Estimated cost for this service: $350 (DDEC IV) to $550 (DDEC V)
Estimated repair time: 24 hours
Back in 2008 anytime I would find a DDEC IV or DDEC V setting the fault code : “RTC backup battery low” it was considered the kiss of death. I had to tell the owner of that ECM that he needed to replace it to correct the problem and he was looking at about $2,000 dollars. I hated giving out that bad news and I hated not being able to offer any other option. So I took some time to look into the problem.
After dissecting several cored and dead DDECs I found that the fact that the RTC (Real Time Clock) circuit was no longer getting power was not the cause of why those ECMs were destroyed. As it turned out the RTC code was not the real problem but just a symptom of a more serious problem. I found one of three causes could kill the ECM. Firstly, oil or fuel in the ECM causes the internal battery to short triggering the RTC code. If you have the DDEC in your hands don't be afraid to shake it around and listen. The second thing that can kill the ECM is if the battery gets too hot. Too much heat (over 250 f) and the battery would leak acid or if the battery was sealed well enough...build pressure and burst getting acid everywhere. The final thing that will kill the ECM if it happens is if the RTC battery shorts internally or it's trace shorts to the case. After that the battery gets hot, leaks acid or bursts. The circuit board's protective conformal coating will buy a little time but there is more than enough acid in these batteries to eat through its protective layer and dissolve most of the board. So if you are getting an RTC battery low code you might just need a battery or you might have a much more serious problem on your hands. Don't chance it. The only way to know is to look.
Back in 2009 I didn't have a source for the Detroit Diesel OE batteries So I found another somewhat practical solution. I spliced into the RTC circuits and powered it with a large coin cell battery such as those found in key fobs. That procedure bought the owners of DDECs with this problem a few more years but it had to be done carefully. If these coin cell batteries popped out of their holders they would bounce around the ECM and destroy it. Coin cell batteries are not made with the quality that the OE batteries are. The coin cells do not provide as much voltage, have less capacity and are more likely to leak than the OE batteries. DCS will not install a coin cell battery. We only use the OE batteries with a life expectancy of 10 years or more. Your ECM is a marvel of modern engineering and deserves better than a key fob battery.