Hello members, first the car 1990 740 with 120, 000on the body and 10, 000 on the rebult engine. . Ok now the problem I let the car sit for a few months now it won't start . I have gone through the usual suspects .. the motors got spark it will run with gas put down the intake .. the fuel pump relay is good (swapped it and put old one in my bros 740 works great) I jumped the wires on the board and confirmed that both pump work .. with pumps running it won't run because the injectors aren't coming on and when I plug the pump relay back in the pumps do not run .. and the fuses are all good.. so with that being sead what turns on the power to the relay and injectors? Is it a crank sensor or something ?

Thanks

 

there's a relay under the hood called the 'radio suppression relay', which powers the fuel injectors. its supposed to come on when the 1/2 of the fuel pump relay powers the ECU (fuel injection control unit), as soon as the key is turned on (II). if you unplug an injector, one of the two pins of the plug should be hot (relative to engine ground) when the ignition is on (the other pin is pulsed to ground by the ECU to fire the injector).

on a non-turbo 740, the radio suppression relay should be on the RIGHT fender, somewhere near the radiator overflow bottle (on a turbo, its on the other side).

if you're getting spark, then the crank position sensor is working, this provides the timing pulses to both the ICU (Ignition Control Unit) and ECU (Injection Control Unit)

if you unplug the Radio Suppression relay, and if you can temporarily jumper pins 3 to 4 (red wire to green-red wire), that will act like the relay is working, see if the car starts, if it does, then you got a bad relay. remove that jumper as soon as your done testing as its leaving stuff powered directly from the battery otherwise.

 

Ok I will try that but what about the fact that the pump relay isn't coming on ? Will that not come on if the other relay is bad? Also what tells the pumps to get power? And I forgot to mention the system won't prime either so something is not telling it to power up..

Thanks

 

the ECU tells the fuel pump relay to come on when it sees pulses from the crank position sensor. *SOME* ECU's will also run the pump for 1-2 seconds when the ignition is first turned on.

since you said you get spark when you turn it over, that says the crank position sensor is working.

 

Yes and it will run with fuel poured down the intake ... do you think Ecu is bad ?

 

pop the passenger side kickpanel off, and the one on the outside... the ECU is mounted behind that, inside the fender, forward of the door hinges. get the number off it, last 3 digits is what counts, its a number like 0 280 000-561 thats the -561. early pink label 561 ECU's are suspect. you can use various newer ones, direct drop in.

Engine ECU

 

unless you have one of the suspect ECU's, I'd be more likely to think it was a wiring or connection or ground problem, than a bad ECU.

 

Ok ill look into it... thanks for your help