Misleading DVM volt meter test results

[Ecomike]

I have been around electrical and electronic stuff all my life all that way back to repairing tube style radios and TVs in the 60's, and I thought I knew how to interpret DVM test results under a car hood. :laugh2: yep, Good one!

I was about to chop off a female two wire connector on my daughters Saturn...when I got an on line hint from the SaturnFans site guys.

I had tested the electric solenoid, with a 9 volt battery, and it worked. I had tested the female connector and it had 14 volts (reading volts across the top of the two exposed metal contacts on the end of the female push on connector, one of those new ultra tiny do-jobbers from GM/Saturn), so I decided the inside of the female connection must be bad on one side. Tried adjusting it with a tiny jewelers screwdriver, NO JOY.:rattle:
Then this guy tells me the meter (High impedance DVM) may be lying to me (Yah, right) , tells me one wire is always hot, the other is grounded through the PCM by a solid state switch, that may leak enough to make my meter show a complete circuit, that is not complete and functional.

So I check, and sure enough the ground leg shows 500,000 ohms to ground, when it should be about 1-2 ohms. Now my real problem is the wiring to the PCM or the electronic solid state ground switch that is in the PCM.:tears:

IIRC our fuel injectors are grounded to fire them via the ECU/PCM on our jeeps. So moral of this story, is that one needs to check and see if the ground is actually firing, actually a solid ground, or just a poor ground that fools the high impedance DVMs. Testing the contacts at the end of the wiring harness connection may not be telling us the entire story folks!

 

[PaulJ]

Re: Missleading DVM volt meter test results

Yes, a DVM with high input impedance is a poor tool for checking grounds through a wire harness. It's even worse if the wire harness is on a 747 and it drives the techs nuts when you tell them they need to find an old analog meter.

 

[Ecomike]

Re: Missleading DVM volt meter test results

I knew there was more than one reason to hang on to my 35 year old analog Cadilac meter, LOL, other than testing the TPS on Jeep for bad spots, or the live O2 sensors on a Renix, that is!

 

[old_man]

Re: Missleading DVM volt meter test results

Yes, a DVM with high input impedance is a poor tool for checking grounds through a wire harness. It's even worse if the wire harness is on a 747 and it drives the techs nuts when you tell them they need to find an old analog meter.

Don't understand where you get this from. I'm and electrical engineer. If you are checking grounds, you are usually using the ohmmeter setting. In this situation, high impedance is not even in the equation. You are putting a voltage across what you are measuring and then it reads the current. Now if you are reading a voltage across a connection or cable, the connection is very low Z so no meter out there will load it down so either meter will read accurately.

Reading a voltage, you want the highest Z meter you can get to prevent loading of the circuit under test. Having said that, auto's have very few circuits that are high Z and need anything more than an old school analog meter.

If you are reading a TPS (variable resistor) as you cycle it through its range, an analog meter is nice. I run a high end Fluke 189 and it has a bar graph that responds at 5000 readings a second if necessary and gives a nice smooth readout.

 

[5-90]

Re: Missleading DVM volt meter test results

Yes, a DVM with high input impedance is a poor tool for checking grounds through a wire harness. It's even worse if the wire harness is on a 747 and it drives the techs nuts when you tell them they need to find an old analog meter.

Which is why I never got rid of my old analogue multimeter - there are times when it's more useful than my DMM!

I wish I still had that old VTVM I built about thirty years ago...

 

[djgrayxj]

Re: Missleading DVM volt meter test results

5-90 do you mean one like this
http://shop.ebay.com/items/heathkit%20vtvm?_dmd=1&_sop=12

 

[thebrick]

Re: Missleading DVM volt meter test results

Hey guys...in some DVM, if the internal battery is getting low, you will get 'false' readings.

I once used a DVM to measure the voltage of a car battery and go 20+Vdc...WTH? Measured another car battery and got the same measurement. I though the DVM lost it calibration. But the internal DVM 9V battery was just low...7.95Vdc (measured with another DVM). After replacing the battery, retook the measurements and got 12.3Vdc.

By the way, the DVM uses its internal battery to measure resistance and make continuity checks (finding grounds).

 

[O-Gauge Steamer]

Re: Missleading DVM volt meter test results

I am a retired Engineer and have used just about every type of meter that has ever been made. Anyone here besides old_man and possibly 5-90, ever hear of a Megger? And yes, I have a VTVM from before time began. I just can't get rid of equipment...

Every meter type has strengths and weaknesses. For all around troubleshooting on a vehicle, I use a Fluke 87 typeIII DMM. I also own several other meters all of which have their specific usage. For instance, I use a oscilloscope when checking power supplies as I want to know how much ripple is riding on the DC. I can give a "for instance"... I had a friend who's car was eating batteries. The Volt meter said it was charging at a nice 14.2VDC. What my o'scope showed was a huge amount of AC leaking past the diode pack which, in turn, cooked every battery put in the vehicle. Replaced the diode pack and the problem was solved. An analog meter did not find this. A modern DMM would have as you can set them to look for DC Hz which will find ripple. Alas this was in 1974 and DMMs had not been invented yet.

To claim that a quality DMM is not an appropriate tool to use, in my estimation, show that the person does not fully understand how the meter works. When measuring voltage, you want the highest Z (impeadance) you can find to lessen the loading on the circuit. Loading the circuit will alter the voltage read. In ohmeter mode, it is using it's battery just like an analog meter. Yes, you do have to keep the battery good but that is part of caring for your instruments.

You know, those old Radio Shack kits worked extremely well. There was a Heathkit store in Oakland and I had a room mate that built the Heathkit Digital FM tuner bought there in 1973. The logic was honest to God TTL and DTL. No IC's anywhere to be found. Sensitive though... As I remember, he was able to get KSAN out of SF from the fourth floor of the Barracks on McClellan AFB located in North Highlands, Ca. That's Sacramento basically. Yah, I was Air Force at the time.

 

[old_man]

Re: Missleading DVM volt meter test results

My Fluke not only reads the DC on that range but also at the same time shows the AC ripple riding on the DC. It spots a bad alternator in a second.

 

[Ecomike]

Re: Missleading DVM volt meter test results

Yes, a DVM with high input impedance is a poor tool for checking grounds through a wire harness. It's even worse if the wire harness is on a 747 and it drives the techs nuts when you tell them they need to find an old analog meter.

I get the distinct impression you and 5-90 are the only ones that understood what I said. Sad.