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​Excessive Current Draw From ECMs: Is Your ECM Draining Your Truck Batteries?

By Fernando DeMoura

November, 2018

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My daily driver for the last 10 or so years has been a 99 F350 with a 7.3 and a 6-speed manual ZF6 transmission. A CD player, a couple dials, some toggle switches, a few extra gauges and a rubber floor are about all I’ve got inside the cab. I love the simplicity of this truck. The only thing I don’t like is when the snow starts to fall here in Pennsylvania the truck’s 7.3 HEUI engine gets hard to start. When it gets cold both the original 7.3 and the 2002 7.3 that I replaced it with need lots of fast cranking to build enough pressure in the high-pressure oil pump to fire an injector. I’m not always able to plug in my block heater and battery tender so the lower the current draw on my batteries the better my chances of getting it started. 

I take current draw into consideration when evaluating the condition of a DDEC. Part of my quality control process involves not only testing the injectors, jakes, fan, the cruise, and the sensors for functionality but logging the DDECs current draw while it’s happening. When DDEC owners ask me to check how much current their ECMs are drawing I can tell them down to the milliamp.

Excessive current draw in DDECs didn’t used to be all that common but recently I’ve been seeing more of it. I get plenty of work from owners who’ve recently bought DDECs from eBay stores and rebuilders who don’t use quality testing equipment. I know how they’re testing these boxes because years ago I used to test ECMs the same way. The first ECM bench harness I ever used was powered by a 12-volt battery charger. One of the senior guys at the company I was working for at the time said, “That’s dirty power”. He was right. A battery charger is for charging batteries not powering ECMs. It wasn’t until I invested in precise, laboratory grade equipment that I was able to see imperfections that the other guys couldn’t. Operators who run their trucks every day might not notice a minor current draw problem. Slow cranking speed and short starting battery life might be the only symptoms. For RV owners and trucks in farming industries who only run a few times a year excessive current draw becomes a no start problem unless a battery tender or a cutoff switch is used. Battery cutoff switches are fine if you have a DDEC III but if you have a DDEC IV you’ll be exchanging one problem for another. When a battery cutoff switch is used with a DDEC IV the ECM switches over to its internal battery to keep the data stored in the memory intact. This shortens the ECM battery life.

Here are two DDECs both being tested with the key off. One is drawing 0.0226 amps. It would take this DDEC over a year to draw down a full battery bank. The other DDEC is drawing almost 20 amps. Electricity, just like fuel, makes heat when it’s consumed. The faster electricity is consumed the more heat. This box is getting hot. A shot from my thermal imaging camera shows a damaged ribbon cable as the cause. You don’t see photos like this very often from ECM rebuilders because laboratory grade equipment is expensive and battery charger powered harnesses aren’t. I look at things a little differently. DDECs are expensive, breakdowns are expensive, and if there is a piece of equipment I can buy that improves my ability to find defects in ECMs I’ll buy it.

Written Fernando DeMoura, Diesel Control Service.

Phone 412-327-9400

Email: www.dieselcontrolservice.com