Currently I am getting a very erratic reading with my multimeter.

Whenever the meter is INSIDE the engine bay the volts jump around like crazy. It doesn't have to be touching any connections just in the air!!!

Whats going on?

I've tried different multimeters, two different alts, and bypassing the killswitch





My setup...

Batt in rear
GND from batt to frame (1/0 welding cable)
PWR to kill switch then PWR to distro block in bay (1/0 welding cable)
Alt lead to kill switch
1/0 welding cable from distro block to starter
4 ga. GND strap from frame to motor

Simple setup but nothing is helping!

 

My setup...

Batt in rear
GND from batt to frame (1/0 welding cable)
PWR to kill switch then PWR to distro block in bay (1/0 welding cable)
Alt lead to kill switch
1/0 welding cable from distro block to starter
4 ga. GND strap from frame to motor

Simple setup but nothing is helping![/QUOTE]

 

Wow, thanks for the help! I would have never thought the multimeters would be too sensitive.
One multimeteris a Craftsman and one is a cheapo Harbor Feight but both are digital.

If I don't 'fix' this bad interference won't my stereo pickup a lot of it as well and sound like junk?

 

Heck yeah - double check your settings too - you should be above 15V or so, never under - even if it has a 12V setting, I'd be cautious you may find 13/14V in there too - although most decent ones do have a fuse - but if it's a good one, I would be cautious, you can always move to lower voltage settings as well, better to start high and move down. I'd think digital would be ok, still as long as it's set right. You should be able to read it fine even set to 100V or so - but it might show 0.12 for 12

But that's wise advice too - a cheap analog is fine for the job. Radio Shack might have some under 15 bucks like that. I haven't noticed any interferance with my digital and it's a craftsman, but the plug wires and such are all pretty new too.


Some of them are quite sensitive. Many will even give you an Ohm resistance reading from your left hand to your right.

 

Yea I'll try to pick up a analog meter tomorrow.

The plug wires are brand new MSD Super Conductors. I thought the very low resistance wires might be the culprit but thats not the case since I switched wires with no result.

Also, I was thinking my plugs might be the problem but turns our Autolite 3923's are a supressor (or resistor) plug I think.

The funny thing is my volt gauge in the dash doesn't jump around

 

Look at the range in the display of the multimeters when they behave like that. I'll bet they say mV. When a digital multimeter is in the sub-200mV range it can have infinite input resistance. That means it picks up even the sligthest amounts of static electricity. Try to put it on a table, then move your hand to and fro one of the leads. You'll be able to see the display respond. A measurement in free air bears no relevance/ importance to the state of your car.

 

Will this interere with my stereo though??


I picked up an analog meter so I'll see how that turns out. Plan to start it again tomorrow.

So this may be no big deal? I was thinking about starting the car for a very short time with the belt off to see if it is indeed the alt somehow.

 

You say nothing about what your problem really is, apart from your discovery and surprise about a peculiarity of most digital multimeters.
Like I said; measuring with probes in free air with a multimeter in its highly sensitive 200mV range has no relevance to interference with your stereo, or anything else for that matter (exept the air is full of electricity at all times). Hook it up to the intended power source if you want any information worth a dime.
I sense you are afraid to turn on your precious stereo lest it might fry. If so you'd better get hold of a digital storage oscilloscope to measure the power supply. They only cost like $2500 nowadays. But then there's the matter of knowing what to look for and how to use it.
Relax man.. If stereos were so easily damaged then you'd think you'd hear a lot more about it, and there would be a huge market for protective filters.

 

Well...My precious stereo doesn't exist.

The car is completely gutted and I wanted to know if I would recieve any nasty sounding feedback WHEN I do get the stereo and interior back in the car.
Didn't even think about frying anything but that may have been a good question. Mearly wanted to know if it would ruin the sound.

Thanks for the help though.

(BTW...I'm sure this is what you were referring to but even though the multimeter does go to 200mV I wasn't in that range. Just the 20V range)

 

Now we're getting somewhere.. Getting readings all over the place in the 20V range is indeed surprising. Maybe the radiation from the ignition wires is sufficient to influence the multimeter (esp. if it's in an AC range) but I haven't seen it that serious before. What were the highest and lowest reading you got, and what did it read when you hooked it up to the battery?

 

Multimeter jumps around so eratically and quickly I can't get a reading. When I had the car started with the analog meter hooked up it was not jumping.

Funny thing...I was having low voltage issues and solved that by running a second alternator charging lead directly to the starter which solved my low volts/not charging issues FOR A SHORT TIME. After a slight amount of driving it went back to not wanting to turn over due to low voltage. I'm thinking the second charging lead got too hot next to the headers or something of the sort (blew the motor and got too frustrated to take a closer look).