Entire Vehicle Ground test

Aaron Owner/Technician Holland, Michigan Posted   Latest  

So I was watching Jim Morton P&Qs video on youtube He mentioned a ground test that Ive never heard of. Heres how I understood the test. One lead on alternator casing and one lead on a clean spot on the fire wall. Run engine up to 2000 RPM with every electrical load you can find that the car has. At that time the volt drop is supposed to be no more than 300mv and if it is you can run a jumper between bat ground and firewall to see if you can get your reading below 300. So my question is if this test passes below 300 can you assume every single ground on the car is good and know for certain that you dont need to check any individual grounds to PCM or whatever else? Im going to start testing every car that comes in with this test if thats the case. Anyway just looking for more info on that test.

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Michael Owner/Technician
Boscobel, Wisconsin
Michael Default
 

No, is the answer to your question. All you are doing with that test is checking the electrical connection between the engine and body, both of which should be connected to the battery ground in a negative ground system.

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Aaron Owner/Technician
Holland, Michigan
Aaron Default
 

OK thanks that makes sense then

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Stephen Educator
Bethel, Ohio
Stephen Default
 

I tell about this test in my book. The thought process here is that if the is a bad ground back to the battery the current is going to flow through the body and to the alternator case. Loading the electrical system and having the alternator output increase causes the power and ground circuit to be loaded. Since the body of the vehicle has grounds to the battery not following a path through the…

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David Owner/Technician
St. Albert, Alberta
David Default
 

I am skeptical of this test as you have described it. Seems to me, a bad ground usually indicates high resistance and less current. So, then it seems there would be some devices not getting enough power to operate? This test is supposed to tell me if there is a bad connection on the ground side of some accessory device?Even then, I would want to know if there is another ground path to the…

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Stephen Educator
Bethel, Ohio
Stephen Default
 

A bad ground can cause and is an indication of high resistance in a circuit. We know the body of the vehicle is used as a ground return. The body attaches to the battery negative not through the engine block but primarily from a ground wire or wires going back to the negative terminal of the battery. If there is a problem with any of the grounds, the electrons will look for another path. With…

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Brian Owner/Technician
Charlottetown, Prince Edward Island
Brian Default
   

If what is being tested is between the test leads then in this test the only thing being tested is the alternator case to body grounds which would be thru the engine to body connections​.​In this case the dvom leads are in parallel with the circuit from most positive to most negative​.​Further down in this discussion you mention that the meter is the…

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Craig Technical Support Specialist
Chicago, Illinois
Craig Default
   

Hello Aaron, for a really good and focused test of the starting and charging system, check out this website veejer​.​com/index​.​html. There's a testing guide called first things first, it's a very well thought out process that can be done quickly with practice. Vincent's explanation of what to check, as well as what problems can arise from basic starting and charging issues is rather detailed…

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