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Learning to Trust the Current ProbePosted 1/18/1999By Jeff Bach
I continually get questions from technicians about inconsistencies in readings they get when testing with a current probe. The first thing that comes into question is usually the accuracy of the probe. I can tell you from experience with at least eight different probes I've tested, the problems are not caused by the probes. What I am seeing is that guys are getting different current levels and rpm readings from the same fuel pump, injector, ignition coil, etc. There are those among us that would jump into an otherwise clear stream, wade about with reckless abandonment, look downstream at the muddy mess and deduce that the fishing here will be no good as the water is unclear. I had the same concerns when I first started testing with a current probe. What I discovered was that it is very important to do your testing under like conditions. For example, you wouldn't question the accuracy of your volt meter if it showed you one number for battery voltage while cranking and a different number with the engine running would you? When Ohm meters first came into our industry they were analogue and needed to be zeroed before each use. That didn't take too long to get used to, did it? The same holds true for the current probe. It has to be zeroed before each use and that needs to be done in the location and angle as close as possible to the actual test point. If this simple procedure is followed, you will quickly learn that you can trust the readings, then you can start to realize just how important a diagnostic tool the current probe really is. Here is an example of how different readings can be achieved at the same test point, with the same Scope settings. Figure 1. Note: Figures on this page will open up in a new browser window. Since resistance for the most part is a constant, and voltage is changing, the current has to change also. During cranking this 1998 F150 4.6 liter Ford pick up was showing 10.8 volts at the battery, the injector had a measured resistance of 15.2 ohms. If you work the simple "Ohm's Law in a nut shell" formula, E ( I x R (Voltage ( amps = () you come up with what I call the expected result. 10.8v ( 15.2 = (.710a) . The voltage between the cursors in the cranking injector current was 716 mA or .716 amps. The other one read 889 mA. This one was taken with the engine running at a charging system voltage of 13.8. Divide that by the .889 amps between the cursors and you come up with the injector resistance of 15.52. I'll take it! Next let's talk about the discrepancies found when testing fuel pump current. The problem here is technicians think of the numbers as though they were a given constant like fuel pressure. If they don't meet the expectations of the tester, the data come into question. Some of the things that must be taken into consideration are battery voltage, which will vary as much as 30 percent from cranking to charging in a normal system. Take into account a weak battery and the numbers are even higher. I consider the specifications for fuel pump current draw and RPMs to be taken with the engine running unless otherwise noted. If the testing is being done on a no start vehicle and the fuel pump is a suspect, don't worry about 10 percent or 20 percent if it's that close, the fuel pump won't be the problem. Usually in order for a fuel pump to keep a car from starting it will be in the range of more like 30 percent to 50 percent of normal. If it's a driveability concern, that's different. Here you need to pay closer attention to the specs. One of the things I see techs do a lot is forget to zero their probe. If you don't like having to turn the dial and watch the scope each time you use your probe, I suggest a model the has one touch zeroing. With these you don't have to look at the scope, you simply put the probe at the test point and push the zero button. This step is just as important as turning on the meter. Scope settings are another thing that can cause problems. For most fuel pumps it is widely agreed that running engine fuel pump current and RPM test data be gathered at 2mS 100mV, Most probes have a 100mV per amp scale. At this setting you will get the best overall view of a good or new fuel pump. If you can't get a decent picture and you have to change from these settings to see a decent waveform, you probably are looking at a bad fuel pump. In a lot of my comparison files I switch scales to show the bad fuel pump wave form. This is because I like to frame a good picture then use the wave form software to show me the data. Figure 2 In figure # 2 notice that the data box (lower left corner) for the waveform from the bad pump shows 50mV/div. I had to switch to this scale simply to show a good picture of the bad waveform. This is a gearotor type pump which generally fails due to gear wear. This results in excess clearance between the gears and the pump loses efficiency causing it to spin faster and draw less current. The common misconception here is if the RPMs are good the pump must be OK. Usually this statement comes from a guy who is used to seeing GM fuel pumps on his scope which tend to fail the opposite way. Figure 3 This 1985 Cadillac still showed 10 PSI on a pressure gage, well within specs. But as you can see the cursors have eight of those nasty humps framed taking a full 18.2 mS to complete one revolution. At 1.4 amps this bad fuel pump has slowed to about 3300 rpms. And the car has a difficult time maintaining speed on hills. The eight humps of Mr. Lester's pump framed by the cursors in figure # shows the increase in both current and RPMs Characteristic of the GM TBI pumps. This pump having 25,000 miles is still in good shape at 4800 rpms. Figure 4 Another common mistake I see is using the wrong test point. Choosing a test point is not a problem if you know your circuit. But if you just assume you can break the fuel pump circuit at the relay and hook in a current probe to get the fuel pump current you may be in for a surprise. I have seen this done more than once by technicians who consider themselves to be somewhat knowledgeable on this subject; present company not excluded. The trick is to have all of the current flowing through the probe. For instance if you do a key on current test at the relay, on this typical GM fuel pump circuit you will get a pretty accurate reading of key-on fuel pump current. Figure 5 But if you start the engine expecting to read higher current due to the increased voltage from the charging system, you will get a surprisingly lower fuel pump current. The reason for this is that the oil pressure switch bypass circuit closes creating a parallel current path making the reading at the relay of little use unless the oil pressure switch is disconnected. Figure 6 Another option would be to start the engine, unplug the relay, and take the reading at the oil pressure switch, or as I like to do, at the fuse box. Note, in order for the circuit to be protected, the current must all flow through the fuse. There is also of course, the option of simply taking the reading at the tank feed wire to the pump. As a 25-year veteran technician solving driveability and electrical intermittent problems, I can honestly tell you that this is the most exciting time I have witnessed in this industry and it will only get better. This new technology is catching on fast and there are a lot of technicians improving their skills by using it. If you want to know where the good fishing spots are, ask the guys cleaning the fish, not the ones carrying empty stringers with mud on their boots.
Get a current probe and catch some fish!
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