I've been doing some work on my 2005 Escape Hybrid and as I have one of the originals, the 12V battery is now over 4 years old.

Here's my issue. Twice I have disconnected the ground strap that runs from the firewall to the engine block. Each time, the next morning the car would not start as the 12V battery was reading about 9V. The first time this happened, I started montoring the 12V battery with a DVM. Every time it read 12.5V or around there (and of course the car started) until the next time I disconnected the ground strap. The next morning the battery was below 10V and would not start. Keep in mind the ground strap was reattached, but it was disconnected for a half hour or so.

Here's my questions:

By disconnecting the ground strap did I shut down the DC-DC converter (or other module controlling it) and thereby keep the 12V battery from being charged while parked overnight?

Does the DC-DC converter work with the key off?

If the above is true, then was the DC-DC converter masking a bad 12V battery?

Of course I realize I may have something draining the battery, but no changes have been made to the original car wiring, and if that were true then wouldn't the battery be dead every morning instead of just the morning after I had disconnected the ground strap.

 

Take your VOM and measure both the terminal to terminal voltage (which is what I think you have written) along with the battery plus to frame or other ground. Do this with the ground strap both connected and disconnected.

The reading connected should be a no-brainer with them being the same, but just in case there's something else going on take the reading.

To my knowledge the dc-dc converter is off when the key is off. Doesn't make sense to use the traction battery, which starts the engine, to keep the 12v battery charged overnight...

I'll look at the wiring diagrams (which I don't have with me) when I can and see if something pops out at me.

 

For what reason would you disconnect the ground strap..??

In doing so you are interrupting the ground circuit return for maybe most of the 12 volt powered devices, only the shadow knows what the result from that might be.

 

I disconnected the ground strap because I was using the stud to mount a bracket. No big deal. I just think the results are interesting so I thought I'd share them with the forum. Someone may have the strap break or go high resistance in the future and look at the strange problems it might create.

I'll do the measurements suggested, but I don't think that is where the problem lies. I did take the battery to Sears and they said it was good, but I'm not that confident that the guy testing the battery really knew what he was doing.

On the other hand, I did come up with another possible reason, and that is that when I disconnected the ground (and you can hear relays click and things cycle so you know it did something) it put the car in a high current drain mode. To the best of my recollection, I don't think I restarted the car after removing the ground strap. Maybe if I had, everything would have reset and the battery would not have drained. When I have the time, maybe I'll test that theory.

I wonder if Ford is willing to release their software!

 

The answer is No to all your questions.

When you disconnected the ground wire the Power Sustain Relay (PSR) can no longer be grounded by the PCM which then powers down the TCM and PCM after Key-Off. If the PCM and TCM are not powered down correctly they will continue to drain the 12V system.

GaryG

 

Thanks Gary, your answer certainly makes sense. It also goes to show you just how complicated these cars are getting.

 

What you need to do before disconnecting the 12V battery is wait with the key off and removed, with the hood open for 5 minutes. The FEH service manual says it needs a full 5 minutes to make sure that various modules reach their shutdown cycle times.

Then it should be safe to disconnect the 12V battery and know everything is as "off" as it will get. In normal use, the PSR is actually only active for mere seconds, so it is unlikely that that alone would be the problem, but simply various other modules having to close down properly.

 

When you disconnect the 12V battery ground, it can no longer be drained by the vehicle systems. This poster only disconnected a ground on the firewall to the engine if I recall. The key shuts-Off the HV battery and DC/DC converter and with the 12V battery ground disconnected will disconnect the battery from a drain of any modules period. Now explain to me where you see in the service manuals anything different and what modules would continue a drain of the 12V battery if it's disconnected. I have the manuals so please quote a page section if you can.

GaryG

 

I had forgotten about the modules that have to cycle off.

Good point.