1993 512TR starting problem

Oh, you'll wish it was that simple (and you owe me for just having to look at the GD horrible 512TR schematics).

The way it works is a bunch of sensors feed information to each Motronic ECU during starter motor cranking which, if it decides it's happy, then sends a signal to close a relay that then supplies +12V power back to the Motronic ECU and to run the injectors of that bank. Have you tried looking at the flashing "Morse Code" error codes by using the diagnostic pushbuttons at the Motronic ECUs? My initial "open-loop" guess would be that either the rotational sensors on the back of the cam or the rotational sensors on the toothed wheel at the crankshaft snout aren't working for some reason.

 

Don't be fooled, Gary.... Steve reads this stuff at the breakfast table

Is it likely that sensors have failed on both banks? Everything seems duplicated: Crank sensors, cam sensors, pumps, relays, fuses... Or do the sensors on one side act as a master?

Do you have a strange aftermarket immobiliser system wired in? It would have to wired in in an usual fashion to inhibit fuel injection, but not pumps and starter motor.

 

I agree with you that each bank seems to be very "independent" from each other, but we've had very few reports of 512TR running on only one bank.

 

If my calculations are correct, relays Q & U are the happy relays. However, the fuel pump relays are piggybacked off these relays. If you have fuel pressure on both banks, the pumps must be working and so must relays Q & U.

 

As Ian indicated, Q & U are the "happy" relays -- so both would have to have failed if they are they are the cause of both banks not working, and their closing properly is what closes the fuel pump relays (so, I'll agree with Ian, that if you have fuel pressure, they must be working).

Welcome to the evils of modern complexity where just one electrical connection or one little thing failing can muck up the whole system operation. The way the 512TR injection system works is:

Bank 1-6
Relay Q supplies +12V power to the VN (green-black) wire of each 1-6 injector
The other wire of each injector is connected to the 1-6 Motronic ECU, and the Motronic ECU grounds that wire when it wants to fire that injector.

Bank 7-12
Relay U supplies +12V power to the CN (orange-black) wire of each 7-12 injector
The other wire of each injector is connected to the 7-12 Motronic ECU, and the Motronic ECU grounds that wire when it wants to fire that injector.

Consequently, verifying that any of the VN wires of the 1-6 bank injectors is a constant +12V (relative to ground, not the other wire of that injector connector) during starter motor cranking would be a logical early diagnostic step IMO. Likewise for a CN wire of the 7-12 injectors. (Again, since you report having fuel pressure, this is probably OK, but it's easy to check.)

This link will get you Volume 1 of the 512TR WSM:

https://www.dropbox.com/s/rlu7xdh95888yq5/Workshop Manual 512 TR Vol 1.pdf?dl=0

Section C describes the ignition-injection system operation, and the Self-Diagnosis is on page C53 (and the codes for the check engine light flashing readout). I still think this is something you should try to check.

This link will get you Volume 2 of the 512TR WSM:

https://www.dropbox.com/s/f6ulpclu1dfsmdi/Workshop Manual 512 TR Vol 2.pdf?dl=0

The (horrible ) 512TR schematics are in Section L.

This link will get you a correct page L32 with the translations of schematic item number to physical item:

https://www.dropbox.com/s/ynom1r9swaia32j/L32.pdf?dl=0

Since we've had so few reports of a 512TR running on only one bank (and when we do, it's usually something physical, like a dead fuel pump, and not something electrical), my guess is that there is some sort of intelligent electrical connection between the two Motronic ECUs that prevents both from operating when one is electrically unhappy. Don't know that for sure, but, IMO, the evidence suggests that.

 

For easier reading of the wiring diagrams, try my coloured/translated versions based on the originals.

https://www.dropbox.com/t/IXEOvAnMiK4aNHVm

Because they are based on the poorly scanned originals, some wire connections are unknown.

Link will expire in 6 days (unfortunately). I'm too cheap to buy a proper Dropbox account

 

Thank you!

 

Thanks for those diagrams Ian, an absolute lifesaver when it comes to trouble shooting our electrical systems!