Voltage Drop Testing Modules

How do you properly test power and grounds on modules? First example, ABS module, how do you load the circuit? Second example, PCM on a car the won't start or run?

I feel like there is a way to use a test light as a load but am not quite grasping the concept.

Thanks!

Nissan Technician - ASE Master Tech

Not easy to do avolts drop, usual quick cheat if you can't see green crusties is just to provide an alternative path to ground directly to Bat negative.
Back probing the PCM the connector with an Oscilloscope to ground should show something unless the bad connection is at the circuit board also a DMM might work but I don't think a using Power Probe as a voltmeter would but you could load the ground path with a Power Probe and measure the volt drop with a DMM or scope.

Lets hear how the rest of the guys do it.

Use a multimeter, scope, or test light from batt positive to a power source or from batt ground to a ground source. Like what scannerdanner did with the starter in this weeks video

Doesn’t have to be from a battery post either. A good way to find a bad wire is to check voltage from one end to the other. If you have voltage across 2 points of the same wire, these is a bad spot in between. The circuit does have to be powered up by the source or by a test light

Tutti57 wrote: I feel like there is a way to use a test light as a load but am not quite grasping the concept.

Disconnect the connectors. Back-probe the power and ground with a couple "T" pins. Be VERY careful not to let them touch each other. Take a voltage reading on each "T" pin. Now, use the "T" pins to power the test-light. Check your voltages, again. The power should not drop, and the ground should, still, be zero. A bigger bulb will load the circuit, even more.

Or, you could do them one at a time. (This would be the safer method, as there is less chance of touching the connector POWER and GROUND, together.) Power the test light using the connector POWER and an alternate ground. Then, power the test-light, using the connector GROUND and an alternate power source.

you could load the ground path with a Power Probe and measure the volt drop with a DMM or scope.

Just a few words of caution about the Power-Probe:
Be sure to disconnect the circuit from the module and ground, first. The power-probe supply's un-resisted power. Attempting to energize a ground circuit will result in a tripped Power-Probe breaker, at best. Or, a cooked module, at worst. Power-Probes are great tools, but can cause a lot of damage, if used improperly.

Like others said, I really like alternate loads. I have a 2157 brake light and a 9007 headlight rigged up with sockets and alligator clips for this exact purpose. The 9007 will draw around eight amps with both elements in parallel - perfect for fuel pump, motors, and other high draw circuits.

That's the setup I'd use on your first example of an ABS module, since it's got some pretty heavy gauge wires going to it (for the pump motor). An ECM, I'd be happy with the 2157. More load than an incandescent test light, but not enough to cook wires or blow fuses.

Having recently acquired a "Load Pro" (right here on this forum as it happens), I finally got to use it instead of the traditional light bulb. Subject in question was a PCM that was supposed to ground a relay to activate it. I finally got to where I could use the Load Pro in conjunction with a multimeter to prove that though the PCM was outputting a ground, it couldn't bear any load at all. Not even a little relay. I was also able to verify that the PCM's ground were healthy. So it's worked for me at least once now.

For more info on a Load Pro there are some rather over enthusiastic Youtube sales videos out there. They do show you what it does and how to use one though.

check out dan sullivans video on youtube on the load pro awesome tool i use it all the time

The tool is a powerful handy tool but should come with a danger of module death warning, the videos selling are something I chanced on once and have avoided ever since.