megasquirt - help!

GeX

GeX

GeX

Soldato
Joined
17 Dec 2002
Posts
6,954
Location
Manchester
so.. it's been running fine for 3 months. then it didn't want to start the other night. i blamed the cold start mapping, but then noticed there was no RPM reading and the trigger LEDs on the ECU were not flashing. This indicates a TDC sensor / trigger fault. It started up fine the following morning. Then the next day it was 'iffy' to start but it did fire. Now it won't start at all. It was moved, and then left for around 5 hours and now no RPM reading etc all over again.

Replaced the TDC, same fault. I've checked the cabling, (twin core sheilded) and it is fine. Looks fine, no damage and shows continuity throughout with no cross continuity between cores or sheild (which is earthed). With the multimeter set to AC, connecting it to the TDC sensor (it's a VR type) it generates a small voltage when the engine is cranked over. This voltage is still present at the ECU main plug. So that all appears to be working.

I dont have a scope to check the TDC signal properly for noise, but it *was* working fine and has never had any issues when running.. so i don't know.

Can only suspect the VR circuit on the ECU is at fault. This problem has shown its head in the cold, is there anything on there that might not like the cold. How do i go about testing? I need this car working, as i need to drive 120miles to get home sometime soon!
 
thats what i thought, but i didnt build it. so am a little dubious about poking it with an iron. would like to be able to trace which bits should need poking.. then poke them!


edit: i tried removing the ECU and warming it up a bit first (after threatening to microwave it!). didnt really help :(
 
its a little fiat FIRE 4 cylinder, wasted spark, std 60:2 trigger wheel using the stock VR sensor.
 
There is a small trimmer type potentiometer on the V3 PCB (R56) to set the zero crossing level for the VR sensor, it's possible that vibration could have moved it slightly. Worth giving it a tweak, but make sure you note the original position in case it makes no difference.
 
Dogbreath said:
There is a small trimmer type potentiometer on the V3 PCB (R56) to set the zero crossing level for the VR sensor, it's possible that vibration could have moved it slightly. Worth giving it a tweak, but make sure you note the original position in case it makes no difference.
Click to expand...

thanks, that is similar to what Bill (who built the ECU for me!) has suggested. Car is still stranded at a friends house, so when i get round there next i'll give it a go.
 
sorted now. minor tweak didnt help. but after a call to Bill and further tweaking - it's alive again!! yay.
 
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