coolride
Member
Cranks but won't start
- Thread starter Dave347
- Start date
Bottom right of your photo is battery power at all times.
Bottom left of same pic should be ground at all times.
Top left of pic is to the fuel pump. should read a low resistance to ground.
Now here is the tricky one. Top right of pic goes to PCM. Key off will read a high resistance to ground. Key to run will read about +12VDC from the PCM. WHY!?! It's like this, these are solid state output drivers in the PCM. There is a small resistor across the coil terminals of the relay. With the relay removed there will be seen a weak +12 volt dc signal. The only way to see the voltage rise and fall is with the relay in the circuit.
Bottom left of same pic should be ground at all times.
Top left of pic is to the fuel pump. should read a low resistance to ground.
Now here is the tricky one. Top right of pic goes to PCM. Key off will read a high resistance to ground. Key to run will read about +12VDC from the PCM. WHY!?! It's like this, these are solid state output drivers in the PCM. There is a small resistor across the coil terminals of the relay. With the relay removed there will be seen a weak +12 volt dc signal. The only way to see the voltage rise and fall is with the relay in the circuit.
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coolride
Member
All I can think of is a bad relay (but swapping the relays around doesn't solve the problem) or a weak battery (just seeing 12.2 volts between the terminals.)
@coolride
Take your relay and do a single wire wrap to the one terminal that will be inserted into the top right socket of the picture from the video. install the fuel pump relay. test for 12 vdc on the other end of that now loose wire as the key is cycled. (remove starter realy as per @budwich post)
Take your relay and do a single wire wrap to the one terminal that will be inserted into the top right socket of the picture from the video. install the fuel pump relay. test for 12 vdc on the other end of that now loose wire as the key is cycled. (remove starter realy as per @budwich post)
2002 TrailBlazer FP relay & PCM control signal
I have a wire wrapped on the relay pin from the PCM. On a 4.2 this is the terminal closest to the passenger side rear of the vehicle. The voltage will be see...
youtube.com
Yes... there is one voltage which is "switched" by the relay... it goes to your fuel pump. The other voltage, which I think you are referring to, is sent by the PCM to activate the relay which is ALWAYS grounded at the other pin (not sure what you previous said about 12v on the ground... user problem with the meter).
Anyway, you should see the voltage coming from the PCM EVERY time your key goes to ON. Do that a whole bunch of times (20 plus). IF any time you do not see that voltage, that means you have a control problem which could be the PCM or cabling including the key switch.
As suggested by tjbaker57 and the way you show your "great wiring adaption" capability, you can likely readily wire "pig tail wires" (careful of bare wire points) on all the pins of the relay and reinsert it, then you will have test access to check all the conditions at any moment. Go from the results.
coolride
Member
What I'm wondering is, if I jump the relay like this can I drive normally without worrying about destroying something?
In other words: If the TB won't start when I'm at work, is it safe to use this "jumped relay" setup as a way to get home?
Mooseman
Moderator
When running normally, the pump runs constantly so jumping it like that wouldn't hurt anything but you would need to pull it each time you shut it down as it would keep running. IIRC, the + side does not shut off.
It would be preferable to find the root cause of why the PCM is not wanting to turn on the pump or why the signal isn't getting to the relay. There have been a few instances of traces under the fuse box that have broken. I guess the age of these trucks is starting to manifest itself.
It would be preferable to find the root cause of why the PCM is not wanting to turn on the pump or why the signal isn't getting to the relay. There have been a few instances of traces under the fuse box that have broken. I guess the age of these trucks is starting to manifest itself.
coolride
Member
Here's the test this morning. Pigtail on the control terminal, I was looking for 12V when the key is turned on. (Image a little foggy because it's humid here.)
I find 12V at the correct time. (Looks like 10V, but is not easy to snap a picture and work the ignition.) It's showed 12V every time. (But the engine wouldn't start.)
I find 12V at the correct time. (Looks like 10V, but is not easy to snap a picture and work the ignition.) It's showed 12V every time. (But the engine wouldn't start.)
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coolride
Member
so you have the right idea with the metering. You need to add a few more wires. 
so you have done the test lots of times and always see "12v", right? remember your "log" which showed 14 plus times success. You need to do lots of tests to ensure that indeed the control circuit / wiring is correctly operating.
You need to move or add wires for the other pin, going to the fuel pump. re do your test with your meter on that wire, it should see "12v" with every key "ON" operation as the relay operates and switches the constant 12v to the output of the relay. Again, lots of test runs to ensure that happens correctly EVERY TIME.
so you have done the test lots of times and always see "12v", right? remember your "log" which showed 14 plus times success. You need to do lots of tests to ensure that indeed the control circuit / wiring is correctly operating.
You need to move or add wires for the other pin, going to the fuel pump. re do your test with your meter on that wire, it should see "12v" with every key "ON" operation as the relay operates and switches the constant 12v to the output of the relay. Again, lots of test runs to ensure that happens correctly EVERY TIME.
coolride
Member
I'll do that budwick. But if I do that test and see 12V going to the pump, that means the engine should start during the test right?
Maybe I shouldn't be using the battery as the ground, but instead use the relay's ground.
Maybe I shouldn't be using the battery as the ground, but instead use the relay's ground.
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Any chance that you heard or felt the relay click here at that time??
If yes, the relay clicked, but the pump didn't run, and you have verified 12 volts at the relay going to the pump, that brings you to the fuel pump ground. I cannot remember if this has been addressed, fuel pump ground removed, cleaned, and replaced?
Why G107? If you were jumping the pump at the relay, sending 12 volts to the pump, should you not be checking G305, the fuel pump ground??
A second option at the time that fuel pump is not running when it should, is to read resistance from relay terminal socket the runs to the pump, to ground. You should read a low resistance value through the pump circuit to ground. And this can be done without having to climb under the truck, right there at the fuseblock with the meter you already have on hand
I'm tossing this up here as I have long had a less than clear idea of which ground is which when it comes to the EBCM and Fuel Pump grounds. Fuel pump ground is G305, whose location can very easily be confused with G304. I see @coolride has pictures showing a very rusted ground that might be G304 or G305 I think. So I dug deeper in my literature and found this imagery.
... and chiming in with Post # 209 via @Mooseman ... It would not hurt eyeballing these PINOUT Diagrams and after pulling the relevant PCM Harness Connectors... Back-Probe the Starting-Fueling-Ignition System Circuit(s) for Continuity. It is rare... but there have been some instances involving Hidden either Broken (or Rodent Chewed) Harnessing and Wiring on the GMT360 Platform:
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not sure wth you are referring to about "the battery as the ground, but instead use the relay's ground".....
Further about the comment "about 12v going to the pump, it should start...", testing is about segmentation of the circuit. There are still additional checks there after.... "baby steps" are needed first to ensure that circuits are doing EXACTLY what is expected at ALL times otherwise, you end up where you are currently with "starts some times, and then not".
Still further, your metering should ALWAYS be done to a KNOWN ground.... Other grounds maybe become known IF you test them correctly using resistance and NOT a voltage test.... Ie. the resistance between two ground points should be "0", voltage between two points that you don't if they are connected, means nothing... hold your probes in mid air, its is likely that they will read near zero, that does NOT mean they are have continuity, it means there is no voltage to be measured (withstanding any residual coming from your body / hands).
Anyway, you are making some small steps.
IF you continue down your current test path (with wire pig tails), you will likely find your answer in the next set of tests ...
coolride
Member
From the diagram you posted (Fig: 42) I'll say that the image I posted of the rusty ground was at position 25 (listed as G304.) I was under the impression that G304 was for the anti-lock brakes.
G305 is the fuel pump ground? I thought it was G107 (on driver's side of the engine block.)
It is more likely g107 is the ground for the fuel pump relay... nothing to do with the actual fuel pump itself.
coolride
Member
Mooseman
Moderator
I'd be checking all the major grounds given the condition of this one. The two on the lower left side of the block are important for PCM and other main functions.
There's a reason why the relay isn't sending the power to the fuel pump. Just have to find out the cause.
Out of curiosity, and in case this hasn't been asked yet, is your security light on or flashing?
There's a reason why the relay isn't sending the power to the fuel pump. Just have to find out the cause.
Out of curiosity, and in case this hasn't been asked yet, is your security light on or flashing?
coolride
Member
coolride
Member
budwich, I'll test those grounds with resistance. Not voltage. Picked up a bunch of stuff from the auto parts store today (actually 2 parts stores, HF, and Lowes.)
Is it impossible to find nice multi-meter test lead accessories at the retail level?
Will be thinking on how to proceed with the testing. I have 2 long work days ahead, then some time off. I'm learning much.
Is it impossible to find nice multi-meter test lead accessories at the retail level?
Will be thinking on how to proceed with the testing. I have 2 long work days ahead, then some time off. I'm learning much.
the simple test is to remove the relay, put your red probe on the socket pin that goes to the fuel pump and the black on your battery ground, take a resistance reading. That will tell immediately what's happening with the wiring ALL the way thru the pump back to the battery. I think that's what I suggested back in post 145. :-(
coolride
Member
That was 6 months ago. I was super busy at work and the TB sat untouched for long periods of time. When I had extra time and energy, I removed some rust and painted the frame and underside of the cabin. Just getting my head around the electrical testing now.
Needed something to do last winter. Not perfect but who will ever know. It's flat black on the frame and gloss almond on the body. Rustoleum. It's like this from the rocker to under the drive shaft. Will start the passenger side when this electrical issue is resolve.
Mooseman
Moderator
Do not shove the probe into fuse box relay or fuse slots or you will spread the connector's fingers and cause more problems. Use a piece of wire or a connector pin.
coolride
Member
coolride
Member
What's "low resistance to ground?" I searched that term on YT and did'nt find an explanation. Could you describe how that test is done and what it means?
P.S. It's been sitting for a couple days so I'll head out to try and start it now.
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coolride
Member
I tested the resistance between the upper left terminal and the battery ground. This is the pump circuit right?
The reading is 1. That means open circuit right? Not good?
The reading is 1. That means open circuit right? Not good?
No. That does not mean an open circuit.
Without seeing the specific setting on the meter face I will guess it is set to read resistance in ohms, and best case it is either set to the lowest selectable scale (nothing with a 'k' in it) or it is an autoranging meter.
That is likely a resistance value of 1 ohm through the wiring to the pump , through the motor windings and then on through the wiring from the pump to the frame ground, and then through the negative battery cable(s) to the battery negative terminal.
If you connect to an open circuit the meter reading will not change from what is seen when the meter is connected to nothing.
coolride
Member
The meter is set to 200 ohms.
Here's the reading when the test leads are connected to each other. (total resistance of the leads.)
Here's the reading when the test leads are connected to each other. (total resistance of the leads.)
coolride
Member
Quick look at the actron manual: A left hand 1 indicates infinite resistance.
Here's a better look. The black lead is connected to the bat. neg. terminal (shown way left) and the red lead is on the power supply to the fuel pump.
This measurement is between 12v power at the relay, and the pump power supply. Connecting these two pig tails should start the pump running, but the pump doesn't run.
Here's a better look. The black lead is connected to the bat. neg. terminal (shown way left) and the red lead is on the power supply to the fuel pump.
This measurement is between 12v power at the relay, and the pump power supply. Connecting these two pig tails should start the pump running, but the pump doesn't run.
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most likely... not good. That appears to indicate an open circuit. The hard part is determinin where that occurs. Havng said that, indeed you need to know exactly which pin at the socket is which. Use your meter and determine which is the constant 12v. Then determine which is the one that gets 12v from the pcm. Then measure the two others for resistance, one is constant ground and the other IS the PUMP CIRCUIT. That is the one that you are interested in.
coolride
Member
So to measure the resistance through the pump to ground you must measure between the neg battery terminal and the TOP LEFT relay socket in your imagery.
your "pig tails" might not be as good as you think. Pull the relay and follow the other directions about using "properly size wire" directly into the socket point. What is the measurement?
coolride
Member
That test was done and is documented in a post just before your post. It's just above and with an image. Post #234.
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