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Tachometer Issues - More added! PLEASE HELP

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Old 01-13-2006, 09:23 PM
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Originally Posted by markwb
Originally Posted by BillBoost37
I spoke to Toasty and mentioned he should test his coils and look over the ICM.

Mark..where does tach signal come from on 00+? I think it'* still derived from ICM???
True. Low resolution engine speed (3X reference)-PCM Input-from the ICM The PCM uses this signal to calculate engine rpm and crankshaft position above 1200 rpm. Medium resolution engine speed signal (18X reference)-PCM input-The 18X reference signal is used to accurately control spark timing at low rpm and allow IC operation during cranking. Below 1200 rpm, the PCM is monitoring the 18X reference signal and using it as the reference for ignition timing advance. When engine speed exceeds 1200 rpm, the PCM begins using the 3X reference signal to control spark timing. Additionally, the IPC (Instrument Panel Cluster) uses the engine speed signal from the PCM to calculate engine speed. The tachometer defaults to 0 rpm if a malfunction in the engine speed signal ckt exists. Looking at a schematic, the PCM inputs the speed signal to the IPC. The crank sensor is where it all begins with the hall effect xdcers along with the 18X & 3X reference signals.



so what does this mean to me, other than it seems that my guage defauled to 0
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Old 01-13-2006, 09:34 PM
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Well, it could mean that there is a fault in that part (engine speed) of the PCM, or the Crankshaft Position Sensor is beginning to act up especially since you mentioned stalling & hesitation. If you can look down there and do close up inspection of crank sensor for any dirt or crud, also pull the wiring connector off the ICM and inspect pins & receptacle for any corrosion or dirt.
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Old 01-13-2006, 09:40 PM
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Originally Posted by markwb
Well, it could mean that there is a fault in that part (engine speed) of the PCM, or the Crankshaft Position Sensor is beginning to act up especially since you mentioned stalling & hesitation.
Is there any way to determine which is at fault?? Would scanning the car do the trick like PDad mentioned

Originally Posted by markwb
If you can look down there and do close up inspection of crank sensor for any dirt or crud, also pull the wiring connector off the ICM and inspect pins & receptacle for any corrosion or dirt.
i wiggled around the ICM connector while the car was runnning, with no result. it was already dark out so i couldnt really get in there too much to test anything or see much.
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Old 01-13-2006, 11:00 PM
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Could the connectors at the PCM itself be loose? I'd check the connectors to the PCM for loose, or frayed wiring. Other than that, I can't think of anything else that could cause this other than a bad PCM, or ICM.
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Old 01-14-2006, 05:47 AM
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The scanner is the best approach like Pdad says. Once you establish whether or not the fault lies in the PCM or the Instrument Panel Cluster you can go from there. I'm thinking it'* not the IPC side of the tach. I'm leaning more toward PCM, ICM, or Crank.
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Old 01-14-2006, 08:26 AM
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Originally Posted by markwb
The scanner is the best approach like Pdad says. Once you establish whether or not the fault lies in the PCM or the Instrument Panel Cluster you can go from there. I'm thinking it'* not the IPC side of the tach. I'm leaning more toward PCM, ICM, or Crank.
i dont think it could be in the cluster since i have a drieability issue, and not just a faulty guage
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Old 01-14-2006, 09:04 AM
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I think it could be the crank sensor area, the sensor reads 2 sets of interupter rings on the back of the crank pulley, 1 with 12 notches and 1 with 3 notches.
It maybe the sensor itself or debris or misalgined rings.
But without a scanner its just a guess.
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Old 01-14-2006, 09:14 AM
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Originally Posted by PontiacDad
I think it could be the crank sensor area, the sensor reads 2 sets of interupter rings on the back of the crank pulley, 1 with 12 notches and 1 with 3 notches.
It maybe the sensor itself or debris or misalgined rings.
But without a scanner its just a guess.
with the scanner will i get readings for both stes of rings or just a single reading....? What us the functionality of each of the rings?/ is one tach/ICM related and the other fuel related?
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Old 01-14-2006, 04:32 PM
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Originally Posted by toastedoats
Originally Posted by PontiacDad
I think it could be the crank sensor area, the sensor reads 2 sets of interupter rings on the back of the crank pulley, 1 with 12 notches and 1 with 3 notches.
It maybe the sensor itself or debris or misalgined rings.
But without a scanner its just a guess.
It should have read 18 not 12....sry.

with the scanner will i get readings for both stes of rings or just a single reading....? What us the functionality of each of the rings?/ is one tach/ICM related and the other fuel related?
No the scanner wont be able to see the rings inputs them selfs (unless you get a tech2 scanner, $2500) but you will see engine speed (rpm).
Now if your tach is dead (reads zero) and the pcm shows 1000 rpm (for expample) then the problem is between the pcm and the tach or the tach itself.

The 18x reference ring is used for ignition and the 3x is used by the pcm.
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Old 01-14-2006, 05:33 PM
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Originally Posted by PontiacDad
Originally Posted by toastedoats
Originally Posted by PontiacDad
I think it could be the crank sensor area, the sensor reads 2 sets of interupter rings on the back of the crank pulley, 1 with 12 notches and 1 with 3 notches.
It maybe the sensor itself or debris or misalgined rings.
But without a scanner its just a guess.
It should have read 18 not 12....sry.

with the scanner will i get readings for both stes of rings or just a single reading....? What us the functionality of each of the rings?/ is one tach/ICM related and the other fuel related?
No the scanner wont be able to see the rings inputs them selfs (unless you get a tech2 scanner, $2500) but you will see engine speed (rpm).
Now if your tach is dead (reads zero) and the pcm shows 1000 rpm (for expample) then the problem is between the pcm and the tach or the tach itself.

The 18x reference ring is used for ignition and the 3x is used by the pcm.
Interesting.... this is quite the puzzler of a problem im having here...

If the scanner reads zero, and so does the tach, then i have a pcm, or CPS issue then..



On another note, on the information that mark gave above; If when a fault is detected in the engine speed, then the tach is defaulting to zero, should it be staying there until the next start cycle???
If not, then by the time i get to scan the sensors, the signal would be correct, but the guage will read 0, due to the past fault
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