 14.097 volts
So yeah, my change did in fact increase the voltage: by 8 thousandths of a volt....
Well, crap. It took me just a few seconds to realize what was going on. I run a Premier Power Welder on the rig, and guess what - it has its own 14-volt regulator.
Nothing like wasting an afternoon.....
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So I thought it would be a fun project for an afternoon. I broke out my HP Tuners setup, and went to work:
View attachment 645574
When I updated the charging table, I had to modify my actual values just a tiny bit for the PCM to accept. Here are the actual charging voltages that the PCM would accept:
View attachment 645575
I thought I would be smart and actually test the results. So, prior to making the change, I fired up the HP Tuners data logger and ran a log. Voltage is tracked under "Control Module Voltage:"
View attachment 645576
Then I made the changes to the table, and ran another log. Feeling pretty good about my technical abilities at this point I exported both the before and after log data and dumped the detail into a spread sheet. I then averaged the before and after voltages, and here they are:
Stock Charging Table: 14.089 volts
New Charging Table: 14.097 volts
So yeah, my change did in fact increase the voltage: by 8 thousandths of a volt....
Well, crap. It took me just a few seconds to realize what was going on. I run a Premier Power Welder on the rig, and guess what - it has its own 14-volt regulator.
Nothing like wasting an afternoon.....
Funny how you & I seem to forget this stuff half the time when we start to work on our rigs....
Now I wonder if there is a way to change the voltage of the VR of PPW? My guess NO but who knows.
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