05 TB won't start after head gasket replacement
- Thread starter AirSick
- Start date
Mooseman
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Well, here's what I tried to send 3 different times....lol. I hit submit then some sort of hieroglyphics showed up.
AtlWrk
Member
- Dec 6, 2011
- 674
Mooseman said:
If it was a sensor problem or a faulty 5V reference you would have different codes. The fact that your throttle body does not move ever is key here. You've pretty much ruled out the harness so that leaves the throttle body itself or the PCM (which, as others have stated, is pretty bullet proof). Is there any sign of binding or looseness when moving the throttle plate by hand (with the ignition OFF)?? It should move both directions from rest position(~20% open) with only smooth spring pressure pushing back.
It really is time to swap throttle bodies IMO. Either find one to borrow (they're common to all 4.2Ls) or pick up a replacement.
FWIW I do recall a member on here having his car vandalized by someone cutting the thottle body harness completely. Unbelievably, the truck still started and drove in REP mode.
gmcman
Member
- Dec 12, 2011
- 4,656
AtlWrk said:
If it was a sensor problem or a faulty 5V reference you would have different codes. The fact that your throttle body does not move ever is key here. You've pretty much ruled out the harness so that leaves the throttle body itself or the PCM (which, as others have stated, is pretty bullet proof). Is there any sign of binding or looseness when moving the throttle plate by hand (with the ignition OFF)?? It should move both directions from rest position(~20% open) with only smooth spring pressure pushing back.
It really is time to swap throttle bodies IMO. Either find one to borrow (they're common to all 4.2Ls) or pick up a replacement.
FWIW I do recall a member on here having his car vandalized by someone cutting the thottle body harness completely. Unbelievably, the truck still started and drove in REP mode.
The vandalized TB harness was ironically mine. Unfortunately I didn't see it before the technician did but I think a few wires were still touching or were about 99% severed, allowing only minute movement from the TB. I could raise it off idle and that was it, if I encountered a small hill it wouldn't pull it in 2nd, had to keep trans in 1st. I have had a few REP moments and it would at least maintain hwy speeds, but not this time.
Does the TB not operate from 5V? Either case, you don't have fuel into the cylinders and at least I had that. A Tech 2 will tell right away but unfortunatley at a cost.
AtlWrk
Member
- Dec 6, 2011
- 674
gmcman said:
<thinks about unplugging the throttle body connector and trying to start, ya know, just to see what happens
>The two sensors in the throttle body are operated from two independent regulated 5V sources provided by the PCM. The cooling fan comes in to play because one of those 5V sources is also wired to the fan clutch to power the fan speed sensor. If there's a fault in the fan clutch harness it effectively disables one of the throttle body position sensors. This causes the PCM to go into REP mode since it won't rely on just one sensor 100%.
The electric motor that actually drives the throttle body is electrically independent of the sensor circuits and is driven by the PCM--though I can't speak to exactly how this is done (would guess a 12V PWM). It is this circuit and/or a mechanical problem in the throttle that I think the codes point to.
gmcman
Member
- Dec 12, 2011
- 4,656
gmcman
Member
- Dec 12, 2011
- 4,656
AirSick said:
Yes, but when he called me over to look at it, the wires were close together. I think the diagonal cutters used were kinda dull so somehow just enough wires were making contact with each other to be barely operable.
This is before he started stripping back the insulation.
My goodness I never noticed that in the schematic before! It was on a reference distribution page, not the one with the throttle body and the APP on it. Thanks!AtlWrk said:
Now you have me curious, and I'll have to drag out a scope to look. Not sure if the TB motor is an up/down stepper, or a unidirectional motor that gets closed only by the mechanical return spring. One phrase in the manual makes me think it's not a stepper, which would stay where it was put when not energized: If the ignition switch is left on for more than ten seconds without starting the engine, the throttle blade is allowed to close so as to not heat up the TB motor unnecessarily.
AtlWrk
Member
- Dec 6, 2011
- 674
the roadie said:
I google-imaged "electric throttle body disassembled" and came across some very similar styles of throttle bodies opened up. What they showed appeared to be a simple DC motor and two stage reduction gear train with the final gear attached to the throttle plate, a spiral spring and electrical brushes contacting carbon traces on a circuit board to make up the position sensor potentiometers. In effect it's basically a strong servo motor with an integral spring. I doubt there's much logic inside compared to, say, an RC servo since it can all be incorporated into the PCM logic. This is why I propose that it's a reversible (H-bridge style) 12V PWM signal. This would be fairly straightforward electrically. I say reversible because the "rest" position is 20% but I believe the throttle plate can be driven down to 0% (as well as opened obviously). PWM would be able to vary the torque of the DC motor to counter the spring at whatever position the PCM needs. Simple really
Some GM design decisions:
Others:
If I'm right about this the OP may be able to actually test the throttle body by momentarily applying 12V/GND to the TAC motor pins.
(DON'T DO THIS until the above can be confirmed)
AtlWrk
Member
- Dec 6, 2011
- 674
Confirmed. It is a simple DC motor driving the throttle plate.
Briefly applying a voltage to pins E and F should open or close the plate (disconnected from the PCM of course). I used 5V from my bench supply but 12V should be fine as long as you don't let it stall. Polarity doesn't matter since the plate can move both directions. This simple test will confirm that the throttle is in good working order.
Briefly applying a voltage to pins E and F should open or close the plate (disconnected from the PCM of course). I used 5V from my bench supply but 12V should be fine as long as you don't let it stall. Polarity doesn't matter since the plate can move both directions. This simple test will confirm that the throttle is in good working order.
AtlWrk
Member
- Dec 6, 2011
- 674
Not sure if the OP is still monitoring this thread but I did a little more testing out of curiosity and for the sake of completeness:
The PCM generates a (remarkably accurate) 10kHz low-side PWM signal to drive the throttle body. The really useful bit is that when the PCM releases the throttle body (after a few seconds of ignition on without a start) both motor control lines are held near 12V--even with the throttle body disconnected.
Point being: pins E and F should see ~12V relative to chassis ground (with a multimeter, not test light) with the ignition on if the PCM and wiring are functional.
The PCM generates a (remarkably accurate) 10kHz low-side PWM signal to drive the throttle body. The really useful bit is that when the PCM releases the throttle body (after a few seconds of ignition on without a start) both motor control lines are held near 12V--even with the throttle body disconnected.
Point being: pins E and F should see ~12V relative to chassis ground (with a multimeter, not test light) with the ignition on if the PCM and wiring are functional.
Well here is the update we have been waiting for. FINALLY got the damn thing running. Here's the kicker, come to find out the harness plug going to the tb and the harness plug going to the fuel injector harness are identical. Some how they got crossed and were plugged into the wrong connectors. Swapped them around and bam, she fired right up. Now this makes me feel like an idiot for not noticing this in the first place.
On a side note. It still threw the 2 codes as stated previously. It also threw another code. P0526.
On a side note. It still threw the 2 codes as stated previously. It also threw another code. P0526.
AirSick said:
Glad to hear you got it going. When I first started out working on vehicles, my dad gave me a tip. Take some masking tape and label connectors with a letter. Wrap the tape around where the connector goes on the sensor or motor and use a magic marker to write an A on it, then wrap the connector around the wiring and mark an A on it. Then put a B on the next one, a C on the next one... To this day I still do it. I also still use a piece of cardboard with holes poked in it for cylinder head and intake manfold bolts in the corresponding pattern they go on the motor. It's served me well and helped keep things clean when working on vehicles.
The code you got... looks like it's for the fan. Search Results
Normally i do that but a majority of the plugs only go one way, of course except for those two. She did fine for about 30 mins of drive time then it just fell flat on its face. It acted like I barely had power.
