How does electronic timing advance work on a TBI distributor?

How does the computer timing advance work on a TBI? I understand how the vacuum advance works on the old HEI distributors. For the life of me I do not understand how the electronic advance works on the TBI distributors. The rotor is on the shaft and points to fixed pickup on the cap. I do not see any parts inside that can move relative to each other to advance the timing. I know its a dumb question, I just wanted to know. Currently, I just have faith it works.

Coil trigger timing changes

The main spark table is based on MAP and RPM, so similar to a carb'd engine with vacuum and mechanical advance. But it has modifiers for coolant temp as well. If it had an IAT, which speed density does, then there'd be modifiers there as well. But for some reason I have it in my head these TBI engines got by without.

The pickup coil sends a signal to the module 90 crankshaft degrees advanced. The module recalculates the pickup coil signal--retarding it so that the spark occurs at a reasonable time when the crankshaft RPM is below 400. This gets the engine running. Above 400 rpm, the module sends the 90 degree advanced signal to the computer, which electronically delays the spark signal back to the module according to it's programming--a full 90 degrees delay so it arrives at TDC-Compression when the set timing connector is unplugged, or somewhat less than 90 degrees of delay (resulting in varying amounts of spark advance) depending on RPM, load, etc.

Example:
If the pickup coil signal is generated 90 degrees advanced, then the computer delays that signal to the module by 60 degrees, the actual ignition timing would be 30 degrees advanced by the time the spark hits the spark plug.

The knock sensor system can modify (override the normal computer-generated spark advance) the spark signal even more, which retards the spark at the spark plug even more, to combat detonation.

So, yeah...Pure Fookin' Magic: PFM.

Schurkey said:

Pure Fookin' Magic: PFM.

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That's my line. ha ha.

I knew I stole it from someone on this site. Just couldn't remember who.

Thanks for the reminder.

yevgenievich said:

Coil trigger timing changes

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So I have a question regarding my vehicle I recently swapped the engine with another running one and the issue I'm having is that my ECM will not send 5v to the ICM in order to allow it to advance I have base timing set to zero and I can start the truck and limp it around but without timing advance I know it isn't safe to drive every sensor is brand new besides the knock sensor which it starts and idles albeit roughly but with or without the knock sensor plugged in I was hoping someone can tell me what tells the ECM to send that 5v reference Google has been no help and I'm new to these forums so I'm not even sure if I'm describing the problem correctly I've poured hundreds of hours into research but electrical is where I'm really weak at so I don't know what I need to be checking

MANIAC said:

I was hoping someone can tell me what tells the ECM to send that 5v

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It s/b hot with key on. Is your EST bypass wire connected back up after setting it to 0°? This drawing may not be exactly like yours but, you can look in the FSM for your truck and see which terminal (D5 in this example) at the ECM/PCM feeds that 5V.

In all your research did you read the chapter 3 , ECM operation, control. In FSM CK-2, Driveability and Emissions. It breaks down all the input data , who does what , how it's controlled. A bit of math and magic.

GoToGuy said:

In all your research did you read the chapter 3 , ECM operation, control. In FSM CK-2, Driveability and Emissions. It breaks down all the input data , who does what , how it's controlled. A bit of math and magic.

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I haven't seen anything like that but I do still struggle to comprehend electrical diag but what I have done is a vortec swap and yes I know I need a custom prom burnt for it to run properly but I'm just trying to figure out why I have no timing advance either way I deleted the EGR valve and purge solenoid ac compressor is not wired up because I have a busted condenser I can't afford to fix yet I have replaced the ECM with a ems7427 plug n play unit I even tested the ppl/wht wire and tan/blk for breaks through continuity test and those checked out fine I don't know what all sensors are on the same circuit as the ICM but I figure thats probably where I need to start but I also have no communication from the OBD1 port i can't get the truck to go into diag mode where it will flash the codes assuming it still has any due to me disconnecting the battery a few times