if you only have one battery their is lotsa drain, glow plugs, starter, accessories. it's almost impossible to fire it up on one battery unless it's in really good shape. best to replace both with identical batteries. although i replaced one and two years later replaced the other with an identical battery, so it can be done.
Due to the high compression of diesels, typically 18-19 to 1 or more, two batteries are needed to crank it fast enough to start. as for replacing one battery, I've been told if you only replace one it puts a hardship on the weaker battery. Not sure how true this is but I can see the logic there.
Yep, it takes two batteries. GM tried diesels with one battery in the early 80's and quit right away. The battery had to be so big that it became a really high price battery and still didn't have enough to do it right, and that was on the car motors.
I just had one go bad on my truck last fall. The thought of replacing only one never crossed my mind. I put both in new at the same time and bought the best batteries I could. If you look at a wrecker bill, it will be more than the price of the battery that you didn't replace. And besides, just about everyone has a need for the OK battery that maybe didn't needed to be replaced. So fix your van up right and have a spare battery around for what ever else may need one.
One good battery and one bad equals problems. Bad battery will be pulling the good battery down when shut down and the alternator will always be playing catch up charging both when restarted. Price of fuel, it pays to have two good batteries.
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