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2004 Chevy Trailblazer LS 2WD 4.2 Inline 6 cylinder (Blue)
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Discussion Starter · #1 ·
2004 LS Chevy Trailblazer. keeps blowing fuse 28 PCM 1. It's a blue 15 fuse. Frist time it blew and replaced car run a few days. second time had to be towed. third time towed replace fuel pump, replaced fuse with 15 breaker, ran most off the day made it to the next county and back ok. Died a Walmart less than 10 min away breaker was hot, trailblazer would not start, blown fuses repeatedly. wait 30 min or so started drove home. Now want start at all.
Any suggestion ?
 

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2002 chevy trailblazer_lt
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So for openers that fuse doesn't have anything to do with the fuel pump!!

Fuse 28 powers all 6 injectors, all 6 ignition coils, and sends a signal voltage to the PCM.

Rectangle Slope Schematic Font Line


Possible causes would include one or more failing coils, a bad spot in one of the 8 or so pink wires that come from that fuse (like where the harness might rub against a metal bracket etc.), or lastly even a faulty PCM.

I have to say I know of a gentleman that had this very same issue, chased it for over ten years! Finally the truck flat out died and had to be towed and they discovered a failed PCM, or at least that was where their testing led them. Haven't heard from him for about a month now.
 

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2004 Chevy Trailblazer LS 2WD 4.2 Inline 6 cylinder (Blue)
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Discussion Starter · #3 ·
The ignition coils are new changed out a couple months ago because code said one was bad. All but one was changed, one of the new ones was crashed in the mail, the old one being uesd tested good. There are no codes at all right now.
Thanks for the help so far.
What else to check?
 

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I'm afraid there are no simple ways to go about this.

Does the fuse now blow as soon as the key is turned to RUN? (Not started)

Are you comfortable with electrical testing?

Do you have a multimeter and know how to use it?
 

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2004 Chevy Trailblazer LS 2WD 4.2 Inline 6 cylinder (Blue)
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Discussion Starter · #5 ·
It says 187 views. Anybody got a clue what would cause 1.5 volts resistance at fuse 28 ? There's a purple plug that makes it go away when disconnected. Serval things are wired to this purple pug, so far disconnecting any of these has not gotten read of the resistance. All the wires look good so far. There is a little bit of what looks like oil on one of these but the oil sensor is on the other side of the engine ? Really confused, not sure what else to do.
 

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OK, a couple of questions/requests first. Please describe exactly how you measured the 1.5 volts of "resistance" at fuse 28. Include exactly where you placed the black lead from your multimeter and exactly where you placed the red lead from your multimeter. If you can supply a couple of pictures of you measuring the 1.5 volts of "resistance" all the better.

Regarding the purple plug that makes the 1.5 volts of "resistance" go away when you disconnect it, exactly where is this purple plug located? Again, if you could take a picture of the purple plug where it is plugged into that would really help out enormously.

As far as one of the wires being oily, and the oil sensor being on the other side of the engine, please do not worry about it. The wires will collect oil, grime, etc., wherever they are simply because that what happens to wires and other things in an engine compartment.

Now when you say that now the engine does not start, do you mean the engine does not crank at all when you turn the ignition key to start, or the engine does crank but does not run when you turn the ignition key to start?

Also, as TJBaker asked, when you turn the ignition key to Run (and not going to start first) does the fuse or circuit breaker blow?

Thank you and good luck!
 
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Discussion Starter · #7 ·
Will try to do some pictures. Yes the fuse blows. Makes nosie like trying to starter but will not start blows fuse. No fuel pressure, in the process of replaceing fuel pump. Fuel inlet check vale came half way off, after market repair kit on the way. Yesterday checking for damage wires with no luck, same thing today. Best discritpion of 1.5 reading red lead where the fuse goes, black lead grounded to body.
 

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Best discritpion of 1.5 reading red lead where the fuse goes, black lead grounded to body.

For reference sake:

reading resistance in ohms (symbol Ω) between the chassis ground (black test lead) and the fuse 28 terminal closest to the engine (red test lead) should yield about 1970 Ω with the key in the OFF position and the PCM inactive. If the PCM is active you may see about 900 ohms. At least this is what my 2002 reads.

The fuse 28 terminal closest to the fender is where the 12 volt positive power is supplied from ignition switch terminal C. Checking resistance at this terminal is meaningless as it is simply reading through all the other circuits fed by terminal C of the ignition switch.
 

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OK.

First thing to say is that when you took your multimeter and connected it between vehicle ground and fuse 28, it appears you actually were measuring some kind of voltage and not resistance. On your multimeter there are either push buttons or a knob to move that selects what you desire to measure. For resistance one measures ohms. For voltage, one measures volts, and for current, one measures amperes.

OK on the fuse blowing when you turn the key to the Run position.

How did you determine that the fuel pump was not producing any pressure? Also, exactly where is the fuel inlet check valve you mentioned that you need to replace?
 

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Discussion Starter · #10 ·
OK.

How did you determine that the fuel pump was not producing any pressure? Also, exactly where is the fuel inlet check valve you mentioned that you need to replace?
Fuel pressure was check with a fuel pressure gage at both posts, the 2004 Trailblazer LS has a port near the tank and one up top under the hood marked with a green cap. Oil pressure was zero at both with key turned on.
The fuel inlet check vale (also called roll over vale) is actually part of the gas tank (defect was on recall in some modules, GM would replace gas tank for 15years or 100,000 miles) After market fix is Fuel inlet check vale Dorman 577-106.
 

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Discussion Starter · #11 · (Edited)
OK.

First thing to say is that when you took your multimeter and connected it between vehicle ground and fuse 28, it appears you actually were measuring some kind of voltage and not resistance.
Yes it was volt, sorry about that. Volts measured on driver side of fuse 28 is battery voltage 12.5 , voltage on passenger side is 1.5 , all wires look good, is this normally, son says it needs to be under 1/2 a volt .
 

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OK on how you measured the fuel pump pressure. Actually there is only one test port for the fuel pump - the one by the fuel filter under the vehicle. The port with the green cap is the EVAP system test port and is used to inject smoke into the system in order to find vacuum leaks.

Do you know if the fuel pump can be heard for the first 3 - 5 seconds when the key is turned to the Run position? If it cannot be heard (remove the gas cap and stick a screwdriver through the spring flap to keep it open), then either the fuel pump is not getting power or it is dead.

Unless you have the EXT version of the TrailBlazer (longer wheelbase) then there is no rollover valve.

Regarding the voltage reading on fuse 28, if you measure 12 VDC on one side of the fuse and only 1.5 VDC on the other side of the fuse, the fuse is bad. You should 12VDC on both sides of the fuse when the ignition key is in the Run position and the fuse is good.
 

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Regarding the voltage reading on fuse 28, if you measure 12 VDC on one side of the fuse and only 1.5 VDC on the other side of the fuse, the fuse is bad. You should 12VDC on both sides of the fuse when the ignition key is in the Run position and the fuse is good.

But if I have been following correctly then fuse #28 blows as soon as the key is turned on. In this case what is needed is to measure resistance from the load side socket of fuse 28 to ground with key OFF.
 

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You are correct TJ.
 
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You are correct TJ.
And if that resistance is seen to be low there are then something like 13 possibilities, 6 ignition coils, 6 fuel injectors, and a signal line to the PCM are the loads I know of for fuse #28.

For isolation of the possible fault, removing the intake resonator would gain access to the 6 coil connectors as well as the main harness connector for the fuel injectors (near the throttle body/resonator connection).
 

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According to the factory service manual, each fuel injector should have resistance between 11 and 14 ohms, so for 6 injectors, they would add between 66 and 84 ohms to the circuit. I am unsure how much resistance each ignition coil would add to the circuit.

For circuit I am referring to measuring the resistance between the load side of fuse 28 and ground.
 
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According to the factory service manual, each fuel injector should have resistance between 11 and 14 ohms

This agrees with my own tests of fuel injectors.


each fuel injector should have resistance between 11 and 14 ohms, so for 6 injectors, they would add between 66 and 84 ohms to the circuit
Remember now, you would be adding resistances in parallel here.... If the injectors were grounded. BUT!! ... the injectors are only grounded by the PCM as required. Thus for our test here at the load side of fuse 28 we will be looking only at the 6 ignition coils and the PCM ignition 1 input at pin 19 of connector 1. My tests of the original Delphi ignition coils shows between 11000 and 12000 ohms each and pin 19 of PCM connector 1 to ground is about 11000 ohms so say 11000 ohms x 7 units in parallel and we get up around 1600 ohms or so.

If I sound familiar with all this it is because we just recently troubleshot this very same issue on another website as I mentioned very early in this thread!!
 

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Thanks for catching/correcting my "glitch". It did not dawn on me that the injectors are wired in parallel hence the resistance presented by the fuel injectors will be lower - likely around 1 to 2 ohms. Hence, the majority of the resistance will be from the ignition coils.
 
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Fuel pressure was check with a fuel pressure gage at both posts, the 2004 Trailblazer LS has a port near the tank and one up top under the hood marked with a green cap. Oil pressure was zero at both with key turned on.
The fuel inlet check vale (also called roll over vale) is actually part of the gas tank (defect was on recall in some modules, GM would replace gas tank for 15years or 100,000 miles) After market fix is Fuel inlet check vale Dorman 577-106.
I've got a 2004 TB, and the fuel pressure schrader valve is near the fuel filter on the rail. If I remember correctly, the one with the green cap is for a smoke test.
 

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According to the factory service manual, each fuel injector should have resistance between 11 and 14 ohms, so for 6 injectors, they would add between 66 and 84 ohms to the circuit. I am unsure how much resistance each ignition coil would add to the circuit.

For circuit I am referring to measuring the resistance between the load side of fuse 28 and ground.
Hey! How can injector resistance be additive? They are never all on at the same time --- right?
 
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