Hello all, I have a 1989 f150 2wd 5.0L V8 almost 120,000 miles. She is a picky old hag from time to time. I have something drawing constant power from the battery when the ignition is off, so if i dont unhook the battery cable it will be dead. I have had this truck for almost a year now. When i first got it i put in an aftermarket stereo, and thats the only electrical part i replaced. it was just fine until about a month ago then one day i went to start it, no dice. is this a common problem? what could i do to try and fix it?

I also noticed that the oil temp fluctuates drastically after the engine warms up. Lower temps in the low end of all gears, temp rises with rpm's (the tachometer was an option in 89, mine does not have one) then drops when it shifts. Again, any tips or tricks are welcome.

Thanks,
The Super Pooper

 

On the battery drain, alternators have been known to drain back when not running.

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Maybe start by ruling out what's not causing the drain & isolating the circuit w/ the drain.

Make sure all switches & accessories are off.
Disconnect the Negative connector from the Negative battery post. Have someone monitor draw between the connector & the Post w/ a volt meter. While they watch the meter, pull each fuse individually. When the draw falls off, that will mean you have likely isolated the circuit responsible.

You didn't mention an after market alarm system, but they are notorious for developing phantom drains.

 

How are you measuring oil temperature?

 

We checked all of the fuses and checked the readings from the battery. Nothing really happened. It stayed around 24 volts the whole time. This is where we ran out of ideas.

And as for the oil temp, i don't... It seems to run fine while fluctuating, the throttle response is pretty slow though. I don't really know a whole lot about vehicles, You think it's damaging it at all? Everything works just fine when its cold.

 

And jim, Thats what i suggested but my brother is one of "those" mechanics that aren't really meachanics yet they think they know all. He told me theres no chance its the alternator. Is there a chance i get to tell him he's wrong?

 

none of the gauges other than the fuel sending unit, tach, and speedo are accurate. Rest are switches, if you have more than 6 psi of oil the oil pressure gauge works.

google or youtube, battery draw test. ericthecarguy has a good vid on it.

 

Yes it could be the alternator, if it has a bad diode it can drain the battery just like leaving the key on. The tricky part is it won't do it all the time.

 

It sounds like it might be the alternator. I forgot to unplug it the other night and it was fine in the morning. Thank you for the pointers, it is appreciated greatly.

 

There is no oil temperature gauge in your truck unless it's been added. Exactly WHAT is fluctuating?

There is NOTHING in that truck that should read 24 volts.