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
Marlin Default

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
Randy Default

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
Albin Default

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
Scott Default

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
Rudy Default

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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