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No love just yet. Found and cleaned all the grounds. At first all was good on initial drive but as soon as I got out of my 1/4 mile long driveway and got on the main rd it started bouncing, then about 2 miles later it was perfect for another 3-4 miles. When I was stopped and I’d,ing it started acting up again and continued to do so the whole way home. So…not sure what’s up with it.
When I first got the car the battery had just been replaced about 200 miles prior. After I fixed pigtail on negative post I assumed pigtail is what fixed it but maybe the fact that it just came off a 200 mile road trip and slightly charged the new battery is what fixed it then and now that it sat for a week in single digit temperatures the charge is simply low. I guess if that’s true it will cute itself out shortly.
Also I found this note from a service bulletins from GM:
PIC6299A: Intermittent Tachometer Fluctuation While Driving
Some customers may complain that the tachometer has erratic operation or that it flares between 1,000 - 7,000 rpm. The engine RPM's will remain constant and drivability is not affected during this concern. This is strictly a gauge issue and is in no way related to engine performance. This condition is not dependent on engine speed, vehicle speed, or the current gear in which the vehicle is operated in. Technicians will also notice that with the scan tool, the Engine RPM parameter in the Instrument Panel Cluster (IPC) and Head Up Display (HUD) will remain constant, even when the concern is happening.
The tach signal that is displayed by the IPC / HUD on these cars originates from the Camshaft Position Sensor (CMP). This sensor reports to the Engine Control Module (ECM). The ECM then sends a signal out on a dedicated Replicated Camshaft Signal circuit, directly to the IPC. The IPC interprets this signal and displays an accurate engine speed signal. A problem may arise when Electromagnetic Interference (EMI) makes its way into this circuit. This will cause the IPC to interpret phantom engine speed counts that are not actually present. As a result, the IPC / HUD tachometer reading may appear to be jumping wildly or acting very erratic. The interference can be caused by any aftermarket accessories, depending on what it may be, and where the wiring harness is installed in the vehicle. Some other known sources of interference are the anti lock brake solenoids and the engine active fuel management actuators. If this Replicated Camshaft Signal circuit has been routed incorrectly and now passes by one of these components, this may induce interference into the circuit and cause the concern. Another common cause of this symptom would be any number of poor grounds on the vehicle.
Depending on previous service work, some technicians may notice that the concern is no longer present after a new IPC has been installed into the vehicle. This will be true as long as the new cluster had not been SPS programmed yet. The base software in these clusters allows the IPC to read the tachometer signal from the ECM over serial data. Not until the cluster is SPS programmed does it look to the Replicated Camshaft Signal for the tach signal. In similar fashion, the scan tool is NOT obtaining the engine RPM signal from this Replicated Camshaft Signal circuit. It takes the information from serial data. This is why the concern may be present and the scan tool reads a consistent, accurate, engine RPM reading. This may lead a technician to believe that the IPC is faulty and require replacement.
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So it appears magnetic interference could be causing it too. This car is modded with an alky controller etc. Could be something with that too I guess. Just have to see how this all plays out. A big FU to GM for a sub-par design.
I even cleaned up the positive side connections at the fuse block at the battery and engine bay. I can’t remember if I cleaned the other end of the battery ground cable where it attaches to the frame. When I say cleaned I mean green scotch brite pad, brass wire brush and dielectric greased them. I can’t remember what I did last but it has not returned for almost a year now. If you have aftermarket stuff wired in I’d disco that also….
Bumping back up. Still got this going on. All grounds were located and all were secure but I re-seated them anyways. Hoping someone posts with more info.
ECM ground is G100. I doubt that ground is an issue since you would have more serious problems than the tach bouncing. Here is where G100 is located:
The IP and HUD ground is G202 which is located at the bottom of the passenger side A-pillar (NOT the left A-Pillar). Here are the schematics that probably should be considered.
If the inline connection X100 Pin 5 is not clean and tight the replicated camshaft signal could be intermittent and cause the tach and HUD rpm display to jump. X100 isn't an easy connector to work with as it is located here:
However, that is probably not as high of a likelihood of failing as a ground.
Now back to grounds. The ground schematic looks like this:
Notice the IP and HUD grounds are spliced together at J238 and then routed to ground point G202. The problem is G202 is on the passenger side of the car which doesn't seem correct since the IP and HUD are much closer to the driver's side grounds.
This is what the back of the IP looks like when you are observing it from the front of the car. Notice J238 and how it's routed:
That is a large harness passing from the IP area to the passenger side of the car. Not sure why they did this but it could have something to do with a single point grounds for circuits that function together.
So the G202 ground point is here:
AND NOT HERE!!!!!
The first thing to do is clean the grounds for the IP and HUD. Then maybe consider the signal from the ECM to the IP. If the ground wire splices at J238 are not making good contact then you are looking at a big mess trying to repair/replace the harness. The ground for the ECM is the least likely problem area since the car would probably barely run if that ground is screwed up.
The HUD gets its signal from the IP and you say that both are doing the same thing which means the IP is sending the wrong data to the HUD so the serial data bus from the IP to the HUD has screwed up data in the tach data frame.
Wow, Bill! Thanks for that solid information! Those two grounds at the passenger footwell I held in my hand and shined them up pretty. Most valuable to me is that (of course) hard to get to section on the passenger side …that X100. That almost looks like a connector for the bottom of the fuse box or something. I’m going to get the car on my lift and have a look at that. The car does have headers so I’d be interested to see this connector a little better and maybe even a reseat.
Wow, Bill! Thanks for that solid information! Those two grounds at the passenger footwell I held in my hand and shined them up pretty. Most valuable to me is that (of course) hard to get to section on the passenger side …that X100. That almost looks like a connector for the bottom of the fuse box or something. I’m going to get the car on my lift and have a look at that. The car does have headers so I’d be interested to see this connector a little better and maybe even a reseat.
where you able to source the issue down? I’m having the exact same issue.
