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Old 11-24-2022, 03:35 PM   #1
Bus Nut
 
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ECM wonít boot, canít turn in headlights until I check battery volts with multimeter?

Hi all, I have a weird one.

The common circumstances leading up to this issue: laptop running with ungrounded ac adapter plugged into my house inverter, USB nexiq plugged into bus data port and laptop.

The issue: turn the key to ON, dash lights up for maybe a second then goes dark. Transmission touch pad stays on. Obviously canít start the engine. If turn on the parking lights I hear relays buzzing, turning on headlights kills the shift pad too.

The only fix Iíve found both times this happened: use my analog multimeter to check the battery voltage at the battery box. It usually says 12v. After that I can crank and start no problem.

In the process I checked the main battery ground, unplugged the laptop ac adapter and nexiq, a variety of other things to no avail.

So what the heck is going on? Do I have bad batteries? Theyíre about a year and a half old.

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Old 11-25-2022, 07:31 AM   #2
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it almost sounds like bad batteries or a bad connection to one of them. the telltale is the fact the shift pad dies when you turn more stuff on indicating a voltage drop. you can check the voltage at the battery terminals when the key is ON and see if it has dropped.. if it hasnt then I would start checking voltage at various places in the bus..if it has dropped way down then give the batteries a good DEEP charge.. meaning a good quality charger on them overnight..




in a vehicle one time I had a bad ground.. when I tested voltage from a spot in the fuse box to a metal dash support I had 8 volts.. (12 volts acros the battery).. so then I ran a wire =clipped on the battery ground itself to my meter and testd from that to the + point in the fuse box and came back with 12.. fixed the frame ground and the car drove great..
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Old 11-25-2022, 07:35 AM   #3
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Quote:
Originally Posted by cadillackid View Post
it almost sounds like bad batteries or a bad connection to one of them. the telltale is the fact the shift pad dies when you turn more stuff on indicating a voltage drop. you can check the voltage at the battery terminals when the key is ON and see if it has dropped.. if it hasnt then I would start checking voltage at various places in the bus..if it has dropped way down then give the batteries a good DEEP charge.. meaning a good quality charger on them overnight..




in a vehicle one time I had a bad ground.. when I tested voltage from a spot in the fuse box to a metal dash support I had 8 volts.. (12 volts acros the battery).. so then I ran a wire =clipped on the battery ground itself to my meter and testd from that to the + point in the fuse box and came back with 12.. fixed the frame ground and the car drove great..
I was inclined to think battery ground issues too but I did already clean up the main battery ground. Thereís various other terminal blocks in between I havenít fully located yet, it may be time.

I bought the batteries from a local battery store (battery wholesale), maybe I need to have them test the batteries just to make sure I donít have a dead cell or something? The fact that Iíve been able to resolve the issue by putting a multimeter across the battery terminals made me wonder if thereís a BMS or something else weird in these batteries thatís getting reset by the very high impedance load of the meter?
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Old 11-25-2022, 10:55 AM   #4
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I'm not a fan of analog multimeters, unless they're pretty accurate. A cheap dvom is much more accurate then most analog multimeters.

Like Christopher, I believe your batteries are just that low, and your multimeter isn't telling you correctly. At least that's what your symptoms sound like.

You can check grounds and powers by doing a voltage drop test. It gives you a much more realistic picture then an ohm reading. There are plenty of videos on youtube that describe the hows and whys of it.

I don't know how you putting a multimeter across the batteries change anything, unless the terminals are loose and touching the batteries with the meter disturbed them enough to get a better connection.
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Old 11-25-2022, 04:04 PM   #5
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Quote:
Originally Posted by Booyah45828 View Post
I don't know how you putting a multimeter across the batteries change anything, unless the terminals are loose and touching the batteries with the meter disturbed them enough to get a better connection.

and ive seen that happen!
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Old 11-25-2022, 04:40 PM   #6
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I had a perfect start today, drove maybe 10 miles, stopped for fuelÖjust got relays buzzing when I turned key to on. Tried the multimeter trick, didnít work. Made sure all my accessories were unplugged/turned off, tightened all the battery connections just a tiny bitÖfired up.

My frame is rusty around the grounding points, I think when I get home I need to disconnect those grounds, hit the metal with a flap wheel until itís shiny, reconnect and torque, then throw some paint over it for a little extra protection.
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Old 11-28-2022, 08:17 AM   #7
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That sounds exactly like sporadic contact then. Sound like you have a good plan of attack. If that doesn't work, check for voltage drop across the various connections while trying to crank.
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Old 11-28-2022, 09:47 AM   #8
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Check the starter connections are tight while your at it, that's where I found the mystery problem on my bus. Space is tight in the area so best disconnect the battery beforehand.



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