No RPM when running but have when starting
 

Hi new to this so here goes. I have a 2007 HHR and I have put 2 new ECM's in and new crank sensor from dealership, a new cam sensor from dealership. When you first install crank sensor and go to start motor you will see RPM on tach and on a scanner until motor starts then RPM's go away on scanner and tach. I have 5 volts to sensor, I have a good ground to sensor and have with a volt meter a good 5 milli volt signal from sensor. The car will code 336 I have checked all wires from the sensor to the ECM and all show good all the way to the pin in the ECM connector.I have had a GM tech try to relearn crank sensor but can't cause it shows no signal. Car runs great but get tired of seeing that yellow check engine light. Anybody else out there had anything like this. To add to this checked all wires from crank sensor connector to ECM connector and all read good and are in good shape. I know the importance of the grounds would like to know which ones to check and their locations I don't use chassis ground when I checked the 5 volt ref on the green wire I used the purple wire as the common and the yellow wire was my signal from the crank sensor

What engine?

Did you check the other side of the ECM connector? I.E. the actual pins.

The signal is in pulses, not milivolts.

Technically speaking the crankshaft position sensor has 5Vref, low ref and signal. You need continuity for all 3 to the ECM, not to chassis ground.

Are you sure that that is where the tachometer gets it's signal? You have low speed LAN and High speed LAN involved.


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Welcome.

You say the code is 336 - I gather you mean the downloaded code from the OBDII system - what is the whole code including the alpha character at the start?

I was assuming P0336 which means the crankshaft sensor is malfunctioning.


Which pretty much narrows it down to one of the 3 wires is loose. Or somebody thought low ref and ground are identical in a digital circuit and re routed the lowref to chassis ground.


I'm not sure how there could be RPM displayed when the motor is not running, perhaps 2 wires are touching.

I read it as he is getting RPM while cranking but once it catches and starts it goes away.

Yes that is correct I get the RPM when Cranking the motor and it will go away right before the motor will start.

I put a new sensor in and see RPM till it starts then it goes away I can change out the sensor with another I see RPM till motor starts. I can put the sensor that I just took out back in and see RPM till it starts. In both sensors I see RPM but once the motor is started you will not see RPM again. Now when I change out the sensor I have been disconnecting the battery so to take the starter off to make changing the sensor easier. I can once motor has ran and got the PO336 code I can clear it and still no RPM until I change sensor.

From the other post did you check "low ref" wire to make sure it's not grounded anywhere?

It is Pzero336! I am guessing, this is a 2.4L engine? When did this start/what was done immediately before, whether you think it could be related or not? Any splicing, LED installs, power thievery, after market installations etc?



I don't see anything in the wiring diagram that has any connection to running or not. If there was a short in the ref circuits that should show in another code.


There are only 3 wires and they all go directly to the ECM, you have changed ECM and presumably had them programmed to your VIN, so ruling that out. You have replaced crank sensor, so rule that out.



The 5Vref is shared with the throttle body(GY wire) and the accelerator pedal position sensor (WH/BK wire). So you should inspect those connections. If it was any of them there would be codes if not poor running. I fact, there is a code for a bad 5Vref.


The accelerator position sensor has 2 different 5Vref circuits, I guess it is possible for some sort of shorting there.

Any possible damage to the reluctor ring, the crank position sensor wiring , per

https://www.obd-codes.com/p0336

I have been avoiding thinking about the reluctor ring, I don't know how it could get damaged, it is integral to the crankshaft.

I do remember somebody in the waybac had a crank sensor self destruct, leaving 30 feet of wire hanging around inside the engine.


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i am not so sure that the cranshaft position sensor has anything to do with the tachometer.