Strange Motronic electronic problem

[AR4me]

Here is one that has me puzzled!

This is the 164S engine that has been converted and running in the Milano race car for quite some time now.

I took it for a test day at Buttonwillow since the AlfaCup race this weekend was cancelled. The car was running without issues up until today, unloaded it from the trailer at the track this morning, fired it up and let it warm up in the pit. After getting suited up, getting in the car, I drove towards the pitlane to enter the track. As I was driving at constant speed of 5 MPH down the pitlane the engine stalled. Huh? Check around, the fuse for the ECU blown Call pits on intercom from car to bring fuse, blows again. Roll car back to the pits. Start looking around and quickly identify that if I pull the plug on the ECU, nothing blows. Also, if I pull EITHER of the two Motronic relays (with the ECU plugged back in), the fuse doesn't blow, but if I leave both in, the fuse blows

It behaves similarly even if I disconnect the wiring for the fuel pumps (got dual fuel pumps since it is a race car, but that is another story) up front.

What is your take? Where do I start? I'm suspecting the ECU developed a short. The car was moving straight at 5 MPH without any particular inputs when it blew the fuse - this out of nowhere seems to indicate a component failure and not a wire shorting out (I could see that happen if I had monkeyed around and then it blow fuses, or if there has been some mechanical input, or, ...). Furthermore, it is kind of odd that removing either relay will prevent the fuse from blowing. It is suspect since both relays are related to the ECU...???

Any ideas? I could try track down an S ECU to borrow. Any thought whether that is a likely failure? Can I measure on the ECU whether an input power pin has been grounded by an internal ECU failure. Note that the fuse blows as a turn the starter to start the car, i.e. when the ECU powers up, I suppose.

Did I mention that we had the entire Buttonwillow track practically to ourselves, but couldn't get a single lap of testing in Very disappointing, particular since we had advanced data-logging ready to assist.

Jes

[Alfisto Steve]

Quote:

Originally Posted by AR4me Here is one that has me puzzled! This is the 164S engine that has been converted and running in the Milano race car for quite some time now. I took it for a test day at Buttonwillow since the AlfaCup race this weekend was cancelled. The car was running without issues up until today, unloaded it from the trailer at the track this morning, fired it up and let it warm up in the pit. After getting suited up, getting in the car, I drove towards the pitlane to enter the track. As I was driving at constant speed of 5 MPH down the pitlane the engine stalled. Huh? Check around, the fuse for the ECU blown Call pits on intercom from car to bring fuse, blows again. Roll car back to the pits. Start looking around and quickly identify that if I pull the plug on the ECU, nothing blows. Also, if I pull EITHER of the two Motronic relays (with the ECU plugged back in), the fuse doesn't blow, but if I leave both in, the fuse blows It behaves similarly even if I disconnect the wiring for the fuel pumps (got dual fuel pumps since it is a race car, but that is another story) up front. What is your take? Where do I start? I'm suspecting the ECU developed a short. The car was moving straight at 5 MPH without any particular inputs when it blew the fuse - this out of nowhere seems to indicate a component failure and not a wire shorting out (I could see that happen if I had monkeyed around and then it blow fuses, or if there has been some mechanical input, or, ...). Furthermore, it is kind of odd that removing either relay will prevent the fuse from blowing. It is suspect since both relays are related to the ECU...??? Any ideas? I could try track down an S ECU to borrow. Any thought whether that is a likely failure? Can I measure on the ECU whether an input power pin has been grounded by an internal ECU failure. Note that the fuse blows as a turn the starter to start the car, i.e. when the ECU powers up, I suppose. Did I mention that we had the entire Buttonwillow track practically to ourselves, but couldn't get a single lap of testing in Very disappointing, particular since we had advanced data-logging ready to assist. Jes

[AR4me]

Thanks for the suggestions.

I'll look at the O2 sensor wiring and report back. Though, just last weekend I actually moved the O2 wiring further from the header and downpipes. Anyway, I will check.

Yes, 164 engine harness. The fuel pump output from the 164 double relays is only used to trigger two other relays with their own heavy duty wiring+power and fuses for the dual pumps - did not want to draw 26 amp through existing fuel pump wiring (which was only dimensioned for 1 pump). BTW, this setup has been running on the car for a long time with the 164 engine. Also, I did try disconnect the fuel pumps - no difference. Additionally, I have cockpit switches to manually turn-off fuel pumps regardless of motronic ECU output (they only turn on if motronic ECU AND manual swithe(s) "say go").

With ignition off and 164 fuel pump relay out I did try measure into the relay socket for any shorts to ground in the wiring for that relay, but nothing...

Jes

[Alfisto Steve]

Another wire off that same fuse is blue-red wire off pin 86 of red stripe relay which is for carbon cannister evaporation solenoid valve. I doubt you hav it hooked up to anything but it could be shorting to something maybe. That pin is also where diode is located in red stripe relay. Do you have another red stripe diode relay to try? 164 starter relay is the same red stripe relay if you have that in your parts stash.

[AR4me]

Steve,

You are the man! You nailed it the first time - the O2 sensor wiring. I disconnected the Bosch plug and the single wire for the O2 sensor, and it doesn't blow the fuse. Reconnect it, and flash, fuse burned. Interestingly, the tail does not look melted, but somehow it is shortening. I may pull it and see if I can repair it. Though, the O2 sensor is the one off the wrecked donor 164S and cold have years and miles on it. Will any 3-wire Bosch O2 sensor work?

Steve, I still have many 164 parts. Let me know if there are any particular parts you may be low on, and it is yours as thank you. I have exterior door handles as well as 3 of the 4 interior door handles (if I recall correctly), airbag, airbag ECU, ...

Jes

[Alfisto Steve]

Quote:

Originally Posted by AR4me Steve, You are the man! You nailed it the first time - the O2 sensor wiring. I disconnected the Bosch plug and the single wire for the O2 sensor, and it doesn't blow the fuse. Reconnect it, and flash, fuse burned. Interestingly, the tail does not look melted, but somehow it is shortening. I may pull it and see if I can repair it. Though, the O2 sensor is the one off the wrecked donor 164S and cold have years and miles on it. Will any 3-wire Bosch O2 sensor work? Steve, I still have many 164 parts. Let me know if there are any particular parts you may be low on, and it is yours as thank you. I have exterior door handles as well as 3 of the 4 interior door handles (if I recall correctly), airbag, airbag ECU, ... Jes

[AR4me]

Yup.

Turned out that the O2 sensor I used off the wrecked 164S had been spliced into the harness at some point. It had not been done properly and now shorted.

I found a generic 3-wire Bosch O2 sensor locally, soldered it into the tail with the 164 plugs, and the car runs again

Jes