[V8] V8 Fuel Pump

[Dave Saad]

That is some great info.  
I have a simple motto that only seems to work for me, but when troubleshooting anything, "start at the beginning, step to the end".  I tried to teach technicians to fix equipment used in semiconductor manufacturing and ran into the same problems you do.  They all want to run a diagnostic, then swap out a part.  Sometimes that works.  Often it does not.  Most of the techs could not use a voltmeter to save their lives.  Many of those machines used ECL logic, which has a -5.2 volt supply and a -2 volt supply, with the -2 volt used for the output stages of the logic chips.  This logic family is power hungry (the current draw of the -2 volt supplies for a single machine was over 1000 AMPS, the -5.2 volt supplies was easily 3 times that) It was common to solve intermittent failures by bumping up the -2v supplies by 50 millivolts, yet techs would spend literal days swapping boards out needlessly - and at great expense.

Last week I had to get my tired old carbureted '83 ranger past emissions.  I went down for the first try and failed miserably.  Went home and found the carb was rattled apart (again) and had big air leaks.  I tightened it back up and tried again.  Still bad.  Time to start at the beginning.
Check plugs and ignition
replace dirty air cleaner
put voltmeter on O2 sensor and adjust mixture until I see the sensor "switching".
Go back for third test and it passes with flying colors.  The techs at the shop were amazed - they were laughing at me and were certain I would never be able to get this beat up old truck back on the road without paying a shaman with a magic gas analyzer a few hundred dollars, and without buying a new carb (another $500 - I paid $1 for the truck 14 years ago).  When it passed, I showed them how I did it, but they could not grasp the notion of "switching" to see that the system was in control limits.  In fact, the idea of attaching a volt meter to the O2 signal wire was over their heads, and these guys were the tune up techs.
At least in the end they had a little respect for the do-it-yourselfer.


Dave


On May 20, 2011, at 4:36 AM, Professor GT gmail wrote:

> Okay, you got it running, but is it fixed?
> 
> Voltage drop, voltage drop, voltage drop! This is basic electrical. Yea, you can SWAG (scientific wild-ass guess) at it, taking all kinds of pokes at what it might be, or you can measure powered voltage drop across the load (the pump) to know exactly what's going on.
>   As an automotive field engineer and instructor for 30 years now I get crazy when folks recommend all kinds of swags, with no real methodical process. Yes, I realize that they are trying to help, and no malice toward those trying to help is intended, but does leading one on a wild goose chase really help? Getting lucky and finding the problem by accident does nothing to help one understand what's really going on, and can lead to future mis-diagnosis. 
>   One seemingly oversimplified pearl of wisdom is: "If You Understand How It Works, You Can Figure Out How To Fix It!". This of course assumes some basic knowledge and aptitude already exists.
>   One of the most mis-understood, and yet most basic foundations of electrical diagnosis is proper "voltage drop" testing, and of course having the bandwidth to understand what the test results indicate.
>   Sorry for the rant (not really), but I cover a 2-3 hour basic electrical review in EVERY one of the classes I teach, whether it's an advanced Allison Transmission Hybrid class, or a basic engine course, and EVERY time, when done, I ask the students if the time just spent was worth it. In EVERY case, the response is a resounding YES, even from the experienced experts who say it was an insightful review, from which even they learned something.
>   I do not say this to brag or boast, believe me, i have better things to do, but just to offer up insight and to possibly help you through your consternation with this fuel pump issue.
>   When an electrical load in a series circuit (which this is) is energized, the load will use all the available voltage to perform it's work. So, the first step is to measure "available" battery voltage. Then, the energized fuel pump (whether it works or not) should "drop" "available" voltage (with no more than 0.3 volt loss in the rest of the circuit). (With high current circuits like starter motors 0.5 v is the allowable loss).
>   Anyway, if available voltage is dropped or consumed by the load when powered, by completing the circuit, which can be done by jumping the relay, if the pump is not working then this test proves that the load (the pump) is bad!
>   If however the load consumes significantly less that available voltage, this indicates that either before or after the intended load (the pump), there is another unintended load consuming voltage. This unintended load, like all loads, has resistance which does two things to the circuit: it steals voltage from the intended load, and it lowers circuit current flow. This is why the intended load either becomes flakey or quits working altogether.
>   Think for a moment about a simple light bulb circuit. If the bulb does not work when powered on, the most common fault is a burned open filament. When testing for voltage drop we turn on the bulb, even though it doesn't work, and we put our voltmeter across the load. If the filament is open, and that's the only problem in the circuit (remember there can always be more than one issue in any given instance), we will have available voltage up to the open filament on the "hot" side of the circuit, and we will have 0 volts or "ground" on the ground side.
>   The key to understanding what a voltmeter does is not only understanding that voltage is electrical pressure or push, but also that the voltmeter ALWAYS displays the DIFFERENCE between the leads, or what we refer to as the voltage drop, or Delta V. So, now if we go back to our bulb, with an open filament, and the power turned on, the voltmeter will show the difference between available voltage and ground, or 12 (nominal VBatt) minus ground or 0V which is = 12.
>   Hence, if we jump the relay to power the fuel pump and the voltage drop across the pump is available voltage, this means the pump has extremely high internal resistance or is totally open. (the load is bad)
> 
> Now, if there is significantly less than available voltage consumed by the pump, this is an indication of an unwanted load (resistance) either before on the power side, or after on the ground side, causing the problem. So, we move our voltmeter leads to check both sides of the circuit individually. Once we have "found" the voltage that doesn't belong there, we have found the problem. Remember, only loads consume appreciable amounts of voltage, so the circuit leading up to the load, and back to ground should not be consuming voltage. To check the hot side turn the circuit on and check from power to the load - we should not see more than 0.3 v. Do the same on the ground side.
> 
>   This will prove out the pump, and the power side of the relay. Then we'd move on to the control side, which other than the wiring and the relay itself (the relay coil is the load on the control side of the relay circuit) those things that energize the relay must prove out also.
> 
>   Getting back to voltage drop, remember resistance that does not belong there causes problems. This can be from corrosion ( green wires which I like to all Chiapets - remember those?), or a wire hanging on by just one strand. These are the ones that fool most, especially those that improperly think they can find this type of fault with an OHM meter! 
> When I hear, "I Ohmed out the wire", it sends a chill down my spine because I know immediately I'll have to spend some teaching time to help that tech unlearn the bad stuff clogging up his brain. It all has to do with how an ohmmeter works, and what's really going on in the circuit!
> 
>   Well, I could go on, but hopefully this small insight will begin to help you gain a better understand of how to perform electrical diagnosis. No matter the circuit, electricity is electricity, so voltage drop tests apply across the board, even with sensor circuits.
> 
>   Maybe this year at S-Fest if there's any interest I can do an Electrical Diagnosis Seminar. As an automotive technical training professional I pride myself on helping others share my passion for all things automotive!
> 
> If you want additional assistance do not hesitate to get in touch.
> 
> Thanks,
> 
> ProfessorGT
> (Sent from my iPhone4)
> 
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