Voltage Drop Test

[Dan]

I have a question about one of Eric’s battery testing tips.

http://www.ericthecarguy.com/faq/solving-automotive-no-start-problems?start=1

“In fact, you can do a little trick if you suspect a bad cable. Take a set of jumper cables and bypass the suspected cable. If you suspect a faulty positive cable, you can just hook one end of the jumpers to battery positive and the other to the positive terminal on the starter.”

Just to be totally clear, I’m going to leave the battery completely connected and I’m going to clamp one jumper to the positive terminal post and the other jumper to where the positive cable is connected to the starter?

After letting my car to sit and cool off for 8 hours or so, the engine is slow to turn over. This is with a fully charged battery. If I’m driving around with a couple stops and starts here and there, the engine turns over immediately when started. I suspect that the cable is the culprit.

If I run this test with a cold start and the engine turns over immediately than my suspicion is probably correct?

Topic

[EricTheCarGuy]

Yes, you can leave the cables connected for that test. However, the first thing to check in that situation is the battery itself. It may be charged, but it may still be bad. You need to load test it to find out for sure. Anytime you have an issue like that start by load testing the battery and checking the battery connections.

More info on that can be found here.

http://www.ericthecarguy.com/faq/solving-automotive-electrical-problems

Good luck and keep us posted.

[Dan]

Hi Eric,

In “Diagnosing Starting Problems” you say that no more than .7 volt for the positive cranking test is acceptable.

Another (I think reputable) source states that no more .2 volt is acceptable in this test.

Does it possibly differ from model to model, for whatever reason?

[Dan]

Well I’ve tried every which way to reach down and in and up and in, in order run the jumper cable test, but there just isn’t enough space to clamp a jumper to the starter positive.

I’ll rustle up an assistant and load test the cables and battery with my dmm.

[Austin]

Use a high current inductive clamp and see what amperage you’re pulling through it. Voltage drop is great, but if it shows voltage drop it can also be resistance in the starter itself, as well as resistance in the ground.

[EricTheCarGuy]

I would forget that test if it’s too difficult to achieve. You can always do a voltage drop as outlined in this article.

http://www.ericthecarguy.com/faq/solving-automotive-electrical-problems

As for the readings, I got those straight from the company I partnered with for that video. Bottom line, you don’t want to see a big voltage drop in a cable. In fact, there should be VERY little voltage drop in the cables and connections. The higher the voltage drop, the greater the resistance in the component. You want your resistance to be in the load of the circuit, which would be the starter in your case, NOT the wiring going to it.

I would stick with a straight up voltage drop test for now and not worry about bypassing the cable with jumper cables.

[Dan]

Well I did never get around to fully testing the cables, but I also think the “slow turnover” might have been a product of my (inexperience with cars) imagination. The battery seems to be charging normally and I can’t complain about the performance at startup. Thanks for the tips, Eric.

[EricTheCarGuy]

Not fixing something is often better than having to fix something. I’m glad you have it sorted. Thanks for the updates and for using the ETCG forum.

[Author]

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