I used to believe I was fairly competent at parasitic draw diagnosis and repair. Until, that is, my C3 kicked my ***.

Let me share a few tips if you've always got a flat battery!

Generally you do parasitic draw testing first by testing a vehicle's fused circuits, and then unfused circuits

There are actually several techniques to fuse-pulling.

You can pull them all and put them back one by one until the draw comes back. (Or if it doesn't go away, move to the unfused circuits)

Or you can pull 50%, and if the draw does NOT go away, U know it's in the other 50%, pull half of those, and .. unless you're very unlucky, minimize the total # of fuses you pull.

After you get done with the fused circuits, if you haven't found the parasitic draw, it's time to go to the unfused circuits, including

Starter
Alternator
Ignition circuit
Anything, car stereo, amp, alarm,... a previous owner (PO) has HOTWIRED to the battery or other unfused circuits. And oh, can they be devious.
Voltage regulator on older models
(Stuff fused by way of "fusible links" rather than fuses!

I found a failed power antenna pulling 40mA had been hotwired to the always hot clock circuit in my C3.

But wait, it gets better!

The washer/wiper circuit in the C3 is HALF fused, HALF unfused! Absolutely certain my draw could NOT be in the wiper circuit, since I"d pulled that fuse, boy was I surprised
when unplugging the wiper motor made my 2.5A draw go away. I still don't know if it's the park circuit, a failed wiper motor, or what. Haven't had time to work on THAT.

So...techniques. I generally connect my meter directly between the battery post and battery terminal, using alligator leads. This won't work if you've already blown the amp fuse in your meter! And
most meters can only handle 10 or 20A, intermittently! So..pull fuses..check..pull fuses...check...

Another technique: Fuses have a small, but finite amount of resistance. That means there is a VOLTAGE DROP across them if even only a tiny parasitic draw (50 mA) is flowing through them! V= IR. Say a 50mA draw. Even if R=.001 ohms your meter can measure V= .050 (50 milliamps) * .001 ohms and display .05mV. That's a lot more than ZERO, and DVOMs CAN measure low voltages quite accurately. The fuse with the voltage drop is energized!

Doing it this way you may be able to identify the problem circuit without pulling a single fuse!

I've seen techs that connect massive, heat-sinked 1 ohm or 10 ohm resistors in series with the battery cables and simply measure the voltage drop across that to infer the parasitic current. (Guys who blew the amp fuse in their meter a long time ago and never replaced it???)

When my C3 was really busting my chops (because of the 50/50 wiper circuit) I went to the horn relay and started pulling cables off it. The draw had to be on one of them! I was pretty riled up by that point!

Rule of thumb is anything under a 50mA draw is acceptable, anything over needs investigation. I have an older Lexus with a 70mA draw. A C3 should be far less (clock, replacement radio station memory, alarm, etc)

You can also get some CLUE as to where your draw is by the size of it. 30mA? small bulb. 75 mA? Relay. More than an amp is a LOT of current. Something "out there" is warm from all that current. So you can go out to the garage on a COLD morning with a thermal (IR) camera, and may be able to spot the warm component!

(Although I likely would NEVER have spotted the power antenna, buried as it is above the muffler!) You gotta be RELIGIOUS with that camera and go over every square inch, every place wires run under carpet, etc!

Lots of good Youtubes on chasing parasitic draws. Hope this helps you!

 

Hi again,
I've learned a lot from following along on electrical problems and solutions.
I believe that you are working on a '68, yes?
Seems that year has 'special' problems with the wiper system.
Willcox has a correct schematic for that unique year.
After you pulled the wiper fuse where is the second power supply?

Others have enlightened me to the fact that the BLACK ground wire from the wiper case to the heater fan motor and ending at a starter bolt gets inadvertently connected to the battery terminal on the solenoid and causes a battery drain.

Keep us informed with your progress.
Pete.

 

On your 68-

Have you checked the wiper service switch- if left on even with the ignition off it'll drain the battery-

Here's the service bulletin on it-

https://www.corvetteactioncenter.com...print-697.html

Richard

 

Yes, I have a copy. Invaluable.

>After you pulled the wiper fuse where is the second power supply? I read somewhere it comes off the headlight switch. Have not confirmed. My goal was to get the 2.5A draw killed so I could "safely" drive the car back to Phoenix for the winter. After that I had bigger fish to fry.

Others have enlightened me to the fact that the BLACK ground wire from the wiper case to the heater fan motor and ending at a starter bolt gets inadvertently connected to the battery terminal on the solenoid and causes a battery drain.

I've examined that wire down at the starter, it has two wires coming off a single ring terminal so I'm fairly sure it's wired properly but I'll double check it.

Thanks for the tip Pete!

 

All I know is that back in '95 when I bought this '72 vert, the entire wiring harness was a casualty of the DPFI install, and the auto overdrive trans and headlight mods, and wiper door being electrified ....SO, I wired my car MY way, the correct way.....

that yellow wire to the wiper motor should be switched hot,.....the way I remember the color code anyway, it's the middle terminal of the 3 inline on the motor.....

course I cheat, olde tyme ET here from the vacuum tube daze yet.....most of yawl never even SEEN one, unless in a museum, I bet.......

 

I haven't had time to diagnose my misbehaving wiper system YET. Sometimes it won't lower the doors, right now it won't raise them. The malfunction changes depending on what day it is, lunar phasing, etc. I've checked / hotwired the door limit switch, tested the wiper switch, tested the service switch, even tested the solenoid for opens/shorts. (Maybe its sticking internally even if electrically it's ok) It's narrowing down to vacuum line problems, failed motor, or intermittent / chafed wiring as it was working 100%, then when I shoved the dash back into place, it didn't work! Argh!

So the short answer is I'm going to have to 100% fully understand the intended operation to diagnose why it ain't working. Thanks for the helpful Service Bulletin.

 

You really should not be installing a Diesel Particulate Filter on your Vette, buddy.

/SARCASM