LMTV '04 M1078A1 starting issues

[Ronmar]

I only glanced at this, and mainly read your description and missed the low current. I agree with Lostchain, that this does not make sense.
alternators do NOT control current, only voltage.voltage in relation to a batteries state of charge in turn controls current delivered. When an alt cannot deliver enough current to satisfy the load, the voltage drops.

Yes the batts should have been drawing more current, the current a battery draws is dependent upon the voltage it is presented. I did not note where you were measuring the voltage, but since you were seeing more than adequate voltage to deliver full current into a battery, thirsty 100AH AGM batts should have been drawing ~45% of their AH rating.

this speaks to me of a bad connection between where you were measuring voltage and the batts, or bad/sulphated batteries. do these batteries pass a CCA load test? I suspect you were measuring voltage at the alt? Did you also measure voltage AT the batteries, and was it near the 28V you measured elsewhere(within 3%). A larger than 3% voltage drop would indicate bad wiring/connections… A connection with enough resistance to drop from the high 14 and 28v you were measuring to down to just slightly above the 24.8v resting voltage of the batteries which would limit the current draw of the batteries... not a fan of jumper cables. Got any better wire and connectors to try this test again?

 

[MatthewWBailey]

I only glanced at this, and mainly read your description and missed the low current. I agree with Lostchain, that this does not make sense.
alternators do NOT control current, only voltage.voltage in relation to a batteries state of charge in turn controls current delivered. When an alt cannot deliver enough current to satisfy the load, the voltage drops.

Yes the batts should have been drawing more current, the current a battery draws is dependent upon the voltage it is presented. I did not note where you were measuring the voltage, but since you were seeing more than adequate voltage to deliver full current into a battery, thirsty 100AH AGM batts should have been drawing ~45% of their AH rating.

this speaks to me of a bad connection between where you were measuring voltage and the batts, or bad/sulphated batteries. do these batteries pass a CCA load test? I suspect you were measuring voltage at the alt? Did you also measure voltage AT the batteries, and was it near the 28V you measured elsewhere(within 3%). A larger than 3% voltage drop would indicate bad wiring/connections… A connection with enough resistance to drop from the high 14 and 28v you were measuring to down to just slightly above the 24.8v resting voltage of the batteries which would limit the current draw of the batteries... not a fan of jumper cables. Got any better wire and connectors to try this test again?

The DMMs were measuring at the batts since the leads like to fall off on the vibrating alt. There was not a ton of current flowing. Spot checks on the alt were the same volt numbers. The batts were showing 45-50% cca test, so not new. These are 3 yrs old +/- a few months. My issue is that the amperage has no reason to fluctuate like that, and does the same in the truck but at a higher amp value, and also creeps up on voltage.
I previously kept the truck topped off at night so I would have never seen any indications of poor charging output. And the long drive I took was during daytime with 4 topped batteries, so very little power draw overall.

Probably could pull it and take it to Napa to test on their bench tester for a cleaner test. My electrical lab is not up to SAE standards.

 

[Ronmar]

At those voltages, 50% SOC AGMs should have been gobbling current… your current was fluctuating because the voltage was fluctuating…

 

[MatthewWBailey]

At those voltages, 50% SOC AGMs should have been gobbling current… your current was fluctuating because the voltage was fluctuating…

That was the expectation. I put my regular old box charger on them after this test and it was running ~15amps to start (12v individual charger) on the analog gauge. On my pulse-tech conditioners, they're showing 75% today so maybe some sulfonating? Might as well let napa test the alt since it'll be out of the truck. Maybe my jumper cable leads were flaky. But They were sparking when I put them on and off a couple times. This is why I love welding, there's never any doubt about current flow.

Does your alt grown loudly when the AIH kicks on? I hate mysteries but this is still one.

 

[Ronmar]

I don’t have an intake heater, but that symptom also speaks to a bad connection between alt and batteries. As the inlet heater pulls current, the voltage drops and the battery and the alt should share that load. A high resistance connection back to the batteries would shift more of the inlet heater load to the alternator… So the heater power feed is connected to the 24V battery terminal on the polarity portion of the LBCD. Do you also have the manual and remote disconnects? A bad connection or switch contactor between the LBCD 24v batt terminal and the 24v+ terminal on the battery could cause this, and would be indicated by a lower alt voltage than battery voltage while the heater is running…

i will be surprised if Napa can test that alt, as it needs a series battery string to connect to perform properly…

 

[MatthewWBailey]

I don’t have an intake heater, but that symptom also speaks to a bad connection between alt and batteries. As the inlet heater pulls current, the voltage drops and the battery and the alt should share that load. A high resistance connection back to the batteries would shift more of the inlet heater load to the alternator… So the heater power feed is connected to the 24V battery terminal on the polarity portion of the LBCD. Do you also have the manual and remote disconnects? A bad connection or switch contactor between the LBCD 24v batt terminal and the 24v+ terminal on the battery could cause this, and would be indicated by a lower alt voltage than battery voltage while the heater is running…

i will be surprised if Napa can test that alt, as it needs a series battery string to connect to perform properly…

I finally found the place in Junction that does alt rebuilding. They do starters as well. That's their M.O. Im taking the old starter there to have that rebuilt or at least serviced beyond what I can do. I'd like that around as a backup. So maybe they can test this alt too.

I still have the remote relays and manual disco in there. I didn't check those with the megger of course. I totally ignored those ugh. I wonder if thats it. My megger only runs 10a. I should run the truck on batts only tomorrow and do a v drop test across those terminals when the AIH is on.

 

[MatthewWBailey]

I still have the remote relays and manual disco in there. I didn't check those with the megger of course. I totally ignored those ugh. I wonder if thats it. My megger only runs 10a. I should run the truck on batts only tomorrow and do a v drop test across those terminals when the AIH is on.

Even with the alt out of the truck, I was able to do a quick Vdrop test across the 28v circuit between manual disco and the load side of the power relay just using the ignition.

the AIH pulled 74amps and the vdrop at the test point was 0.540v. That comes to 7297 uOhms, or 7.297 milliohms of resistance across that Circuit, my spec on high amp switchgear was always 250 uOhm or less for passing grade. So this seems high in terms of acceptability to me as the the vdrop is 2.25% of the system voltage using 24 as base. That's just 2 sets of contacts. What else is going on down the line!

That resistance at 74amps is like a 40w light bulb. Maybe that's normal for automotive circuits at 24v? It's not great looking from typical industrial numbers imo.

So that bolsters what you're saying about the load shifting to the alt more during warm up.

I'll pull the power relay, reconnect and test it again.

 

[MatthewWBailey]

Found some evidence of melted connections on the 28v side of the manual disco. The bus bar insul is melted. Not sure when that ever happened but some of the post threads are melted away. Oddly, the 2 compression washers were installed on the 14v side. Those are supposed to keep pressure on the connection in my mind. Maybe there should be 4?.

I pulled the relay and flattened the busbars to make up the distance as the cables are exact length. That copper heats easily with a MAP gas torch.

 

[Ronmar]

Thats where the voltage drop test comes in handy. Visuals are great, but voltage drop under load will tell you things you cant see about how well the circuit is performing while passing current…

How To Videos for Starters & Alternators | Delco Remy

Learn how to fix and replace starters & alternators.

www.delcoremy.com

 

[MatthewWBailey]

Thats where the voltage drop test comes in handy. Visuals are great, but voltage drop under load will tell you things you cant see about how well the circuit is performing while passing current…

How To Videos for Starters & Alternators | Delco Remy

Learn how to fix and replace starters & alternators.

www.delcoremy.com

That's a pretty nice video on the topic. Probably Should be required viewing for ownership of this truck lol. I'm already at .5v drop and I only tested across 1ft of circuit. Time to start polishing terminals. I feel like I'm back at breaker maintenance oversight duty.

 

[MatthewWBailey]

Getting close to finishing the wiring changes for the alt and found this today. Ground strap connection did not look good underneath the lug. (That dang nut was near impossible to get to, and get off). The Paint is thick on that connection. Curious whether that's a factory job or field job.

Anyway, I moved it bc the strap is too short to reach the new alt negative term anyway. Good thing. I ground down a fresh spot of frame over that extra bolt hole in the frame. Drilled it to fit the original bolt from the old hole.

So thats 2 power connections that are/were questionable so far.

 

[GeneralDisorder]

Prime example of why it is imperative to clean every connection.

That is VERY LIKELY factory. I've run into a severe voltage drop between the cab and the chassis on a 2021 A1P2 due to excessive paint under the cab to chassis ground strap. You are starting to see the situation you are dealing with.

 

[Ronmar]

Yep, clean connections and volt drop test to show you the problems you couldn’t see in the inspection(bad crimps and corroded cables)…

 

[MatthewWBailey]

Prime example of why it is imperative to clean every connection.

That is VERY LIKELY factory. I've run into a severe voltage drop between the cab and the chassis on a 2021 A1P2 due to excessive paint under the cab to chassis ground strap. You are starting to see the situation you are dealing with.

I need to track down those other 2 chassis connections. I can see the one for the cab and I should be able to rework it once the cab is down. I need to find TL64/65. Is that right next to the starter? I can't remember when I had that apart but I thought there was a braded cable there. Those frame bolted connections are hard to get to since there's so much stuff going back and forth in the frame rails.

 

[GeneralDisorder]

Yes there is an engine block to frame ground near the starter IIRC.

The chassis to cab cable is behind the grill on the passenger side. The chassis side of the cable is unreachable.... you will see. I haven't had an issue with it yet but my plan is to simply add a second braided ground strap and attach it to the same point on the cab, but make a new chassis connection point that is serviceable.