ALPOC Message Board

[FelipeT]

Hey guys,

While this little story is actually applicable to general automotive work and isn't strictly a second gen thing, I figure that I would post it here because the story comes from a second gen. I know that most of you here probably don't need help with this sort of thing, but I figure that I'll throw this up here anyway just to expand the knowledge base. Who knows, it might help some passerby.

I was working on a '93 PGT for a local guy who found me (apparently, I'm "the Probe guy" around here, so that happens sometimes.) The owner was complaining that the battery in the car drained down in the span of about three days to the point where it would barely start the car, if at all. He wasn't kidding, either--It sat here for about a day before I was able to get to it, and the car would barely start.

Finding this kind of fault is actually really easy if you follow a plan and have the right information. The first thing, before anything, is to verify the problem. In this case, I saw for myself that the car, left unattended, will drain down the battery, but I still had to make sure that had a "clean" starting point for the diagnosis. This means eliminating anything that might invalidate tests or be the actual cause of the apparent problem itself. In this case, the first two things to check are the battery and the alternator. Both were good, so I could begin.

There are actually a couple of ways that you could do this. Probably the easiest and best way would be to start with charged battery and put your volt meter across it's terminals with the battery disconnected from the car. It should read somewhere in the neighborhood of 12.2V. (If it reads lower than that, the battery isn't charged!) Now, connect it to the car and check again. If you just saw the voltage go down a few tenths, then, yes, you have a parasitic drain.

(Note: I actually did my tests a little differently, but more on that later.)

Now, the fun begins. Start disconnecting each fuse, one at a time to isolate circuits. The exception to this is the 100 AMP main fuse. It's much easier to disconnect the wire right next to it. (The one after the fuse, or on your right, as you are standing over the driver side fender, looking down at the under-hood fuse box. If you disconnect the one before it, it disconnects every circuit from the battery other than the starter!) Just pull one fuse and put it back while watching the meter. You're looking for an increase in voltage when a fuse is pulled. I got as far as the 40 AMP engine cooling fan fuse before I saw the meter twitch. Double checking it, and indeed, pull the fuse, drain goes away, put it back, the drain comes back, too.

This is where you will need wiring diagrams. I was lucky, because there isn't a whole lot to this circuit. Mostly, it's the cooling fan motor (driver side) and two relays (high and low speed.) The motor is the easiest to access, so I started there. I pulled the connector, and the drain goes away. Too easy? Yes. When each positive terminal of the motor (there are two) are tested, there is no short to ground. Besides, there shouldn't be any current flowing to the motor if the rest of the circuit is working properly.

Next I checked the low speed relay. Pulling this relay kills the drain, too, but if you look at the diagrams, it doesn't actually isolate anything, so this might be the problem, it might not. I have to admit that at this point, I did something which I hate doing. I just swapped it with a known good part. If I had not had one available, bench testing the relay could have just as easily told me what I needed to know--this wasn't the real problem.

Now, we get to the high-speed cooling fan relay. If I had studied the wiring diagrams more closely before beginning, I probably would have checked this first, because this is the most likely place where such a fault could occur. Besides, a quick visual inspection also made this suspect. Disconnecting this relay makes the problem go away, but with one big difference--someone has taken this relay off of it's mount, so it's hanging upside down, and it's protective boot has been peeled up, away from the connector. This is where I should mention that you should have been inspecting every connection as you went along, because this one looked terrible. Water had obviously gotten into this relay and corroded it and the connector. Big red flag. Interestingly, when plugged back in, the fault didn't return. The short was in the crud between the connector and the relay.

I still replaced the relay as a precaution, but first cleaned the connector. with that kind of corrosion, I didn't want to take any chances. A healthy dose of dielectric grease and mounting it in it's proper position, with it's boot in place should keep the problem from coming back.

Finally, a quick double check is in order: does pulling the fuse show the same changing readings on the meter as before? No? Great--then it's fixed!

Questions, comments, ideas?

I may follow up on here with some additional notes...
_________________
--Felipe I Torrejon
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[PSE driver]

Good Job Phil. I've just got my probe back from my folks house. It's been sitting there for over a year. I think I've got a few similar problems like that as well. I'd like to look it to them. I just washed her up this afternoon and got the first layer of dirt off so I need to start getting her back in order.
_________________
~ PSE driver - James
'96 PSE-T

If it takes a chicken and a half a day and a half to lay an egg and a half, then how long does it take a grasshopper with a wooden leg to stomp a hole in a 2x4?

[FelipeT]

Ah, thanks!

Tracking down problems like these isn't incredibly difficult, it just takes a methodical approach and a little persistence. The truth is, I really like electrical diagnostic work.

One thing which I did a little differently from the procedure described above is that after I did the initial voltage drop tests, I took the battery out of the car and did the rest of my testing using resistance. Thus, when I connected the meter leads across the positive and negative and selected the lowest Ohm rating, I got a resistance of about 100 Ohms across the two terminals--way too low! (Remember Ohm's law: it's an inverse relationship, so you want to see a really high resistance which will indicate a very small current draw.) Pulling the cooling fan fuse basically broke the circuit (short.) I'm sure that if I had an auto-ranging meter, or if I selected a much higher Ohm range, I would still get a reading of some sort, but I was just looking to eliminate the big current draw item. There will always be small current draw items like the radio clock and computer memory.

Anyway, that is an alternate test method.

I suppose that you could also select Amps on your meter and test it out that way, but I wouldn't recommend it simply because it is very easy to damage a meter in that mode if you are not careful.

I called the guy to let him know that he could pick up his car, but he hasn't been by yet. That's fine, though, because it gives me a chance to test things out. After sitting for over 24 hours, the car still starts up just fine.
_________________
--Felipe I Torrejon
Visit My eBay Store!