Ohms & Continuity Question

This question is regarding some testing I did one a very basic fuel pump on my Kubota BX25 tractor (diesel). There are two pumps and the one I was testing is a simple two wire pump. Using my auto ranging DMM I went to measure resistance and I got 9.75 M ohms with the DMM set to the ohms setting between the disconnected connector terminals (pump side). The service manual says to check for continuity between the connector terminals. If no continuity, replace pump. Setting the DMM to the continuity setting I got absolutely no continuity reading at all. None, zero, zilch. DMM display just showed the "OL". So my question is how can I get a reading on the ohms settings but not get continuity on the continuity setting?

Moved your topic to the diagnostic tools section.

Sorry Dylan, working on my phone and forgot to pick the right forum. Thank you.

No problem!

because on most meters the continuity test only measures 200 ohms. or atleast it does on mine. this is the setting that "beeps". ohms checking you have different ranges that can go into the mega ohms.

So Cheryl based off the readings I'm getting do you believe it would be safe for me to say I need a new pump? P.S. I just momentarily connected the pump and it did operate. My symptom is the tractor is starving for fuel. That is the definite problem. I'm just trying to determine if the pump is the issue. Trying to understand my testing.

Basically I don't understand why I have so much resistance, but yet the pump will operate.

A resistance in the mega ohms range is normally considered an insulator, as at normal automotive voltages almost zero curent will be flowing through it. If you put 12.5 volts across 9.75 ohms roughly 1 micro amp will flow.
Also normal digital voltmeters cannot measure resistances that high. Good Voltmeters and Oscilloscopes have internal resistances of between 500 M.Ohms and 2M.Ohms. Some volt meter tools sold for car work have lower internal resistances and can give misleading readings.

A fuel pump is an electric motor * it generally consumes anywhere between 5 and 25 amps depending on the vehicle and the condition of the fuel pump.
V/I=R 12.5/5 = 2.5 ohms, 12.5/25 = 0.5 ohms.

* N.B. some fuel pumps such as the S.U. and Facet (aka Bendix) pumps use solenoids rather than motors but the resistance is in the same range. This type of pump is called an interupter pump and is rarely found on modern production vehicles.

rockp2 wrote: So Cheryl based off the readings I'm getting do you believe it would be safe for me to say I need a new pump? P.S. I just momentarily connected the pump and it did operate. My symptom is the tractor is starving for fuel. That is the definite problem. I'm just trying to determine if the pump is the issue. Trying to understand my testing.

Basically I don't understand why I have so much resistance, but yet the pump will operate.

Interesting. :huh: Is there any convenient way to check for fuel volume out of the pump? I can imagine a situation where the pump is spinning, but not actually moving fuel.

Andy is talking about current flow, which is probably where I'd go next. If you have a significant amount of current with the pump running, then I'd say that ohm reading is inaccurate, or at least misrepresenting the condition of the pump. You could use the ammeter function in your meter to measure running current, but you take a chance of blowing your meter fuse if the pump draw is too high.

I do know that on automotive fuel pumps, the motor itself will carry enough current to light an incandescent test light. You could try the same thing by providing a ground on one pin of the pump, connect a test light to B+, and touch the other pin. If the light shines bright, then you're flowing 150mA or more.

So using Andy's response I tested the pump using my Vantage Pro. This time I got 2.74 M ohms resistance and the continuity measured at 1.764v. What does that tell me about the continuity? That it will only allow 1.764v of current through it? As I mentioned before, this diesel tractor has two pumps. The one I am testing is known as the "feed" or "lift" pump. It is very basic looking. Very basic circuit. A second question I have concerns the voltage drop from the battery to the pump connector (disconnected harness side). Battery read 12.2 volts and at connector it read 11.8 if I remember correctly. Is 0.4 volt of drop acceptable? Battery is low because of trying to start it too much I'm pretty sure. Thanks to all for your input. It is highly appreciated and helping me learn.

You need to do your voltage drop test with the circuit loaded(backprobe the connector). You can also stick a headlight bulb between the power and ground where your pump plugs in to load the circuit.
Does the pump run when you crank it? Have you disconnected the hose from the outlet of the pump and checked to see if you have any volume of fuel coming out of it?
I would say your resistance is too high.

gav09 wrote: You need to do your voltage drop test with the circuit loaded(backprobe the connector). You can also stick a headlight bulb between the power and ground where your pump plugs in to load the circuit.
Does the pump run when you crank it? Have you disconnected the hose from the outlet of the pump and checked to see if you have any volume of fuel coming out of it?
I would say your resistance is too high.

Thanks for the headlight idea. I'm going to try that out. The pump would run but it would run poorly. It started when I was moving snow. The engine bogged down severely so I and regardless if the throttle I gave it, the engine would rev on its own to a very low rpm then almost stall then surge back and forth between just about stalling to barely running. I determined it was a fuel problem because when it did stall I could hear the pump doing the same surge. It has a very loud clicking when operating properly. I've taken the whole fuel system apart up to before the injector pump (not the pump in question) and have cleaned it and am replacing the filters. I'm sure I could have troubleshot this much faster by just hooking everything back up to see if it was a bad fuel problem, but I'm using this as a "teaching moment" to learn if I can be 100% accurate in calling a bad pump or not. Trying not to be a "parts changer"...especially for a $120.00 pump. Putting the system back together now and will test the pump for flow disconnected at the output. Once again, know I could have done this faster, but want to learn what good or bad looks like for my future endeavors

With circuit loaded using a headlight:

Backprobe at connector(+) to B(+) post 0.343v. I believe less than 0.5 volt is OK.

Backprobe connector(-) to B(-) post 118mv. Is that okay? Should it be less than 100mv?

Well, the pump is putting out fuel. Just have no idea what the correct volume should look like. It's nor dribbling or anything. Seems to be a good flow, just don't know what a good flow is. On to buttoning up the rest of the system.

I wanted to follow-up and not leave this thread hanging. I'm not quite thru with my troubleshooting yet but this is the first I've had time to actually post a follow-up. So I buttoned up the rest of the fuel system, bled it and then fired the tractor up after a couple attempts. I only drove it about 20 yards to put it back in it's "parking spot". So I am not comfortable yet calling whether it was gelled diesel or the pump is bad. Until I have a chance to put the tractor thru it's regular work paces I don't know if the pump is bad or failing. It kind of drives me a little nuts because based off the measurements I'm not sure how the pump can be working at all really. I just don't see how the high resistance (2.75 M ohms) and weird continuity reading allows it too function. I would appreciate if anyone could tell me what the continuity reading of 1.764v tells me and how that would effect the current flow?

I did get a new pump anyway. Figured I'll have this tractor until I die and having a spare pump on the shelf isn't a terrible idea. Tonight I am going to take measurements on that pump and will post those results.

I very much appreciate the responses as I feel I learned a little bit more from each one of them. So, my continued thanks.

rockp2 wrote:

So using Andy's response I tested the pump using my Vantage Pro. This time I got 2.74 M ohms resistance and the continuity measured at 1.764v. What does that tell me about the continuity? That it will only allow 1.764v of current through it? As I mentioned before, this diesel tractor has two pumps. The one I am testing is known as the "feed" or "lift" pump. It is very basic looking. Very basic circuit. A second question I have concerns the voltage drop from the battery to the pump connector (disconnected harness side). Battery read 12.2 volts and at connector it read 11.8 if I remember correctly. Is 0.4 volt of drop acceptable? Battery is low because of trying to start it too much I'm pretty sure. Thanks to all for your input. It is highly appreciated and helping me learn.

I think you may be accidentally using the diode function of your meter, not continuity bud. This 1.74 volt number you are saying is right on par for most low end meters maximum diode forward voltage readout.

Thanks for your response Sniper Dan. Just so you know, I have been using my Vantage Pro after my initial post (on "Digital Meter" setting). I just tested my brand new pump and it showed ohms resistance of 2.41 M ohms and when measuring continuity it shows 1.175v and doesn't beep. When I touch the scope leads together it does goes to 0.00v and beeps. So this raises a couple questions that are frustrating:

1. Based off the readings on the new pump it looks like my old pump is fine and the problem was the fuel. However, the service manual states to test for continuity and if the test fails, replace the pump. The meter I first used was a Actron auto meter and did not show continuity. Even with the Vantage Pro showing 1.174v on the continuity test that would strike me as bad continuity...no? So I would have replaced the pump needlessly based off the service manual.

2. Now that I have a known good pump, I now know what "good" looks like. But how could I have tested the pump without that info using a meter and been able to call a bad pump? The resistance and continuity results would make me think the pump had too much resistance. I'm really confused on this.

Ahhhh now I understand. The "continuity" function of the vantage is somewhat broken. On normal meters, continuity displays an audible tone, as well as circuit resistance up to a certain range. On the vantage they rolled the continuity buzzer into the forward voltage diode tester, so you can't actually view resistance while using it. To be clear, this voltage number is absolutely meaningless unless you are testing a diode. As for the high ohm's you are recieving during your resistance test, that's definitely concerning. What does your meter say when you touch the leads together on ohms?

It's also worth noting that typical resistance of a brushed DC electric motor, like the one in your pump, is well under 1 Ohm.

When I have the Vantage set to "Digital Meter>Ohms" I calibrate the leads and they show 0.00 ohms when touched together. On the pump it shows 2.41 M ohms on the new pump and 2.74 M ohms on the old pump (the pump is removed from the tractor so only measuring the pump itself).

With the Vantage set at "Digital Meter>Diode/Continuity" I calibrate the leads and when touched together they show 0.00v and it beeps. But does not beep and shows 1.174v on the new pump and 1.764v on the old pump. I understand that you are saying to disregard those readings, but the Vantage does not beep...indicating no continuity. Which is the same thing my DMM did. So with the service manual stating replace the pump with no conitinuity...= potential wasted money apparently.

Do you see my confusion? Why the high resistance and why no continuity on both the new and old pumps? I'm now confident that gelled diesel was the problem, but I can't let it rest on why I can't use a meter on the pump and know whether it would have been the cause or not.

Don't mean to interrupt but isn't that a solenoid pump? I don't know that it makes a difference as far as its continuity or resistance, just thought I'd mention

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