I have a 1986 f-350 dually diesel. I just bought it, its in rough shape but runs real good and when i first got it a week ago it started right up. On the way home its started to start a little slower and by the time i was home, after i turned it off i couldnt start it at all.

I noticed two things... one that the alternator belt was loose, so i tightened it so the alternator is definitely spinning, and i replaced a broken battery terminal on the battery to the left (I dont even know why the truck has two batteries... diesels are new to me).

The previous owner wired in a doorbell type puch button ignition, so i turn the key to on and push the button to start. It seems like the battery is dead, it will only turn over slowly a couple times, so I jump it and it starts hard but then runs fine. After running it a while i can turn it off and wait a few seconds and start it right back up. But if I come out an hour or two later it wont start again, but there is enough juice to have the cab light on.

I am going to try and sell it, and probably wont get much more than $1,000 or so for it so I dont want to spend hundreds of dollars on new batteries unless I have to. I read somewhere that it may be a bad relay draining the battery? Is there other things I can try and problem shoot before going out and spending a ton of dough on batteries?

 

Have your batteries tested. You might have dead cells in them that drain your batteries if the vehicle sits very long. You could also have your alternator checked, but I'm guessing it's ok if you can start the vehicle back up after having it off for short periods of time.

 

I agree with Jed, a dead cell or two in one (most likely both) batteries will cause the problems that your having. Batteries with weak or dead cells have the tendency to read 12+ volts with a multimeter, but once under significant load like starting, the voltage will quickly plummet.

The truck is equipped with two batteries due to the fact that diesels have much higher compression ratios than gas burners. The high compression makes these motors much more difficult to turn over. To overcome this, the trucks have heavy duty starters that pull a lot of amps. You'd be very lucky to get a cold motor to started on weak batteries.

On top of this, the glow plugs will also add a heavy load to the batteries when starting cold. Which may explain why you have trouble getting it restarted after it sits for a couple of hours.

Have the batteries tested at a local automotive parts shop. Once you can rule them out, I'd start looking elsewhere.

 

Barty88,
Welcome to FTE and the IDI diesel forum.

I would say fix the battery clamp that is broken, which one is it?

Passenger side clamp is very important, that is where everything ties together for the electrical system.
Alternator charge goes there, cab power goes there, starter load goes there and the glow plug load goes there.

200 amps for the glow plugs, 300 amps to crank the starter.

Two batteries of at least 850 CCA are needed.
Winter time starts, two 1000 CCA batteries make getting it started a bit easier.