l28et spark advance doesn't change

[boostin280zx]

I'm trying to figure out why the spark advance won't change when i rev the engine. I have done the l28et swap before but didn't have this problem. I searched and tried a lot of the things in a previous post i found but still no change.

 

I swapped 3 other ecus, changed coil and igniter, took the dizzy off my other car but i didn't get a spark for some reason with that one, TPS sensor works, injectors cycle faster as the rpms climb, checked all grounds.

 

Can anyone think of any other possible causes of this? How much authority does the AFM play in controlling the spark under no load?

 

The car starts on first crank every time and idles and revs fine, just no spark advance or retard.....

[woldson]

Ummm,, vaccume advance?? That is alot of work for you not to have mentioned that. Hopefully that might put you back on the road.

[Six_Shooter]

Ummm,, vaccume advance?? That is alot of work for you not to have mentioned that. Hopefully that might put you back on the road.

 

I didn't think the ET dizzy used vacuum advance.

[jamunm90]

It isn't...The vacuum and mechanical advance is only used on the NA cars. The ECU controls the advance on the turbo. No idea why it wouldn't be working....makes me think it's a crank angle sensor issue, or maybe a bad ECU, but you've checked that. BTW...the air flow meter doesn't control the spark at all. Even at idle, it just gets the mixture close, and the oxy sensor/idle air bypass fine adjusts the idle mixture. After thinking about it more, I'm pretty sure you have a crank angle sensor. Anyone correct me if I'm wrong, but the increase in injector speed as the engine increases is controlled by the tach/ingition pulse, not the crank angle sensor.

[woldson]

Opps, sorry did not see the "t".

[boostin280zx]

How does the ecu know to retard the timing under boost? Is it the amount of airflow through the AFM or does that just let the ecu know to richen up the mixture?

[jamunm90]

Simply put, all the AFM tells the ECU is the amount of air being used by the engine. This is a simple system, and the computer doesn't actually know the system is under boost, but only knows it's using a given qty of air, and so it adjusts the fuel mixture accordingly.

 

Now, the spark curve is part of the same map, and is mapped against other parameters such as Airflow (AFM), Engine RPM, Cylinder Head Temp, Intake Air Temp, Crank Angle Sensor, etc, and then adjusted down a percentage based on the knock sensor. If it senses detonation, it drops the spark curve to a lower curve on the map.

 

So it’s not quite as simple as saying the AFM adjusts the spark. By itself, it doesn’t, but it is an input into the ECU map and is used to calculate the spark/fuel curve.

 

So, a bad AFM won’t cause the spark advance problem you are seeing. If the AFM is bad or going bad, it also affects the fuel flow, and that will be very noticeable. The engine will hesitate and actually cut out when driving down the road. Or if it’s completely gone, the car won’t start. It’s a simple device with a air flap and a electric meter/sliding contact, and when that begins to fail, it starts to cut off the fuel completely during normal driving..

 

The Crank angle sensor is used by the same map to tell the computer exactly where the crank is, and uses this info to determine when to fire the spark. My thinking is that it is not functioning, and you are getting the idle spark advance all of the time, so the spark timing is not moving as the RPM's increase.

[boostin280zx]

I don't know what would be wrong with the CAS.

 

I took the dizzy off with the harness still pluged up and powered up the car. I slowly turned the rotor and it fired the coil 6 times and the injectors clicked every full turn.

[woldson]

Simply put, all the AFM tells the ECU is the amount of air being used by the engine. This is a simple system, and the computer doesn't actually know the system is under boost, but only knows it's using a given qty of air, and so it adjusts the fuel mixture accordingly.

 

Now, the spark curve is part of the same map, and is mapped against other parameters such as Airflow (AFM), Engine RPM, Cylinder Head Temp, Intake Air Temp, Crank Angle Sensor, etc, and then adjusted down a percentage based on the knock sensor. If it senses detonation, it drops the spark curve to a lower curve on the map.

 

So it’s not quite as simple as saying the AFM adjusts the spark. By itself, it doesn’t, but it is an input into the ECU map and is used to calculate the spark/fuel curve.

 

So, a bad AFM won’t cause the spark advance problem you are seeing. If the AFM is bad or going bad, it also affects the fuel flow, and that will be very noticeable. The engine will hesitate and actually cut out when driving down the road. Or if it’s completely gone, the car won’t start. It’s a simple device with a air flap and a electric meter/sliding contact, and when that begins to fail, it starts to cut off the fuel completely during normal driving..

 

The Crank angle sensor is used by the same map to tell the computer exactly where the crank is, and uses this info to determine when to fire the spark. My thinking is that it is not functioning, and you are getting the idle spark advance all of the time, so the spark timing is not moving as the RPM's increase.

 

Dang dude!! Nice write up!!!!!!

Well put and very concise:)

[jamunm90]

Remember that when you did your test, you were only checking that it sends a spark signal to the ECU. That we already know.

 

Unfortunately at this point, you're getting to the point of finding the issue by elimination.

 

I'd swap out the CAS with a known good unit. If it still does not time correctly, then you've eliminated one major component and you move on to the ECU. Since this is a likely suspect, you can't move on in your troubleshooting until you've eliminated it. Start with the CAS, then do the ECU, then check all of the values of your sensors, etc. This is easily done with a volt meter and is best done at the ECU connector to also identify shorts and grounds.

 

Also, and it's a big also, I've been going on the assumption that all of your wiring is correct, all of your other sensors are hooked up and they are functioning normally. It's possible you have a wiring issue or possibly a ground someplace. If both the ECU and CAS check out, check all of the other sensors. If they check out, then you likely have a sensor or sensor wire that’s grounded or shorted somewhere in the ECU wiring. Grounds and shorts will create back circuits and do all kinds of weird things. I know, this is a lot of time consuming grunt work and it sucks, but once you're eliminated the above, that's what you're left with.