TBI Fuel Pump Waveform

Juan Technical Support Specialist Illinois Posted   Latest  
1989 GMC C1500 Sierra 5.7L (K) 5-spd (MG5)

Performing a pre-trip inspection on a 89 GMC P/U 5.7l TBI. I was checking the current waveform of the fuel pump. It had 5 A peak draw during start up, steady at 3 A. Compared to what I have studied, been told and seen. I suspect a weak/failing pump. I am aware that TBI systems only require 9-13 psi of pressure, so a low amp draw is not very surprising to me, I keep this in consideration due to the low demand of the TBI system. The main issue I have is that I saw no visible commutator segments in the wave form. I was originally testing at the fuse and suspected there was more than 1 load on the circuit. Then jumpered the F/P relay and tested there, same results. Went to tank, same results. I didn't capture the waveform, due to being mobile and my versus battery died. Has any one seen a flat line on a fuel pump wave form with no drive-ability issues. I did not measure fuel pressure or volume either. Im suspecting a dirty commutator gaps, or possible shifted brushes?? I will try my hardest to meet up with the customer this weekend to capture the waveform. 

Thank you, 

Juan …

Marlin Technician

An OE-style pump cannot draw a steady amperage. Your scope is set up incorrectly or your current adapter is wacky.

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Randy Curriculum Developer
North Carolina

The low current level doesn't surprise me on a TBI. Let's see what the waveform looks like. Does the vehicle have any symptomatic complaints? I will try to find an old capture and post it for comparison.

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Albin Diagnostician

I have a terrible time examining a waveform with no waveform present :) That being said, I am also a little on the lazy side, and will use my scan tool to squeeze as much information from a vehicle as possible. In the case of fuel flow and delivery analysis, we are interested in volume & pressure. Given a fuel filter that is capable of flowing the correct amount of fuel, it is a little hard

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Scott Owner/Technician

Albin's test is valid, as for a flat line for current, if we think about it's impossible, unless it's brushless. In my experience I used to scope ton's of pump's back in the day , even if they were ugly they still did there job. Now a no start with a bad com bar would show up beauty on the scope. I'd look at the rpm , pattern uniformity and current. We need a capture to go on here. Being it's

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Rudy Technician

Id dump that practice of using FP wave forums for accurate diagnosis. Its not very practical and can be misleading. It is very much like ohms testing......

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