I need to buy a scope...

[Supercharged111]

ABSOLUTELY!!! You'll need a low-ampere probe (inductive would be most convenient.) I got mine from https://www.aeswave.com/low-current-probes-c496/ although the one I bought doesn't seem to be listed. It's very similar to the 0--60 Pico probe. It comes with a BNC connector, I had to buy an accessory harness to convert BNC to banana plugs--not too expensive.

Then you set up the 'scope to show the instantaneous current draw of the pump while it's running. Each armature bar produces a "hump" of current draw as it becomes aligned, is perfectly aligned, and then goes out-of-alignment with the motor brushes. The "average" current draw may be--for example--about six amps, but the peak and minimum current draw will be a little on either side of the average.

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It's best to connect your 'scope in conjunction with a fuel pressure gauge. Correlate the amperage draw with pressure produced.

1. Maybe you find out the fuel pump is wearing-out, and now you can plan to change it at your convenience instead of as an emergency repair.

2. You get familiar with the process, you get familiar with your scope, and you learn what the waveform is supposed to look like.

When you get the pattern set up and "Frozen" on your 'scope, you'll find that the waveform of each of the armature bars are like fingerprints. (Every fingerprint looks the same to the person who has to clean the window, but every fingerprint is different to the detective trying to find evidence.) You need to be more "Detective" than "Janitor". At some point, you'll find that the pattern of individual "humps" repeats. You'll count the number of humps before the repeat--8 humps on a fuel pump is common, but some brands have 12 or some other number. That represents the number of bars on the motor armature. Each bar has it's characteristic current draw, which shows up on the 'scope.

Then, using the magic of digital storage oscilloscopes, you'll find the amount of time it takes between repeating patterns. Divide 60,000 by the milliseconds for one pattern to calculate fuel pump motor RPM.

Keep a notebook of results from your tests--Vehicle, pump brand, average current draw, peak current draw, number of armature bars, fuel pressure developed, anything else that draws your attention--as you check more fuel pumps, you'll develop a history of known-good and known-defective pumps.

This technique works for other automotive electric motors besides the one powering the fuel pump--the HVAC blower, for example, or the motor that raises and lowers a power antenna. With appropriate set-up, you could get a pattern from AC motors, too--refrigerators, window Air Conditioners, fans--whatever.



Guilty as charged. My Trailblazer is due again.



Here's my thread--on another forum--with photos of my Snappy Counselor II scope and the fuel pump from my K1500. I just checked, the photos are visible. If you don't see 'em, it's likely due to an ad-blocker on your browser.
www.thirdgen.org/forums/tbi/612473-tbi-fuel-pump-upgrade.html

TBI--Fuel Pump Upgrade w/Photos - Third Generation F-Body Message Boards

TBI - TBI--Fuel Pump Upgrade w/Photos - This seems to be the finest web site in existence for TBI information. While I recognize that this is primarily a Camaro/Firebird site, much of the information applies to GM TBI vehicles in general. In my case, I have an '88 Chevy K1500 work truck with the...

www.thirdgen.org

Question for ya: I can see the pattern of 8 above, what would be an indicator that that pump is getting iffy? Inconsistent humps?

 

[Schurkey]

Yeah, one or more humps RADICALLY different from the others. That armature winding could be open or shorted.

Slow pump RPM.

Too much--or too little current draw (if the pump voltage is acceptable.)

(This is why you need a notebook/diary of pumps and other items tested--manufacturers aren't going to tell you the specs of their pump motors, or windshield-wiper motors, or power-antenna motors. The more stuff you can test that WORKS, the bigger your "database" becomes when comparing to suspect parts.)

And if the pump motor checks-out--decent, largely-symmetrical humps of expected current draw and RPM, but the pump pressure is too low, the motor may be fine but the pump section could be failing.

 

[Supercharged111]

I had no idea you could get a scope so cheap. The only one I've seen is a big old b@$t@rd like this.

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But now having googled oscilloscopes I see even these old dinosaurs can be had for a few hundo.

 

[Schurkey]

"Automotive" 'scopes tend to come with more and different accessories than "scientific" 'scopes. A "modern" automotive 'scope is probably slower than a modern "scientific" scope. The Snap-On Counselor series were two-channel, my OTC is two-channel, but newer automotive 'scopes may be four- or eight-channel. Scientific 'scopes tended to have more channels than the older automotive 'scopes; but I don't know that that is still true--in part because automotive components have gotten enormously more-complex than they were twenty years ago.

The low-amps probe (fuel pumps, small motors, older generators and alternators, etc.) and high-amps probe (modern alternators, starter motors) could be the same between automotive and scientific 'scopes. In general, an automotive 'scope will have probes to measure ignition output; and a way to caluclate engine RPM from the firing of the spark plugs. The more-complete automotive 'scopes will allow cylinder shorting--individually and as every-other-cylinder groups to check the two planes of a dual-plane intake manifold.

The Snappy Counselor II scopes have much more for engine diagnostics. But they were $10,000 and up when new; about one-tenth that used on eBay last I looked (which has been a while) so they're not real common. But if they work properly (they're getting old) AND have the various optional accessories, they're unbeatable for engine work.

 

[stutaeng]

Question for ya: I can see the pattern of 8 above, what would be an indicator that that pump is getting iffy? Inconsistent humps?

Some the videos I have seen, most of the little humps we're basically missing, maybe one or two were still there.

I think that's the case when you can beat the fuel tank and sometimes you can get them to work briefly, similar to a starter that is dying. Again, that's on a completely failed unit. A working unit would probably take a more trained eye to evaluate the condition of the commutator.

Thank you @Schurkey very good advice.