NEED HELP Pulsing battery drain???

fixit

Original poster
Member
Apr 27, 2021
15
Austin, TX
2004 Chevy Silverado 3500, Extended Cab, Long Bed, LLY Diesel, Dual Battery, 6 Way Electric, Heated Seats:

I have a problem that I have never encountered before. After driving my truck from Austin to San Antonio to have a Maxon liftgate installed, I returned to Austin two days later and parked the truck. At this point, everything seemed fine. Unfortunately, for the next two it rained and the truck stayed parked. On the third day, I was going to get busy, but when I punched the key fob, nothing happened. I put a digital Voltmeter on one of the batteries and read 1.9 Volts! So much for $500 worth of six month old, AGM Batteries. I put the best charge that I could on the batteries using a power supply and then started the engine. The alternator put out 14.7 Volts for a couple of minutes and then began to taper off the charging, (normal behavior). From this I took that the alternator was not the problem. Next, I made a test connector out of a "J Case" fuse and connected a 20 Amp current shunt and meter to it. With the truck's key off and removed, driver's door open and all visible accessories off, I began swapping out the "J Case" fuses one at a time. The LBEC 1 and LBEC 2 buses showed only a few milliAmps of battery drain, but the MBEC(1) bus indicated something strange. This bus exhibited an approximately 6.5 Amp current pulse lasting about one second and repeating every six seconds.
As far as I can discern, the only things that are served on this bus are the electric seats and the door modules. Unfortunately, tracking the wiring beyond the fuse box becomes more difficult, especially if you are an elderly gimp, so I am hoping that someone has seen this symptom before and can save me some effort. I don't know if I can still hang upside down under a dashboard, or not. So, does anyone know what can cause this pulsed battery drain?

fixit - I ran, screaming form the grease pit.
 

Jfcdeuce

Member
Feb 6, 2022
2
Oregon
Any more to this one? I had a very similar situation.
I just traced drain to the MBEC 1, it was bouncing .125A up to 6.81A. Up for roughly 3 sec, down for about 3 Sec.

I ended up it the wrong fuse box but when I pulled the IGN-0 fuse it dropped and everything went to .008A and stayed.... weirdly after reinstalling the fuse this drain did not come back.... 24 hrs later Ichecked again and the battery while lower than when I parked it appears to be holding and certainly not dying overnight.

From reading IGN 0 is the PCM/TCC so I basically hard rebooted the PCM not just pulled the battery....

Prior to the battery dying I can think of 2 things I did: I may have left the passenger door ajar over night, but after that I set the battey up an charger and ran it a bit, then took it for 3-5 mile drive, it was dead the next morning again.

The other thing was I cleaned the throttle body with MAF cleaner, on the vehicle. Just a quick spray and wipe (ignition/key off). There was a rough start as I may have gotten some cleaner as far as the cylinders, but can’t imagine there was enough near the control electronics to short them, plus that stuff evaporates in minutes...


So it’s a mystery, right now all seems good.
But I really wonder what could be bouncing a 6A draw, seems big for a sensor or something doing a search.

Thoughts?
 

fixit

Original poster
Member
Apr 27, 2021
15
Austin, TX
Hey!

I did eventually track down the problem. I built a hall-effect current probe, small enough to sense current within individual wires, without cutting the wires or puncturing the insulation. This made it easy to find the wire that was carrying the pulsing current. That MBEC 1 bus supplies power to the electric seat controls, seat heaters and, strangely, the passenger side power window. The problem is likely to be an always hot power supply wire shorted to the body. The wires from the MBEC 1 distribution block under the dashboard to the power seats runs under the carpet and insulation on the floor. There are a couple of places that the wires are likely to rub the body and abrade the insulation, creating the short circuit. The 3 second pulsing is caused by the thermal circuit breaker, in the MBEC 1 block, cycling on and off. The current being pulled during the ON half of the pulse is determined by just how far down the wire from the distribution block the short occurs. I would start looking for the problem by lifting the carpet and insulation on the driver's side floor pan. Find the wires that run through there and carefully check them for having shorted to the body. With luck the fix is simply some electrical tape over the wires. If the wire insulation is "fried" you will have to splice in some new wire. I would suggest wrapping the wires with a couple of layers of tape and applying a strip of non-conductive duct tape to the body. along the path of the wires and another over the wires. That's about all of the insulating and protection that you can do without leaving a lump in the carpet. One other point: it would be a good idea to replace the thermal circuit breaker in the MBEC 1 block. That cycling on and off will have taken it's toll on the device and you can expect to have it fail sometime in the future.
To be honest, I still have not repaired my short, though I know where it is. I'm still pulling the 50 Amp MBEC 1 bus fuse, under the hood. I'm partially disabled and there is no playing BAT and hanging up under dashboards, anymore. It'll have to be a summer project.
Best of luck, Fixit
 
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