Resistance When My Car Is Running ?!?!
 

Hey All,

I have a question for you guys...

Here's what I did recently:

I did the ground the hell out of your RX-7 mod 2 days ago... I put a ground from my starter housing to the negative post on the battery. I put one from the neg. post to the firewall and an extra alternator wire to the positive terminal. I am also planning on putting one more from the top of the intake to the firewall tomorrow. Not to mention everything stock was reconnected. (All new wires are 4 gauge)

Now, what I have found with some testing is that I get about 20 ohms of resistance when I touch one of my DMM test leads to the negative post of the battery or a good metal part on the firewall and the other test lead to almost any part of the engine. BUT, this is only the case when the car is *running*. If the ignition is off I get 0 ohms as well as when the ignition is on *without* the car running I get a reading of 0 ohms. Also, just for reference I do get 0 ohms when I touch any part of the firewall and the negative terminal at the same time even when the car is running. So it's only between the firewall/neg. terminal to the block.

I never checked the resistance before so I don't know if I caused this or not. Unfortunately I have nothing to base my findings on.

Is this normal or should I be getting 0 ohms at any time whether the car is running or not?

If this is a problem do you have any suggestions to what it could be?

Thanks a ton!
-Mike

you could always unbolt one side of the wires and try testing again. wouldn't take more then a few mins.

Yeah, I think I'll try that. Although, I doubt that me adding *better* grounds would cause my resistance to rise.... But it wouldn't hurt to check I guess.


Any other comments?

You know checking the resistance of something while current is going through it is a very quick way to fry the voltmeter, dont you? The current flowing through the voltmeter(that really shouldnt be) is most likely causing the discrepency in resistance.

If the resistance is 0 when no current is flowing(engine off), then the grounds are good. Period.

oh I just noticed that you hadn't added the engine to firewall one yet. I'm sure that would get rid of your resistance since it is between the engine and battery. you probably need it. stick that on and test again.

I added one from battery to fender, intake to firewall, and a pos from alternator to fuse box to battery (following the stock wire).

edit: just read above post, don't retest after adding the manifold one :)

MazdaSpeed7 : No, I didn't know that testing with the engine on could fry the DMM. Does current even move through engine grounds? Could you explain for me a bit more?? I'm no electrical guru that's forsure :)

Scott: I kinda do have one to the firewall already. The one that goes to my fender from the neg. terminal and the one from the starter touch at the battery terminal. (Does that make sense?) So I figured that would be virtually the same thing as going directly from the firewall to the engine. I just thought I was killing 2 birds with one stone. Either way, I'm still going to put the other one on.


Thanks for your help so far guys! Keep the info coming !
-Mike

Lets think about this for a min.........Of course current flows through the ground wires. If current didnt flow, you wouldnt need them, now would you. And yes, checking the resistance of anything with current running through it will kill a volt meter. Voltmeters check resistance by sending a small current(thats why the voltmeter needs batteries) through whatever it is youre checking the resistance of. By comparing the signal it sends out with the signal it gets back, it figures the resistance. Now what happens is more current comes back than the volt meter send out? At best, it throws the reading off, at worst, it overloads the circuit, and you have a dead voltmeter.

The ground on a car is just to simplify the wiring. its not really a ground, its just a common -. By connecting the - terminal of the battery to the body of the car(consuctive metal), you make it unnecessary to run the - lead from everything that requires power all the way to the - terminal on the battery.

OK, thanks for the added info. It makes sense.

That means my grounds should be ok... what a relief.

Thanks
-Mike

You might want to run one from the engine directly to the negative terminal of the battery.&nbsp Remember, the spark plugs fire due to the + off the spark plug center electrode and the negative ground off the ENGINE.




-Ted

Thanks for the suggestion Ted. I have one question though:

Shouldn't the extra 4 gauge wire I put on from my starter housing to the negative battery terminal be good enough to accomplish that?

Thanks,
Mike

1) You're assuming you've got good ground contact between the starter and the engine block.&nbsp I've seen starter bolts get loose enough to prevent the starter from working correctly.

2) I use one of the number of largish bolts/nuts on the driver's side front part of the engine and/or AC/PS.&nbsp This places the ground point a lot closer and more secure versus the starter in the rear.



-Ted