Fuel pump not working

[DadevilleBimmer]

so restoring a 1987 BMW convertible and rebuilt engine. So today we tried to start engine and have spark and compression but no fuel. Checked relay and we have 12volts on one side of the coil constant but unable to get 12v across coil and feel that some thing is preventing the ECU from taking pin #3 to ground. We have continuity from the relay to the Pin 3# on the ECU and also have a short on fuse #27 and was wondering if something related to the central locking mechanism shuts off the fuel pump. We are able to jump #30 to #87 on relay connector and fuel pump runs and sends fuel to the rail.
Hope I have explained this well enough, had an EE helping me today and feel like I am in Kindergarten!

Thanks

[superj]

Check your crank sensor and make sure it's not accidentally in the wrong plug as it and the plug next to it by the round diagnostic port above the alternator look the same

[jclausen]

Then you don’t have power at the fuel pump? Depending on the production date split you may have 1 or 2 fuel pumps if the main pump is on the frame rail in the back then you have a pre pump in the fuel tank if not then you just have one in the tank

[DadevilleBimmer]

So the CPS measures 519 and I get 519 at the ECU plug at pins 46 and 47 I read where it is 48 and 49 but my 2 cars have no pin at 48! so trial and error probing got me identical readings at 46 and 47. And shouldn't the pump come on with the switch in the ON position prior to crankshaft turning? we are able to get the pump working by jumping the relay. But the ECU pin #3 is not sending it to ground and energizing the relay across the coil.
looking for what would be preventing that form happening? I do have 2 fuel pumps but again I can get the pump to run cause we have a bad O-ring on Number 1 injector causing leak, amazing how a tiny cut can cause such a mess!

[hugegoat]

So the CPS measures 519 and I get 519 at the ECU plug at pins 46 and 47 I read where it is 48 and 49 but my 2 cars have no pin at 48! so trial and error probing got me identical readings at 46 and 47. And shouldn't the pump come on with the switch in the ON position prior to crankshaft turning? we are able to get the pump working by jumping the relay. But the ECU pin #3 is not sending it to ground and energizing the relay across the coil.
looking for what would be preventing that form happening? I do have 2 fuel pumps but again I can get the pump to run cause we have a bad O-ring on Number 1 injector causing leak, amazing how a tiny cut can cause such a mess!

The pump will only power and turn on when the engine is cranking

check if your fuses/relays are all good, and if your pump is new, or for sure working condition. Also check your fuel filter to make sure it's not plugged.

You said you jumped the relay and it pumps fuel so your issue is either TPS or DME

Check for spark and if you don't get any spark then your issue is the TPS

[superj]

throttle position sensor doesn't effect the pump running. not on an e30

[DadevilleBimmer]

We get fuel when we bypass the relay the pump does not come on unless we jump the relay so we are asking what sensor could be preventing the eco telling the relay to send power to the fuse and eventually the pump. replacing the relays and CPS this weekend and will follow up. thanks for the input

[DadevilleBimmer]

Cleaned relays with wire brush, checked gap on CPS and unplugged and replugged in ECU and voila fuel in the rail! Thought maybe when I checked CPS gap I knocked some metal filings off that allowed it to read the engine turning over? Maybe main relay was too dirty! Idk but it’s working gotta get O ring tomorrow for injector and fire it up! Thanks for the input!🤞🏼