Hi Forum, who can tell me which GM component is responsible for the engine spark advance.

Is the ICM responsible or the Engine Controller responsible for the spark advance.


BR
Bob

 

On 1996 and newer, the crankshaft position sensor generates a pulse and sends it to the PCM. The PCM calculates what ignition timing should be, and sends a pulse to the ignition control module. The ignition control module sends a pulse to the ignition coil and the coil fires. Ignition timing is not adjustable. If the timing cover or crankshaft position sensor is moved, removed, replaced, or disturbed in any way, the crankshaft position sensor relearn must be performed with a scan tool capable of initiating it. After the relearn is performed, ignition timing and injector timing, are correct.

 

Thank you Captian,
maybe you can help me. If the ICM is not responsible for spark advance,
do you know a module which advances the spark by 24 degrees.

Similar to the Mercruiser Thunderbolt IV ignition.

Thank you

BR
Bob

 

The ignition control module does not modify the advance on EDI systems, (like it did on HEI systems). The ICM is simply a switching device for the ignition coil. The PCM determines advance and sends a pulse, (with the advance already applied) to the ICM.

Where are you coming up with the 24 degree figure? If you're watching the scan tool under "ignition timing advance" you are seeing what the PCM is applying to the ICM to fire the ignition coil. Engine at idle, operating in cell 17, ignition timing should be around 18 degrees. It will vary a couple of degrees, but should never approach 24 at idle.

 

I would like to find a replacement for a mechanical certifagual spark
advance which I can use as a backup system for my GM 350 boat motor.
It uses a module which has a 24° spark ADVANCE INBUILD:

The unit is $350 and that is way too much.

In the old carb times and contactless dizzys, wasn´t the ICM on the market with build in spark advance????

Attached some examples out of the marine service manual.

Attache the 350 spark advance curve.

Thanks captain.

BR
Bob

 

On the older HEI distributors, the pick up coil was mounted to a plate that rotated when the vacuum advance diaphragm pulled on it. As the plate rotated, it caused the pick up coil to send the pulse sooner to the ICM, which increased ignition timing. On the EDI system, the CKP takes the place of the pick up coil, and it is stationary. The ICM is just a switching device, a low current relay of sorts. It does what it's commanded to do and makes no decisions on it's own.

 

Thank you Captian Hook for the explanations.
So I guess I have to purchase a Adjustable Spark Timing Control unit. I thought I could avoid this somehow. Wrong thought.

Hook, I am living in Germany. That why my posts are always places on weird times.
Chevy Blazers are rare species on the road. I am running my car with an Landi Renzo LPG sequential system, to make is affordable. (fuel wise) LPG is 50% off regular gas prices.

You must be a GM guy with you detailed experience.


My other question is about AC. My AC stopped working after refilling it. It was complete out of R134a. While we evacuate the system the filling masching could not identify a leak. (Underpressure) I have searched all lines with black light. No evidence for a leak.

I was not able to see in the unit behind the dryer. What is your experience tells you. I do see behind the some reflecting items but it hard to identify what is aluminum oxidization & a leakage.

The hole must be very small. It takes a month unit it dissapears.


BR
Bob

 

If the spark issue is on a 1997 Blazer, you will need to modify the software in the PCM, or purchase a "performance tuner chip".

A very common place for the A/C to leak, is where the two compressor halves come together. If the compressor mounting bracket is not perfectly flat, or the compressor mounting bolts are not evenly torqued in the proper sequence, the compressor will leak. It is fondly referred to as a "belly leak". It's usually on the bottom of the compressor and difficult to see without removing it. You are checking it correctly: UV dye and a black light is the answer.

 

Captian Hook, i am located in Germany and 4L60E tranmission shops are rare to find. A needle in the heastack.

I did not drove my Blazer very rarely in wintern time. Now it makes trouble in 4th. gear. Normally @50mls it has about 1800 rpm. When I run at this speed suddenly it ´make bang for ms. and you get scared. It looks like the
drivetrain torque power gets intermittent for some ms.

This problem occurs only if I am running in LPG mode.
On gas is runs fine.

If my LPG fuel is providing less torqe than gasoline, can be the TCC gets kicket out for a second.

How can I check this theroem.

BR
Bob

 

Hi Bob, I am not familiar with the modifications that are necessary when using LPG. It might be necessary to reprogram the PCM. Did you ask the company that installed the LPG modification?