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fuel pump question

2K views 29 replies 6 participants last post by  Ravalli Surfer 
#1 · (Edited)
ok so i’ve been trying to diagnose this f/p issue, my truck is cranking but not starting so i checked fuel pressure and there is none. i pulled the f/p relay and probed the connectors with a ddm and its getting 12v power (#30) and 12v ground (#86) BUT when i probed pcm control (#85) it comes up but at slightly lower 10.6v

i jumped #30 and #87 and the fuel pump pressure shoots back to normal and you can definitely hear the pump come on but i tried to start the vehicle and it still just cranks..

so i’m confused of wtf is happening now so does this point to the fuel pump being faulty or something else cuz like i said i felt like the pump was fine because i could hear it come on when i jumped the connectors BUT when i jump it still didn’t start just continues to crank.. so i’m just a little stumped
 
#3 ·
no the security light is not on when i jump the connectors (or otherwise)

i’ve drained my tank in preparation to replace the pump just in case it’s the pump..

*sorry i have another thread going on this subject as well i’m not sure if the mods can combine them or not
 
#4 ·
BUT when i probed pcm control (#85) it comes up but at slightly lower 10.6v
That terminal will only have 12 volt power when the PCM has commanded the fuel pump on. It should do this for two seconds when the key is first turned on then it turns off until the key is turned to crank when the engine is starting.

It is possible that you may have read a lower voltage that is not meant to activate the relay but only to monitor the relay coil circuitry.
 
#6 ·
In your first post you said you jumped terminal 87 & 30 at the fuel pump relay and the fuel pressure went back to normal. This would mean the fuel pump is OK and is not the trouble, no??

This is why I asked about the security light. The security system might be able to disable the engine, perhaps by disabling spark. This is a guess by me, I do not know this as a fact but I suspect it might be this way.

You might try to test for spark or at least a signal from the PCM to the coil(s) when cranking.
 
#7 ·
yes exactly and that why i’m hella confused cuz it’s not getting fuel pressure cuz i hooked up the tester and it’s at zero but when i jump the connector i can hear the pump come alive plus the pressure returns back to normal BUT it still just cranks and no start??

so i feel like it’s fuel related to some degree considering there isn’t fuel pressure… i will take your advice and test for spark in the morning tho

a weird side note is that the SERVICE BRAKE SYSTEM message came up on the dic right when the car stalled out and it’s still there.. not sure how the hell that could possibly be related but it’s just a weird coincidence..
 
#8 ·
hella confused cuz it’s not getting fuel pressure cuz i hooked up the tester and it’s at zero but when i jump the connector i can hear the pump come alive plus the pressure returns back to normal BUT it still just cranks and no start??

Just to make sure I know what you have tried here... Do I understand correctly that you have jumped the relay and confirmed fuel pressure (with a test gauge) and then while the relay is still jumped and you can still hear the fuel pump running you turn the key to start and the engine cranks but does not fire at all??
 
#9 ·
WHEN did this show up? ----> "SERVICE BRAKE SYSTEM "

If you jump the relay - the pump runs - but when you reinstall the relay it doesn't pump?

If that's so, change the relay --- or swap it with any other relay of the same design in the fuses/relay panel.
 
#11 ·
Got any sort of code reader?

When you first turn the key to RUN and all the dash lights come on for a couple seconds before going off, does the security light come on then? (just to prove it works)

And when most of the dashboard lights go off, does the check engine light stay on like it should?

The "Service Brake System" seems like it should be related since it came on right at the same time. But that is by itself not proof that they are related. I looked for commonalities between the brake system and the engine and I see that ground point G107 is the ground for the fuel pump relay and the brake fluid level switch but that is about all I found. Might be worth checking that ground though.
 
#12 ·
so yes all the dash lights come on and go out as normal, security light works as well and the check engine light does go out as well.. and speaking of code readers, is there a code reader that can pull old codes?? i know if you clear the codes by disconnecting the battery, they are still stored somewhere tho right? i grabbed a code reader from amazon but it cannot pull old codes

and hells yeah bro that ground is worth checking out.. i found a diagram of the grounds and i’m gonna remove and clean g107 and see what it do, i feel like you might be on to something
 
#16 ·
and the check engine light does go out as well..

That check engine light is not supposed to go out until the engine is started.


is there a code reader that can pull old codes??

It is a common misconception that one needs an expensive scantool to get such codes. Well at least as far as these older GM vehicles that use the SAE J1850 VPW protocol that is SIMPLY NOT TRUE.

I can read all codes from these trucks with a $14 Bluetooth OBD2 adapter that supports OBD2 protocol #2, SAE J1850VPW. How to do this was detailed in this very website over ten years ago but now it is far cheaper than it was back then. And it seems no one today (or then) has any desire to learn anything and only seek a tool that does everything for them at a push of a button.

Is it a plug & play sort of affair? NO, of course not. You need to have at least a passing interest in learning how to do it.
 
#13 ·
OBD2 does not clear codes when the battery is disconnected. They're all still there, waiting for a real scan tool to liberate them.
 
#15 · (Edited)
When it was my money, I bought used-but-usable PROFESSIONAL GRADE scan tools from eBay sellers.

About 20-something years ago, I got a Snap-On MTG2500 bundle with software cartridges from 1980 1/2 to 2005. Various adapters and Personality Keys for OBD1 and OBD2. Eventually it got flaky, wouldn't boot unless I put it under the heater outlet to warm it up, and then I spilled gasoline on it and welded-in the software cartridge. At that point I went back to eBay and bought a Solus Pro (2 generations newer than the MTG2500 and enormously more sophisticated and powerful.)

VERY pleased with the Solus Pro. The 2500s, Solus, Solus Pro, and perhaps the Solus Ultra are no longer supported by Snappy--NO UPDATES, no repairs, no service, no nothing. But if you buy the tool with software new enough to deal with whatever vehicles you intend to use it with, you don't NEED an update. Similarly, buy it with the vehicle adapters/personality keys, battery, recharger, operator's manuals, etc. and you're all set.

My 8.2 software (good through 2007) will not auto-bleed the ABS on my '97 K2500, but seems to do everything else I've asked of it, including auto-bleeding the ABS of my 03 Trailblazer. The 14.2 software can auto bleed the '97.

I got my Solus Pro with the 8.2 software, various vehicle adapters and keys for OBD1 and OBD2, owner's manuals on DVD, a usable battery with a recharger, and a suitcase to carry it around in, for under $350 shipped to my door.

A Solus, or Solus Pro (or newer but of course more expensive--Solus Ultra, Solus Edge, or the current model Solus Legend) is recommended in favor of the older MT/MTG2500 series in part because of the graphing feature that the MT does not have at all, and the MTG has in rudimentary form. Software for the '2500 series is not available newer than 9.x, which should be good to 2008; but even that is RARE; the 6.2 I had was about the newest I saw. The MT2500 dates back to 1988; it was the class act of it's era...but that era is over.

There are competing brands of PROFESSIONAL-GRADE scan tools--OTC and the GM-specific Tech 1 and Tech 2 and others. There are cable-or-dongle-plus-software solutions that run off of a standard laptop, and there are numerous consumer-grade scan tools that may or may not do an adequate job with the engine, but can't talk to the ABS or Body module, or the instrument cluster, or whatever. I have ZERO experience with Non-Snap-On scan tools except to say that OTC is a four-letter word to me due to them screwing me on a "Lifetime Marathon Warranty" on a $400 floor jack that broke, and the offshoring of their entire product line. OTC now means "Oh--That's Chinese".
 
#18 ·
"Code readers" are not "scan tools". A scan tool will read codes, but does a huge amount more.

"Codes" can be useful. But the real information is in the data stream; with bi-directional control being very helpful as well.
 
#21 ·
There is that magical phrase "bi-directional".

What precisely do people think this means?

"Bi", meaning two as in bicycle, biped, bipolar?

Operating in two directions it would seem. Transmit and receive on the serial data bus.

And how exactly does a "code reader" retrieve codes? It first sends a message on the data bus requesting codes from a module. For the subject vehicle of this thread that message takes the form "6C10F119D2FF00" where this is a request from the code reader or scantool (both devices send the exact same message) to the PCM requesting all Current, Pending, MIL requested, and History codes status levels. Other options are available for Old codes, etc.

It then receives responses from that module (or modules). Is this somehow not operating in two directions, transmit and receive?




In reality, on the single wire class B serial data communications network we are dealing with here there is absolutely no difference between a sent message and a received message. All are boiled down to nominally 7 volt pulses on this single wire where each bit is put on the wire one after another. It looks like this.....

Rectangle Font Electric blue Engineering Science


Seen here is a ~200 microsecond high pulse that signifies the beginning of a frame (message). This is followed by a ~64 microsecond low, a ~64 microsecond high, a ~128 microsecond high, then a ~128 microsecond low. That represents the binary equivalent of the character "6".

This is followed by a long low, a short high a short low and a long high. This is the binary of the character "C". @chem_man it's sort of like reading morse code, no?

These are the first two characters of the message I posted above for a request for DTC codes. Sent by a module (code reader or scantool) and received by all modules on the bus. There is no "directional" component to the message, even the module sending the message also receives the message.


"Bi-directional Control": Here are just a few of the things I can do with my little $14 bluetooth adapter....

Unlock my doors should I accidentally lock my keys inside.

Activate my front axle disconnect.

Close my EVAP vent solenoid valve.

"Ding" my door chime any number of dings up to 255.

Activate my Fuel pump.

Move my fuel gauge, volts gauge, oil pressure gauge etc to a given value.

Turn on or off the various warning lights in the cluster.

And just this weekend I figured out how to release the electric brake on the encoder motor and send a PWM value of between 8% to 50% to the encoder motor circuit A or B.

I do get what folks say about the usefulness of a "quality" scantool. Especially for those who must work on a variety of vehicles. And the overwhelming majority of owners simply don't have the time to spend learning how to do the sort of things I just listed above.

And where is the real difference in these tools? Not really in the hardware, electronics are fairly inexpensive in reality. The cost is in the licensing fees that must be paid yearly to GM Ford, etc. Over ten years ago it was said that GM required about $50,000 annually for this proprietary information. Can't begin to imaging what it may be now.

But for the owner who just needs to pull codes every now and then to assist in diagnosing an issue with their one or two older GM vehicles it may be worth investing some 20 or 30 minutes to get that far and not have to resort to depending on O'reillys or Autozone or whomever to try to scan codes. Particularly since they more often than not will be unable to read codes from the BCM, EBCM, TCCM or the like. The method I am speaking of will read all those codes and at all status levels to boot, Old, History, Pending, Current, MIL requested, etc.
 
#24 ·
Inundated with facts are we?

I don't like that the VEEP Dongle-thingy forgets every car every time and it has to be setup anew each and every time.
It's a bogus piece of gear that CAN be useful if it's all one has --- but isn't to me.

You've gotta drag and drop the readouts on the screen to keep them from overlapping each other and making them useless until I spend 20 minutes just straightening the screen.

THEN you've got to decide if you want digital or gauge or meter icons and put them in order --- gack!

It's too much setup every time --- and it never remembers ANY vehicles I have scanned in the past - so I don't know what wizardry YOU are using, but to me the device is useless.

After I finish breakfast here --- I think, in homage to you and the wonderfulness of this $hitty device --- I'll go out to my shop and affix this piece of junk to the wall with a 16p nail.
 
#27 ·
I didn't view the other thread but sounds like the PCM isn't activating the relay. I think the OP is chasing a symptom of an issue. PCM needs some enabling criteria before it activates the fuel pump, injectors coil etc. One essential input is RPM from the crank sensor. If the PCM doesn't see any RPM - nada. FYI security cuts injection not the spark. Back in the early 80's scan tools offered very little data so going to each sensor was mandatory for values.Once you had the values you needed to know the ranges to interpret faults or not.Well this isn't the 80's so if you want to efficiently fix vehicles a tool w/ data is beneficial.
 
#28 ·
This is 27 posts long by now and I'll admit I'm not gonna go back and re-read then either.

However, I think he said he jumped over the relay and the pump ran.

The pump on all my GMs run every time the ignition switch passes RUN on the way to START --- but even if you don't go that far, the pump runs for a few seconds if you stop at RUN.

Then --- it'll also run at the moment the ignition switch is turned OFF --- for the same few seconds.

............ so there's that too.
 
#30 ·
That happened to me found out it was the throttle body not communicating with pump

Mine did the same thing it was the throttle body
Can you expand on that?

I don't "see" how a bad or even a good needs to communicate with the fuel pump. I mightta missed that in school --- and I really want to understand this.
 
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