Batteries drain unless block heater is plugged in

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mtboggs

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Just bought a 96 5spd 12 valve and if it's below around 40 degrees if I don't plug the block heater in it seems the batteries get drained but if I plug it in it'll start up no problems. I tested both batteries and both tested good had a new starter put on 2 months ago, I'm clueless I've never had anything like this happen before any thoughts or help would be appreciated
 
The block heater isn't tied to the electrical system so it being plugged in is just masking a problem by helping it to start easier. How did you go about testing the batteries? Sounds like they may be weak even though they tested "good"
 
Harder to turn engine over when everything is cold.

Batteries are less efficient the colder it gets.

Its a win, win.
 
estrada5.9 said:
Harder to turn engine over when everything is cold.

Batteries are less efficient the colder it gets.

Its a win, win.
Click to expand...

Technically that's a "lose, lose". :lolly:

Like Stubz said, the block heater is likely just masking the issue. You can try disconnecting both negative battery terminals when you park it. Then see if that changes anything. If you have a multimeter, then you can check for current between the negative battery terminal and the negative clamp. Just make sure the other battery is disconnected, and the hood light isn't on screwing up your reading. Also check to see what voltage your batteries are charging too with the engine running.
 
I'm a mechanic at a trucking company and we have a battery tester from interstate batteries and that's what I used to test them I have checked them twice and both times both batteries tested good. I know the block heater has nothing to do with the charging system but if I don't plug it in it probly doesn't even crank a full revelation. When plugged in it starts normal doesn't crank slow or anything spins over like it should I'm puzzled none of it makes sence.
 
I bet if you parked it in a warm garage it would start fine to.. id say the block heater is keeping the batteries warm as well as the engine block which will increase there output from the 32 degree rating...try swapping batteries just for giggles.
 
unless the PO added a battery minder to the block heater plug in (I actually like that idea), the block heater plug has nothing to do with the trucks electrical system.

The engine is a lot harder to turn over when it gets cold, plus the batteries can't put out as much cranking amps. Even my one truck with a single 5 year old battery can start @ 40*.
 
I followed the block hearted cord to check that and there are no extra wires or harnesses coming or going from it. I tested the batteries today after it had sat outside all day it was plugged in but the batteries still checked out good both tested at over 900 cca and on the batteries they are 840 cca one battery is new and the other doesn't look like it's very old I have checked all connections and cleaned any corotion that I found but I didn't find any bad conections.
 
And the truck is garage kept every night and even then it will start with out block heater but barely starts
 
(A) Test the bats after letting them sit all night without plugging the block heater in.
(B) If they test good, turn your ignition on and let the intake heater cycle, then test again.
(C) If they still test good and still won't start the truck, I suspect a starter issue.
 
This sounds odd, but if a previous owner put a block warmer/heater in the truck it could drain the batteries overnight where using the block heater plug will not cause the block warmer/heater to run.
 
It's a stretch as well but possibly there is a short in the truck. When the block heater is not hooked up it pulls amperage from battery but when the block heater is hooked up it provide a separate path the ground which prolongs battery life.

It does not seem to be to far fetched but the difficult part now is how would you chase it.

Did you try parking it in a heated garage over night to verify it is not temp related but time related?
 
We go through hundreds of batteries a year here getting concrete trucks started and moved around for customers. One thing I have learned is the battery tester is everything but gospel. Plates can be taking a crap in the battery and they will test out alright with no load or they will test OK but not great but be just a little too weak to do what you're asking.

Does your interstate tester put a heavy load on them? Our tester gets the clamps glowing red hot by the end of the test and really finds the weak ones typically but as I said not always.
 
Sounds like weak batteries and a worn out starter to me. Add that with the draw of the grid heaters when it's cold and I can see why you're having issues. Might look into a starter rebuild kit
 
I know Ive had more trouble this year with batteries testing just fine, but as soon as ya put a draw to them they shat out. Dam batteries are going to be the death of my checkbook I swear... To the op, I echo everyone else. My money is on your batteries being wore out as well as a starter probably going south. Good luck!
 
OP, I bet your coolant being warm-ish actually keeps your engine bay a few degrees warmer. That along with the engine turning over like a champ and the fuel igniting easier (warm engine = less compression to ignite), probably just makes it feel like the batteries are charged.

I had a similar issue when my alternator was going out on me. Swapped it, and I've been good ever since. My dying alternator would slowly drain my batteries when the truck wasn't running, so if it sat for a couple weeks, it wouldn't want to start.

Another good trick an old timer taught me is to toss a light bulb next to your battery. Run a splitter out at the truck so when you plug it in, you are plugging in an old light bulb sitting next to your batteries. The little bit of heat from the light will do more good than you think. I only had to do that at subzero temps though.

Finally, get some good batteries. I picked up the biggest Red Top Optima's they had back in summer of 2008 and they have been drained dead more than a few times by me and are still great. Not as good as they were brand new, but still great.



Cowboy303 said:
unless the PO added a battery minder to the block heater plug in (I actually like that idea)
Click to expand...

I really like this idea too... I doubt a small motorcycle sized one would do anything with around 1800CCA worth of batteries though, and I assume most use a timer so their trucks aren't heating coolant all night.
 
I'm not sure how much of a load the battery tester is putting on the batteries, it doesn't turn anything cherry red I have had it give me a false reading before that's why I tested them several times and each time it said the same thing. I'm going to swap starters out of my 98 12 valve and see if that doesn't change anything, If that doesn't change anything then I'll swap the batteries also Last winter I put new batteries and starter in it and hadn't had any trouble with it starting. And my garage isn't heated but does stay warmer than out side it did seem to make a little difference it would start on it's own in the garage but barley
 
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