Can a ref sensor be bad if Motec diagnostics says OK?

[stownsen914]

I have an older M4 on a Porsche 968 racecar, using a 52002 mag sensor for REF. I am pretty sure that for SYNC the stock Hall sensor is being used.

I am trying to start this engine after the car was completely disassembled after an accident and then sat for many years. Engine is freshly rebuilt. When attempting to start, all I get is belching out the exhaust, no real firing. Leads me to believe that something fundamental is wrong with ignition timing, REF, or SYNC. I've checked a few things to troubleshoot (more to do, I realize, but this is what I've checked so far):
- Motec diagnostics say OK for SYNC and REF (and everything else, save for an air temp sensor that is temporarily disconnected)
- Ignition timing appears to be correct on cyl #1. I verified using Motec Crip test screen setting while connected to the car, while cranking with sparkplugs removed. I used a timing light and TDC for cyl #1 mark on flywheel to verify.
- Fuel pressure appears to be correct (injectors are rebuilt)
- I smell light gas smell in intake stacks. I didn't think it was flooded. No real gas smell in exhaust.
- I detected spark on cyls #1 and 2 using the timing light

I am wondering about the 52002 mag REF sensor. It is set to .015" air gap per the instructions. Can it be damaged/bad even if the diagnostics say OK?

Thank you.

Scott

[David Ferguson]

I think you really need to use an oscilloscope to look at the ref signal.

Later ECU software has a facility to do this without needing a scope (Ref/Sync Capture).

[Stephen Dean]

The ECU will only fire the injectors and coils if it is synchronised, so from what you are saying, the ECU is reading the sensors correctly, and is OK with the information coming from them.

When the vehicle was being repaired, was any work done on the wiring loom? It is possible to have the wires for the Mag Sensor swapped and is still work, the difference will be that it will swap the direction that the edges go. If this is the case, then the ECU will sync up, but be firing in the wrong place.

You need to get a scope on the outputs for the Ref and Sync Sensors and confirm that the edges being generated match that set in the configuration file.

[stownsen914]

Thank you David and Stephen for your input. I notice that both of you suggest getting an oscilloscope to check the signal from the mag sensor. I don't currently have one or access to one. I've seen some cheap ones (<$50) on Amazon and eBay. Would something like that be sufficient to test a mag sensor like the 52002?

To answer Stephen's question about the wire harness, it was removed from the car and stored. It appears to be a nice, professionally built one that is largely contained under the dash. I checked it carefully while it was out of the car and cleaned all the contacts before re-installing it in the car. To the comment about the sensor switching edges, is this related to the rising/falling setting in the Motec software? (It is set to falling edge for my case.) And to confirm, can this be off even if I get a clear confirmation of TDC on cyl #1 while cranking and using a timing light on a TDC mark on the flywheel while in the Crip test screen + "OK" from Motec diagnostics?

Scott

[greenamex2]

Thank you David and Stephen for your input. I notice that both of you suggest getting an oscilloscope to check the signal from the mag sensor. I don't currently have one or access to one. I've seen some cheap ones (<$50) on Amazon and eBay. Would something like that be sufficient to test a mag sensor like the 52002?

Scott

I use a really old version of one of these...

https://www.picotech.com/oscilloscope/2 ... 0-overview

Personally I would probably go for a second hand picoscope rather than a no brand chinese thing.

[stownsen914]

So another question, re: syncing and Crip. I've read in a few places that if the ignition timing looks good but the car won't run, the Crip can be off by 360 degrees. This would be a little surprising in my case since the car ran previously with pretty much the same setup as it has now, but now that I think about it, this could match the symptoms I'm getting - it won't start, but belches out the intake and exhaust.

Is there anything in the Motec software, or that I can check, to know which cycle the Motec thinks the engine is in while cranking? It's wasted spark, so I guess I can't check spark to verify this. Or is the only way to just adjust the Crip by 360 and see if that works?

Scott