tach pegged after intake gasket repair
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Joined: Dec 2012
Posts: 35
tach pegged after intake gasket repair
tach pegged after intake gasket repair, mechanic who did it claims it isnt his fault, i know better...
anyhoot, its a 2002 jimmy 4.2 (2nd gen)
i looked up the scematic, but haven't yet examined the vehical,
scem says its a white wire to-from the coil that drives the tach
i may have the wrong scem...
but it sounds like the tach wire may be plugged into a secondary 12v source on coil...
question is;
are all wires in one plug or seperated, and could the mechanic gotten them swapped somehow?
i read in a different post it may be (CPS), would that be something that would get unplugged during a intake gasket change?
any suggestions before i start tinkering?
anyhoot, its a 2002 jimmy 4.2 (2nd gen)
i looked up the scematic, but haven't yet examined the vehical,
scem says its a white wire to-from the coil that drives the tach
i may have the wrong scem...
but it sounds like the tach wire may be plugged into a secondary 12v source on coil...
question is;
are all wires in one plug or seperated, and could the mechanic gotten them swapped somehow?
i read in a different post it may be (CPS), would that be something that would get unplugged during a intake gasket change?
any suggestions before i start tinkering?
Last edited by Scarakus; Feb 19, 2015 at 07:54 AM.
If I had to guess, I would say that the mechanic has pinched a harness somewhere.
The tach is driven by the PCM which uses data from either the camshaft position sensor or the crankshaft position sensor (I couldn't find a definitive answer as to which one is used).
The first step would be to get a scantool on it and see if the scantool indicates the same information that the tach is displaying. If it doesn't, then there is likely a problem with the wiring between the PCM and the instrument cluster or a problem with the instrument cluster. Again though, I would suspect the wiring given the work that was performed.
The tach signal should be 15-25Hz with the engine idling. You would need to backprobe terminal A3 (white wire) at the connector on the back of the cluster to test the tach signal. This wire runs to the PCM connector 2 (red connector) pin 20 (white wire as well) so you can backprobe there as well.
My information is from my 2000 service manual so hopefully the pin numbers and wire colors are correct.
The tach is driven by the PCM which uses data from either the camshaft position sensor or the crankshaft position sensor (I couldn't find a definitive answer as to which one is used).
The first step would be to get a scantool on it and see if the scantool indicates the same information that the tach is displaying. If it doesn't, then there is likely a problem with the wiring between the PCM and the instrument cluster or a problem with the instrument cluster. Again though, I would suspect the wiring given the work that was performed.
The tach signal should be 15-25Hz with the engine idling. You would need to backprobe terminal A3 (white wire) at the connector on the back of the cluster to test the tach signal. This wire runs to the PCM connector 2 (red connector) pin 20 (white wire as well) so you can backprobe there as well.
My information is from my 2000 service manual so hopefully the pin numbers and wire colors are correct.
Moderator
Joined: Mar 2009
Posts: 8,453
From: Belleville, Michigan
If the primary, (negative) ignition module or ignition coil circuit was grounded or open, the engine would not start or run. Usually tachometers operate off of ignition pulse at the ignition coil primary terminal, but not on this vehicle
The CKS, (crankshaft position sensor) generates a pulse when the crankshaft rotates. Among other things, the PCM uses the pulse to calculate RPM, ignition, and injector timing. The PCM sends a modified pulse to the ignition module and the module fires the ignition coil. The PCM also sends engine RPM, (via classII serial data) to the BCM, (body control module) which in turn sends the data, (via classII serial data) to the instrument cluster, and then to the tachometer. The tachometer uses a stepper motor to operate the needle in the tach.
My guess is the tach stepper motor is stuck at full sweep. It sometimes happens when the battery is re-connected. Here's something you can try: Turn the ignition to the RUN position, and with your finger, rap on the lens in front of the tach. If that doesn't work, drive it down a bumpy road.
The camshaft position sensor has no effect on primary, or secondary ignition. You can unplug the sensor and the engine will still start and run just fine. CMP sensor data, along with CKP data, enable the PCM to detect, and accurately identify, cylinder misfire.
The CKS, (crankshaft position sensor) generates a pulse when the crankshaft rotates. Among other things, the PCM uses the pulse to calculate RPM, ignition, and injector timing. The PCM sends a modified pulse to the ignition module and the module fires the ignition coil. The PCM also sends engine RPM, (via classII serial data) to the BCM, (body control module) which in turn sends the data, (via classII serial data) to the instrument cluster, and then to the tachometer. The tachometer uses a stepper motor to operate the needle in the tach. My guess is the tach stepper motor is stuck at full sweep. It sometimes happens when the battery is re-connected. Here's something you can try: Turn the ignition to the RUN position, and with your finger, rap on the lens in front of the tach. If that doesn't work, drive it down a bumpy road.
The camshaft position sensor has no effect on primary, or secondary ignition. You can unplug the sensor and the engine will still start and run just fine. CMP sensor data, along with CKP data, enable the PCM to detect, and accurately identify, cylinder misfire.
New Member
Joined: Nov 2014
Posts: 61
From: Imperial, MO
On our Blazer both the Tachometer & Speedometer pegged out when I reconnected the battery once (and I did reconnect the (+) side 1st then the (-) side). I disconnected the battery & reconnected it again. The Tachometer reset but not the Speedometer. I was able to reset the speedometer by fishing a piece of weed-wacker string through the odometer trip reset hole and push the speedometer dial back to "0" ( actually it fell back after I reached the top center of the gauge). I don't know if this is the preferred method but it worked for me. 
If the primary, (negative) ignition module or ignition coil circuit was grounded or open, the engine would not start or run. Usually tachometers operate off of ignition pulse at the ignition coil primary terminal, but not on this vehicle The CKS, (crankshaft position sensor) generates a pulse when the crankshaft rotates. Among other things, the PCM uses the pulse to calculate RPM, ignition, and injector timing. The PCM sends a modified pulse to the ignition module and the module fires the ignition coil. The PCM also sends engine RPM, (via classII serial data) to the BCM, (body control module) which in turn sends the data, (via classII serial data) to the instrument cluster, and then to the tachometer. The tachometer uses a stepper motor to operate the needle in the tach.
My guess is the tach stepper motor is stuck at full sweep. It sometimes happens when the battery is re-connected. Here's something you can try: Turn the ignition to the RUN position, and with your finger, rap on the lens in front of the tach. If that doesn't work, drive it down a bumpy road.
The camshaft position sensor has no effect on primary, or secondary ignition. You can unplug the sensor and the engine will still start and run just fine. CMP sensor data, along with CKP data, enable the PCM to detect, and accurately identify, cylinder misfire.
The CKS, (crankshaft position sensor) generates a pulse when the crankshaft rotates. Among other things, the PCM uses the pulse to calculate RPM, ignition, and injector timing. The PCM sends a modified pulse to the ignition module and the module fires the ignition coil. The PCM also sends engine RPM, (via classII serial data) to the BCM, (body control module) which in turn sends the data, (via classII serial data) to the instrument cluster, and then to the tachometer. The tachometer uses a stepper motor to operate the needle in the tach. My guess is the tach stepper motor is stuck at full sweep. It sometimes happens when the battery is re-connected. Here's something you can try: Turn the ignition to the RUN position, and with your finger, rap on the lens in front of the tach. If that doesn't work, drive it down a bumpy road.
The camshaft position sensor has no effect on primary, or secondary ignition. You can unplug the sensor and the engine will still start and run just fine. CMP sensor data, along with CKP data, enable the PCM to detect, and accurately identify, cylinder misfire.
Moderator
Joined: Mar 2009
Posts: 8,453
From: Belleville, Michigan
I stand corrected. Thank you. The tachometer is in fact driven by the PCM on circuit 121, (white wire). The circuit is dedicated so that the PCM can control the tachometer stepper motor. The circuitry is the same from 1997 through 2005.
The negative terminal on the ignition coil is a white wire with a black tracer. It comes from the ignition module with no other circuits spliced to it. This is the circuit that a conventional tachometer would be connected to.
If circuit 121 is open or grounded, the tachometer needle will not move. I'm still stickin' to my "guess" that a voltage spike, such as reconnecting the battery, caused the needle to peg out
The entire instrument cluster is accessed on a scanner from the BCM menu "instrument cluster functional tests". It includes gauge sweeps for all of the instruments. I knew the tach uses a stepper motor, and from that, figured the BCM controlled it. I learned something new, thank you.
The negative terminal on the ignition coil is a white wire with a black tracer. It comes from the ignition module with no other circuits spliced to it. This is the circuit that a conventional tachometer would be connected to.
If circuit 121 is open or grounded, the tachometer needle will not move. I'm still stickin' to my "guess" that a voltage spike, such as reconnecting the battery, caused the needle to peg out
The entire instrument cluster is accessed on a scanner from the BCM menu "instrument cluster functional tests". It includes gauge sweeps for all of the instruments. I knew the tach uses a stepper motor, and from that, figured the BCM controlled it. I learned something new, thank you.
Not a problem at all. I put that caveat in my post about the info coming from my 2000 FSM because I wasn't sure if it was the same for the OPs 2002 model. I was asking solely for my own information to make sure I didn't detail the same information again for someone with a newer model that may have been different in the end. I used to have access to all of the info in the Mitchell1 system, but long since let that subscription lapse.