Extreme Parasitic draw

Asked by May 01, 2017 at 02:10 AM about the 1989 Ford F-350 XLT Lariat Crew Cab 4WD LB

Question type: Maintenance & Repair

There is a massive draw on the battery/ batteries
that will drain it from 12v to 0v in 10 minutes! How
can i test it to find the draw when the volts are
continuously dropping? Can I test for the draw
while the charger is attached? So I at least have a
constant power source, or is that stupid? It's a
referbed 7.3 idi, and there are several home made
electrical add ons so the manual isn't super helpful,
I'm about to just start cutting anything that's
nonessential to run it! Any advice would be so
helpful! Thank you

11 Answers


A draw that massive is not parasitic, your on the right path buy disconnecting all the added on electrical systems first, then if it is still there disconnect the alternator and then the glow system as only those two other than the starter, can pull enough current to kill a battery that quickly just sitting. As to hooking up a battery charger while testing you would need a big one to do that. I would disconnect the batteries and recharge them, and then do this: use just one battery as a circuit tester, hookup the positive cable on one battery and put a test light between the negative battery post and the cable end, if the test light is bright then you have enough of a draw to kill a battery in short order, after you have removed all of the added on equipment electrically and none of those turned out the light, then you have to start on the vehicles systems, start by electrically disconnecting the alternator, if light goes out or just glows faintly you found your draw, if not proced to the glow system, if light goes out then you have either a stuck glow relay or the glow control unit is bad, if the light is still on then start pulling fuses noting where each one goes until the light goes out, once that happens find out what componets are on that circuit and repair or replace the culprit, then start putting the fuses back in making sure the light does not turn back on bright. Note parasitic loads will cause a test light to glow dimly, this is not a full blown battery draw. Hope this information helps, I know a lot of tech us voltmeters for this test but I like using the light, guess I learned to do it that way and it stuck.

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Best Answer Mark helpful

Here is a video on how to test for a parasitic draw... https://www.youtube.com/watch?v=QRso1A0VScw

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I would hook it up to a steady 12V source, and also make sure to pull driver door fuse or computer modules will stay "awake" and draw more current than when they sleep, usually about 10 minutes after closing the door. That's a huge draw from your description, sounds like a motor or one of your add-ons.


I would start by disconnecting the home made add ons and see if the draw disappears.


First, I would disconnect the battery and recharge it. Then test to see if it dies by itself from an internal short. They do that when they get old. Next, I would disconnect the alternator output wire, the heavy red one and hook the battery back up and see if that was the problem. Sometimes alternators develop a "leaky" diode that bleeds the battery down even when everything is "off". Next, I would start testing each of those add on gadgets for drain. Until you get it fixed, park it outside or disconnect the battery so you don't have a fire. A dead battery in ten minutes is a huge drain! I hope this helps.

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Thanks so much for the advice! I'm headed out today to try it out... I charged both batteries disconnected this time and I'll try the test light out. So the previous owner was the one who put in the add ons, and I think I know which one is the problem. There is some kind of coolant heater that he installed inside the cab, behind the back seat. I'm not sure where the power originates but I think it's direct to battery. I didn't ever hear the motor when the batteries were draining, but could that mean the motor was just seazed? Also if I disconnect it, I should be able to cap the coolant hoses without affecting the system right?

Also he has a 4 plug, in home style outlet wired with the tail under the hood for plugging into a wall outlet, with THREE cords plugged in that look as though they all go to the block heater. Is this set up for the block heater normal? Or is that also likely to be causing the problem?


Disconnecting that coolant heater could prove problematical depending how it was connected to the cooling system, you may have to redo the lines, on some of those you will loose cabin heat if you just cap them off. If i remember right those were suppose to be powered by a 110 volt power inverter and there should have been a shoreline plugin mounted somewhere that went to the power box, but maybe the original owner wanted the inverter as they can get expensive.

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Ok, so I messed around with it for a while and figured out that it's actually something to do with the starter solenoid. More precisely one of the circuits connected on the same post as the wire from the battery to the solenoid. I removed that wire and tested it with my meter, it has a short to ground somewhere in that line... I don't know what that circuit controls though but it's a larger wire that splits into 4 and then follows a bus to all the connectors under the steering wheel. I'm not sure how to figure out which circuit it is... Do I have to brake open the wire bus to follow those wires? Or is there somewhere I could get cheap wire diagrams? I also tested the starter solenoid and that works, also tried the screwdriver start and the starter cranked just fine! Is just when that wire is connected, it somehow draws all the power that should be for the starter!


The cheapest place to get wiring diagrams is your local library in the reference section find the repair manual for your truck and have a copy made of the wiring diagrams you need. Or buy a online wiring diagram or a complete repair guide with diagrams and componet locator.


Thanks for the best answer points.

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