That observation of low resistance measured by a multimeter yet very little current flow when 12 V is applied is more common than we would think. There is a clear answer to this apparent paradox. Who can explain it here?
Hi Dave! Great work procedures along with the electrical diagnostics. In the first part where you measured 2 ohms of resistance, you should have some current flow where the motor should have at least moved if maybe only a very small amount. One of the armature windings should have been energized although there wasn't any movement of the motor. If you've got resistance, there has to be some current flow. When you hit the motor, this would indicate that you had a poor connection between one or both brushes to the commutator. And when you finally opened the motor, there was brush material wedged in the slots between the commutator bars. It was hard to see the windings, but they looked a little overheated. One other thing to do would be to measure each armature winding and compare its resistance with the other windings. Or it's possible that one of the windings could have been open. Keep up the good work Dave!
Pretty simple from the perspective of your high level diagnostic videos. Yet even in basic diagnostics there are thought provoking fundamentals hiding in plain sight.
After you've done some of these you realize there are more similarities than differences. The curve ball with some modern cars is the curtain side air bag in some vehicles. Most are in the seat but some use the door itself. If you see one of those, it would be wise to avoid orange wires and probably disconnect the battery before putting yourself at risk.
3:57 I have question about Back EMF. Brushed Fuel pump motor is design to work best around 12-13.7 volts. Let's say i have corrosion in the Fuel pump connector and i get 10.5-11 volts into the pump. The pump works , but gradually get worse and finally burnes every 2 weeks. So i changed 3 pump motors until finally the connector melted. My diagnostic mistake is i tested the motor power and ground with just a simple test lamp(270mA) on the relay side of the connector which is outside the plastic cap/top plate/circle of the fuel pump. So that way i did not confirm good contact point on the connection outside/inside the pump. What i needed to do was leave the connector plugged to the outside cap/top plate/circle and test with equal 5 amp substituted load(maybe a fog light bulb, WHILE at the same time test for voltage drop with DVOM) ON THE LEADS THAT GO DIRECTLY into the fuel pump motor , these leads are on the other side of the cap/top plate/circle - inside the tank. You know the leads that are inside the tank and on fuel tank side of the plastic cap/top plate/circle(i don't know how it's called exactly). I think because the pump was working at lower voltage than design to , the back EMF wasn't enough and the current draw was more than design even if the voltage was lower which is impossible according to ohms law. What i cannot understand is why the after replacing the connector and new fuel pump module(not only the motor this time , because the female side on the plastic top cap was melted) the problem was solved. This was 5 years ago and i still cannot find the answer. Why every new motor kept burning and not just "running at slower pace" because of the corrosion in a connector like a regular headlamp would do let's say. Could this be Back/counter EMF related?
What an outstanding question. Indeed, I spent years misunderstanding that very point, and eventually I was so frustrated I took another motor course to find the answer. As you say, resistance of wires in typical motors is very small, so how could low voltage damage the motor? Electricians will tell you that low input voltage is a definite killer of most electric motors. To get to the answer, note that the rotor won't break free and start spinning until a threshold torque is reached, and the way the motor meets that threshold is by pumping energy into the magnetic fields. Energy delivery from the wires is simply the instantaneous product of voltage drop across the windings and current (otherwise know as power), and the total energy delivered is the integral of that over time. As the motor builds its magnetic field, if voltage is low, then the rotor remains locked for longer, allowing current to spike to a higher level to finally meet that power requirement. So, at startup with borderline voltage you get a longer time to start, and current through the windings is much higher. Those tiny 24 gauge wires inside the windings can't take the heat! After startup the back EMF reduces current so the motor can keep going, but with lower input voltage the motor ends up having to work at a less efficient point on the torque speed curve. In the end the motor dies a premature death. Incidentally, I have done current waveforms for fuel pumps and the exact same sawtooth pattern is seen with those motors. Thank you for making the discussion thread a thoughtful and interesting read.
You can't get to that spot without destructive tools, and in these motors even the brushes can't be replaced. However in the three phase AC world of huge motors, where most are brushless inductive motors, it is not uncommon for repair techs to do an invasive clean and repair.
Hi would it be possible if i can send you 2 images of my Fridge compressor if you have time to identify if its cause of my freezer not freezing. Its got some sort of black solidified liquid on the compressor coils, just wondering what it was
Compressors can certainly leak , and although the refrigerant instantly boils away the oil makes a mess like that. Only thing is, I wouldn't be able to distinguish the mess from oil that came from elsewhere. So I don't think you should rely on my opinion, it wouldn't be reliable enough.
Isn't ACDelco the oem supplier to GM? I always try to establish a part number & search on that to find the lowest price. Though I have been burned by ebay sellers selling third party Chinese crap as oem.
That's a good question. Yes, AC Delco supply OEM parts for manufacturers, and there's a chance that this AC Delco part is identical to the much more expensive OEM part listed by Rock Auto and others. It's why I chose AC Delco here, hoping that the part is better than most generic providers. I think it's also why the AC Delco part is more expensive than most generics. Problem is, no manufacturer is confined to one product line, and they are perfectly allowed to compete with generics by producing a "price point" version that may not meet all of the stringent criteria demanded by a manufacturer. I decided to take the chance, and so far it is working out great.
