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Old 05-24-2019, 06:55 PM   #1
Skoolie
 
Join Date: Sep 2018
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Year: 2007
Coachwork: Thomas Built
Chassis: Minotour
Engine: Chevy Express 3500 6.6l
Parasitic Draw

Hi,

I posted this on Duramax Forums. Figured I should try here too.

2007 3500 LMM Express Cutaway Parasitic Draw
I have a 2007 Express Cutaway that’s a Thomas Built short bus. I have a parasitic draw that’s draining the battery in a day or two. It’s drawing 2 Amps.
I’ve worked on tracking it down as follows:

Measuring current flow through the engine compartment negative cable while pulling each fuse one at a time starting with the bus body, then the fuse block under the seat,’then the fuse block under the hood. No change.

I disconnected the fusible link on the under hood block. No change.

I disconnected the primary wire from the under seat fuse block. No change.

I traced the cable for the wheelchair lift and disconnected it. No change.

Then I decided to work from the positive side. There are several hots connected under the hood. I’ve tentatively identified these as Wheelchair Lift, Bus Body, Cutaway Van Starter/Under Hood Primary/Interior Primary.
I separated each of these, except for the “Cutaway Van” part where the interior/under hood/starter are part of a factory made inseparable cable.

The bus body, no current.

The wheelchair, no current.

The “cutaway van” current.

So focusing on the Cutaway:

Disconnected the primary to the under seat fuse block. No change.

Pull each fuse one at a time and each relay one at a time from the under hood fuse block. No change.
Disconnect one side of the fusible link on the under hood block. No change.

So that’s where I’m at.

Is the alternator fused through the fusible link?

If anyone has ideas for how I can proceed to test further, please let me know!

Thank you!
Danjo
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Old 05-24-2019, 11:20 PM   #2
Bus Geek
 
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I have the same thing.

As best I understand, the ECM draws current even when the ignition switch is off. The drain is not a problem if parked for a few days but a few weeks and the batteries are low enough that I cannot start the bus.

My solution was to turn off the battery disconnect when I park for more than a few days.

I suspect that is why there is a factory installed battery disconnect?
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Old 05-24-2019, 11:29 PM   #3
Skoolie
 
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Engine: Chevy Express 3500 6.6l
Hi, I don’t have a disconnect that I know of. What does it look like?

I’m drawing 2 Amps. It seems high for just preserving memory. It would. Be great to find out I don’t have a problem.

If you disconnect the battery, doesn’t the ECM need to “relearn”?
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Old 05-24-2019, 11:38 PM   #4
Bus Geek
 
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Quote:
Originally Posted by Danjo View Post

If you disconnect the battery, doesn’t the ECM need to “relearn”?
That would be a great question for Christopher. I believe that it does to some extent. It has not been a problem for me.

My disconnect is likely different than on a cutaway. I have worked on similar vintage Ford's and they had a black box roughly 3-1/2" x 3-1/2" with a red lever and a red push button.

If you don't have one, they are readily available aftermarket.
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Old 05-25-2019, 12:00 AM   #5
Skoolie
 
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Engine: Chevy Express 3500 6.6l
I think I’ve disconnected everything except the starter and alternator and haven’t been able to stop the drain. Assuming I haven’t missed something, there’s still the alternator, starter, and primary wire runs to check. I’m still pretty convinced that 2A is too much of a draw for keeping memories charged.

IDK if it’s normal for all their buses, but when I went to inspect, all the auction buses at the bus yard had their batteries disconnected. That had cleared the check engine light. And I wouldn’t have bought it it if had a light on.

I’m going to ask around on the Chevy sites about the disconnect to find out if I can do that. I’ll report back here whatever I find out.

Thanks for your help!
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Old 05-25-2019, 12:04 AM   #6
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Good luck!
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Old 05-27-2019, 06:49 PM   #7
Skoolie
 
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Hello All,

i did some more troubleshooting today. 2.5 Amp Draw. I went back to pulling fuses and found when I pulled one, the draw dropped a few 10ths of an amp. I looked at the legend and found it labeled TCM. Then I found two others labeled TCM1 and TCM2, so I pulled those. Then it said .5 Amp Draw. While I was looking at it, that disappeared. I thought I was having problems with my multimeter connections so I disconnected and reconnected. .5 Amps. Then 60 seconds later, 0 Amps. So I have surmised that something wakes up for 60 seconds and then sleeps again. But then there’s the other 2 Amps.

A quick Internet search says it’s the BCM. I found the BCM behind the passenger side dash panel. I reinstalled the fuses and disconnected the BCM connectors and test again. .5 Amps for 60 seconds and then 0 Amps.

so is it the BCM that’s bad or something downstream? I see posts on the Internet that say it could be both. Is there a way to test? With the multiple connectors is there a way to isolate?

thank you everyone for taking the time to help!

Danjo
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Old 05-28-2019, 12:32 AM   #8
Skoolie
 
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Sorry. I didn’t remember the name of the fuse labels correctly.

TBC, TBC2 and TBC3. I realized just now that there is a TBC4 (you’d think they’d all be adjacent). I didn’t pull TBC4. After collecting my observations, I put all the fuses back and found the BCM. I disconnected all the connectors and tested again. Same. .5A for 60 seconds then to 0A.
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Old 05-28-2019, 07:30 AM   #9
Bus Crazy
 
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Do you have two batteries? Sometimes one of the cells in either of the batteries is damaged and draws it down.

May be an engine light under the hood that stays on?

Could be a GPS / alarm system that the district installed to keep track of the bus when it was in service.

Good luck. J
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Old 05-29-2019, 10:42 AM   #10
Skoolie
 
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Engine: Chevy Express 3500 6.6l
I feel stupid, but I found the drain through the BCM. I’m not out of the woods yet.

While I was hooking up my multimeter with long leads while sitting in the cab, I hear a click. I disconnect and reconnect “click”. A third time and I realize that the accessory lights are on!

But that’s not my root cause. I think that maybe I performed brain surgery to treat athletes foot though.

I’ve been doing this testing with only one battery connected. When I connect the both batteries there’s a draw of 1.7 Amps. If I do the test using either battery alone, 30 miliamps.

So is one battery causing the draw from the other? I tested voltage on them. One is 12 volts, the other 10.5.
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