Testing Defroster Grid Wires Quandary

[ARCHINSTL]

OK to snicker and guffaw at my ignorance, but here goes (At my age, I've lost all sense of embarrassment.)...
Problem: Testing defroster grid wires - some are visibly broken and some just do not work.
Aggravating factors:
a) Electricity is mystifying.
b) I fried the ignition coil/module/pickup/all of the preceding back in '06 when I forgot and left the ignition turned on way too long while testing something else.
c) There is only moi, so having a helper cycle the ignition key while I test the wires with the meter is not an option.
Possible solution:
Since the p'up has a 12V battery, can I hook it, via jumper cables, up to the two prongs of the grid (after removing the OE wires)?
Does it matter which clamp goes on which prong?
Should the negative clamp be grounded to Goldie's body?
Addenda: The relay, breaker, and switch tested OK when I checked them earlier this year. I will check the flexible wire packet from the body to the hatch first.

Will I harm something by this method?
Excuse my paranoia, but I just do not want to fry something.
Thanks,
Tom M.

[c_j]

Since the p'up has a 12V battery, can I hook it, via jumper cables, up to the two prongs of the grid (after removing the OE wires)?

Yes.

Does it matter which clamp goes on which prong?

No.

Should the negative clamp be grounded to Goldie's body?

No.

Will I harm something by this method?

No.

If you remove the Terc wire connectors from the defroster, and hook up a positive and negative to the defroster from some other power source (your truck battery), then the defroster might as well be in your truck as there is no electrical connection to Goldie whatsoever.

You could use Goldie's body as a ground, but that just increases the complexity of your troubleshooting.

Let me draw a diagram...

[c_j]

I might be getting a little ahead, as you haven't proved there is power to the grid yet, but:

If you can't visually see the breaks in the defroster grid, you can test with a voltmeter that is hooked to the negative side (ground), and probe along the grid with the other end of the meter. This is what you will typically see on a broken defroster grid:

troubleshoot_defroster_grid.png

Note the broken grid lines on row 2, 5, 6, and 8 (with multiple breaks on 6).

Now the one side probably won't be exactly 12V, but the general idea is that the full voltage is on the positive side, and the voltage tapers off to zero as you move toward the negative side. If there is a break in the line, then you will have full voltage on the positive side of the break, and zero on the negative side of the break.

[Petros]

I would just unplug the distributor and use the ignition switch. this way you unpower the ignition system, and the defroster system is tested in the car all connected as designed. You have zero risk of over powering it or damaging the contacts by using another separate battery directly on the heating element. there is a reason Toyota uses a relay on the defroster. You may want to jump directly to your battery however with the truck battery, that defroster (if working) pulls a heavy power load.

[ARCHINSTL]

Thank you both, gentlemen!
I feel like smiting my forehead over Petros' suggestion... Whenever I change the oil, I disconnect the distributor lead and turn the engine over by the starter until the oil pressure light goes out. Why didn't I remember this?
I think I will disconnect the dizzy and test the system to make sure the electron guys are getting TO the grid, and if that works, then hook up the p'up's battery and follow c_j's outline.
Tom M.

P.S. Thanks for muting the chortling. I didn't fall off the tech turnip truck yesterday, but I've just never gotten the hang of electricity.

[irowiki]

This is all very useful info for when I end up testing mine soon! Post pictures if you end up needing to take apart the inside of the car to get at the flexi wires!