Odd Battery Voltage Readings on Multimeter

[KidCoronet]

So I took your advice and ordered a Fluke 107 which should be here in a few days and I'll give that a try. It would be sweet if it was just a crappy multimeter!

Also, great information on the Diodes and simple way of testing. I followed the instructions and unhooked my battery and the positive power feed to the alternator. Also unhooked the plug that leads to the VR..the other field plug is grounded to the case. Using the same cheap multimeter (with a new battery) switched on the diode function, I put the black to the power stud and red to the case and came up with 1519. Tried this several times and same number? Is this indicating a bad diode? I reversed the probes - red to the post - black to the case and came up zero. That part is good.

Also, for an experiment, I switched to AC 200V (lowest i have) and checked the battery with the car running and get the same crazy fluctuating number thing - 2.6 to 9.4 ish. It's pretty hard to tell since it changes so fast. Bizarre

 

[65CopCar]

I would have to draw out the circuit to figure out exactly what is being tested in the instructions I posted. But if checking a single diode with a multi-meter in diode check mode, in one way you will get the diode voltage drop which should be around 0.5v plus or minus a few tenths of a volt, and basically open circuit the other location. Zero is bad. Very bad.

Let me draw the circuit out and see if what you measured makes sense or not.

 

[RJRENTON]

So I took your advice and ordered a Fluke 107 which should be here in a few days and I'll give that a try. It would be sweet if it was just a crappy multimeter!

Also, great information on the Diodes and simple way of testing. I followed the instructions and unhooked my battery and the positive power feed to the alternator. Also unhooked the plug that leads to the VR..the other field plug is grounded to the case. Using the same cheap multimeter (with a new battery) switched on the diode function, I put the black to the power stud and red to the case and came up with 1519. Tried this several times and same number? Is this indicating a bad diode? I reversed the probes - red to the post - black to the case and came up zero. That part is good.

Also, for an experiment, I switched to AC 200V (lowest i have) and checked the battery with the car running and get the same crazy fluctuating number thing - 2.6 to 9.4 ish. It's pretty hard to tell since it changes so fast. Bizarre

FYI....a diode is a simple device; it conducts power in one direction......anode to cathode......but not in the opposite direction.....cathode to anode. When checking diodes, most Fluke instruments have a diode check function. Read the Fluke information for your device. Typically, when testing a diode, the TYPICAL anode to cathode RESISTANCE (+ to -) is 10 ohms...but this is not an exact number for all diodes. Conversely there should be infinite resistance (Fluke instrument would read open circuit). Diodes USUALLY fail open circuit...infinite resistance anode to cathode and vice versa. Diodes can fail SHORTED....ZERO resistance (or very low resistance) in both direction anode to cathode and vice versa.
In a typical 3 phase full wave bridge rectifier in a typical automotive alternator, an open diode will exhibit low output amps but ok output voltage (due to the 3 phase design, the loss of one phase, due to open diode, results in loss of half of the output amps of the alternator); a shorted diode USUALLY results in the vehicle's starting battery being drained overnite. The battery is discharged thru the remaining Diodes and stator windings and generally takes overnite. The AC voltage you measured when the alternator is in operation occurs because of the ripple or FREQUENCY of the generated AC wave form because it's RPM dependent and is filtered by the battery of the car and for all intent and purpose is meaningless.
Probably more information than you asked about....but....if you have any comments or questions...just ask.......
BOB RENTON

 

[65CopCar]

Ok - I drew the circuit out. That link I sent you before makes zero sense to me (perhaps I should of actually read it before I shared it?????). After drawing out the circuit, I would expect the assembled alternator test to act the same as a single diode, so if you do measure zero in one direction you have diode problems.

This video shows the exact results I would expect for an alternator with good diodes. Hope this helps.

 

[RJRENTON]

Ok - I drew the circuit out. That link I sent you before makes zero sense to me (perhaps I should of actually read it before I shared it?????). After drawing out the circuit, I would expect the assembled alternator test to act the same as a single diode, so if you do measure zero in one direction you have diode problems.

This video shows the exact results I would expect for an alternator with good diodes. Hope this helps.

READ WHAT I JUST WROTE IN #23).......I've explained it far more explicitly than the video in #24. I'm a degreed Electrical engineer with 50+ years design experience.......but....believe what you wish......btw.....I never saw your marked up sketch or drawing.....
BOB RENTON

 

[65CopCar]

READ WHAT I JUST WROTE IN #23).......I've explained it far more explicitly than the video in #24. I'm a degreed Electrical engineer with 50+ years design experience.......but....believe what you wish......btw.....I never saw your marked up sketch or drawing.....
BOB RENTON

Well I have to somewhat disagree. I am also a degreed electrical engineer with almost 40 years in the industrial world. I am very versed on synchronous generators which is what an alternator is.

The video results are correct. When he tests the alternator, he should see the results that are shown in the video. Its that simple.

You are correct on the resistance in the two directions. I almost recommended taking resistance readings, but felt that could muddy the situation.

I was just looking for some oscillograph captures of an intermittent shorted scr (same concept as the diode 3 phase full wave rectifier) on a large synchronous motor field supply I helped troubleshoot. Unfortunately I can't find the files. But the results were very telling - it showed the expected AC shoot through from that one shorted SCR. It caused the effective DC voltage to be lower which caused the motor to get underexcited and bad things happened. Ripple voltage is very telling.

Although diodes can open, the typical failure mode is they short. They can also begin to breakdown before getting to full short mode. I have changed several larger diodes in my career because of excessive current leakage below their rated reverse voltage rating. The diodes get tested every year and we could see when they started their decline.

I stand by the

 

[69Bee]

That's the FUNDAMENTAL operation of an alternator. FYI....the Mopar alternator uses a THREE PHASE FULL WAVE BRIDGE RECTIFIER DESIGN.... changing the induced AC voltage in the stator windings to DC via the 3 phase full wave bridge diode array.....
BOB RENTON

No ****, that is what I was saying Bob...

 

[RJRENTON]

Well I have to somewhat disagree. I am also a degreed electrical engineer with almost 40 years in the industrial world. I am very versed on synchronous generators which is what an alternator is.

The video results are correct. When he tests the alternator, he should see the results that are shown in the video. Its that simple.

You are correct on the resistance in the two directions. I almost recommended taking resistance readings, but felt that could muddy the situation.

I was just looking for some oscillograph captures of an intermittent shorted scr (same concept as the diode 3 phase full wave rectifier) on a large synchronous motor field supply I helped troubleshoot. Unfortunately I can't find the files. But the results were very telling - it showed the expected AC shoot through from that one shorted SCR. It caused the effective DC voltage to be lower which caused the motor to get underexcited and bad things happened. Ripple voltage is very telling.

Although diodes can open, the typical failure mode is they short. They can also begin to breakdown before getting to full short mode. I have changed several larger diodes in my career because of excessive current leakage below their rated reverse voltage rating. The diodes get tested every year and we could see when they started their decline.

I stand by the

I appreciate your comments.....there are several points to disagree about. It all depends on the service intended. I designed a REVERSING polarity, three phase SCR controlled 6 megawatt power supply (3000 amp @ 200 volt DC) for an electrowinning metallic tin recovery system. It was 2 SCR rectifier circuits, back to back, powered from a common AC 2400 volt, 3 phase primary power source. A unique process requirement. It took several months of wave form analysis and load discharge analysis b4 the reversing SCR circuit discharged the load and ramped up, to turn on the reverse supply. A little difficult to simplify as to the reversing requirement as to the why it was required. And, after several initial attempts, and several 2000 amp "hockey puck" style SCR failures along with numerous AMP TRAP fuse failures, finally got the system to work as intended.
We each can state our reasons for the equipment we designed/worked on and have our own opinions about how diodes fail, why they fail......personally, I don't give much credence to homemade Utube videos.....but to the totally uneducated, they serve their purpose. If you're interested, I could discuss the need for a reversing polarity DC power supply to power an electrowinning tin recovery system....via PM, to not draw criticism from the moderators. Electrical engineering is a unique dicipline.......
BOB RENTON

 

[RJRENTON]

No ****, that is what I was saying Bob...

Then WHY did you not explain it further......say what you mean.....some of us (me) don't read minds.....
BOB RENTON

 

[Nacho-RT74]

... Ballast resistor looks good at 1.8ohms.

Just for the records, Ballast resistor has nothing to do with charging system, just ignition system.

I did check the voltage from the high side of the ballast resistor to the positive side of the battery and get a 2 volt reading.

I’m not getting the reason for that test…

 

[Don Frelier]

I’m not getting the reason for that test…

It tells you how much voltage drop you're getting from the battery up to the ballast resistor.
(Usually through the bulkhead but not necessarily)

 

[Nacho-RT74]

On a side note and sharing one experience I had, once long time ago I was getting discharge on amm but 24 volts at batt. It ended being one of the stator leads arriving to diodes bank was being touched on every spin with some of the rotor vent vanes. Just bent the lead and got it working normally.

So that could support somehow what @65CopCar about a shorting at diodes giving that fail?

Just thinking out loud…

 

[Nacho-RT74]

It tells you how much voltage drop you're getting from the battery up to the ballast resistor.
(Usually through the bulkhead but not necessarily)

I thought on that but againts chassis ground on each point, not between both positives?

Dunno, just asking

 

[RJRENTON]

Just for the records, Ballast resistor has nothing to do with charging system, just ignition system.



I’m not getting the reason for that test…

The reason requires a fundamental knowledge of electrical engineering, specifically, OHMS LAW....E=IxR, or it's variations and Kirchoffs Laws....to determine system losses......suggest you learn them b4 commenting.......just my opinion....
BOB RENTON

 

[Nacho-RT74]

Keep posting your degree and being unusefull for the rest of members on every thread… it seems you found another member also with a degree to be disagree with too.

 

[KidCoronet]

Update!

Aside from those crazy voltage readings at the battery while running...the car seemed fine with battery charging, no parasitic draw, etc. Last night I took it for spin..was running nice and got into it pretty good a few miles from home and then I broke it! LOL. Completely died and lost all electrical's, coasted to a good spot. My fusible link on the starter relay fried to a crisp. Called a buddy and he towed me home in the dark. Felt like I was 17 again.

I inspected every inch of the wiring inside and out and could find no other damage. Felt lucky. Put a 12v test light in series on the + batt terminal and got nothing...so appears no short. Removed all three wiring harness' and the bulkhead and everything was in order. Replaced the fusible link, and started it up. Battery voltage was still fluctuating wildly on my multi-meter (new one hasn't arrived yet). I immediately noticed the ammeter gauge bouncing to the charge side and new something wasn't right because I have an 8 gauge shunt wire from the Alt power stud to the starter relay which should mostly render the ammeter useless (as it has been). So I removed the shunt wire, tested it and NO continuity! I had an installed a 12 gauge fusible link at the relay and guess what? Something didn't feel right about the link so I cut off the outer plastic covering and the internal copper wire was completely split in half and had been arcing (blackened but not melted).

So correct me if I'm wrong but this should explain the wild multi-meter voltage readings from the voltage drops and spikes caused by resistance (not to mention the heat!)

Picking up some new 12 gauge fusible link on Tues so can't wait to hopefully put this one to bed.

Let me know if I'm missing something (highly likely lol) and thanks for all your ideas and inputs here...most appreciated!