Engine Electronic Control Module (ECM)

[Bob Stone]

It is my understanding that the engine ECM is wired such that it is a constant drain on the chassis battery even when both battery switches are turned off. Apparently this is done so the data required by the engine is readily available at startup. I've read that this data refresh is accomplished in less than one minute.

Questions:
1) Is it possible or practical to rewire the ECM prior to the chassis master switch to prevent battery drain during long term storage?
2) If 'yes' to the above question, can the ECM be wired to the key switch so the ECM battery drain only occurs prior to start up and during running?

Thanks in advance for clarification anyone can provide.

Bob

[Edward Buker]

Bob,

You could certainly wire the coach so that the ECM is powered down with the chassis battery switch is off.  A relay could be used to open the 12V supply unless the chassis battery switch is on.

I think the use of the ignition switch is probably not wise given the write time for the ECM. You typically turn the key switch on and then proceed to the crank position. I'm note sure how the ECM would respond or if cranking would occur before the ECM has had enough time to rewrite itself.

Later Ed

[Gerald Farris]

Bob,
The answer to your first question is yes. The answer to your second question is yes it is possible, but it is not a good idea, because you will be adding the possibility for many more problems with virtually no upside. There is no need to cut power to the ECM unless the coach is in storage without shore power, or your "Bird" system is malfunctioning. If the "Bird" system is malfunctioning, it needs to be repaired to make both of the charging systems on your coach operate properly.

Gerald      

[Edward Buker]

Gerald and Bob,

I think the only advantage to cutting power to the ECM is during storage when no outside source of power is available, that is hard on our batteries. I discussed this with Cat Engineering and they did not have a requirement to maintain power on the ECM and in fact there are vehicles that are wired this way. From a reliability viewpoint I considered that CAT may want this ECM in a stable state voltage wise using about 20 watts of power to keep the temperature stable and not pass through condensing temperature shifts. They indicated that they have it well sealed and that was not an issue.

I agree with Gerald that adding complication, like a relay contact, or a manual switch contact could induce a voltage drop or an open kind of fail while driving is an added concern. There is a small chance that things can get messed up in the ECU rewrites with many cycles of on and off power. Still there are folks that do not have a powered option in storage so we do have a real problem to deal with as the question is what is the best way to handle this issue.

Probably the best way to handle this is with a wrench. If you are going to store long term disconnect the main + lead going to the chassis battery until it is time to go. While not the most convenient, it is a safe way to handle this issue without adding wiring complication. It also satisfies that you have a minimal amount of cycles of powering down the ECU memory, you can still use the chassis battery switch to depower all but the ECU. You should not be overly concerned about this ECU rewrite given you have done it already every time you have changed your chassis batteries or removed the terminals for cleaning. When the batteries are reconnected give it 60 seconds or more for the ECU rewrite before turning the ignition key to start....most of us are not that fast anymore anyway so this is not a big concern :-)

Gerald if your thoughts are different then this, jump in if I have overlooked something that you are concerned about that I am unaware of.

Later Ed

[Bob Stone]

Gerald and Ed,
I have increased the number of solar panels on the roof so except when covered with snow they should keep the batteries up to spec. Correct me if I'm wrong but I believe the solar panels charge the batteries even when both master battery switches are turned off?
Your advice and this forum is much appreciated.

Bob

[Edward Buker]

Bob,

I believe that is possible through a coupling circuit (Big Boy or Echo Charger) if the house batteries are being charged and voltage reaches the threshold to cause the circuits to couple the house and chassis batteries. The way to know for sure is with a meter. You can measure the DC voltage of the house and DC chassis batteries with the main switches off in full sun. You should see a voltage higher than 12.8V if charging is occurring. If the voltage has tripped the coupling the voltage on the chassis and house batteries should be about the same within a couple of tenths of a volt. 12.6 to 12.8V is the range of an 80 to 100% fully charged battery. If it is getting dark and the solar cells are minimally putting out a current then the batteries should fall in this range if being maintained.

later Ed