Hi folks. Glad to be accepted here. I have a 1989 740 turbo wagon. It is in great shape and I am now a big fan. It does have one wee problem. It will not start. History: When I purchased this car, it would not start. Batt was bad, old fuel... the usual for a car that sat. I put in a fresh battery, drained the old fuel, put in fresh. Turned it over and nudding. It did fire up one day after a lot of farting around with elect. and fuel system. I never did come up with the solution. It ran for about 2 months then one day when I shut it off, it refused to fire up. I read where the fuel pumps sometimes lock up so I tapped it and it (after a while) fired up again. This happened 3 or 4 times (would not start) so that I could not rely on the car. I stored it for over a year with other pressing projects taking my time. I now am back and determined to sort this out. So far, I have removed the pump/filter assbly and proven it on the bench. (I previously checked motor for spark, ok) I used jumpers to remove the last fuel from the tank, via the injector rail input line. Seemed to pump with lots of volume and pressure. I removed the injector rail with injectors intact, connected the grounds to the manifold and spun over the motor. Not one injector fired. (fuel pump runs when motor is spun over) I removed the harness to the air mass sensor and heard the pump running up to pressure. I have checked the temp sensor via the harness at the ECU and it was about 1K Ohm over the 20°C listed reading... should not effect starting. I checked the Air mass sensor as per Chilton's repair manual #7040 and found the sensor wire cct. (pins 6 & 7 of the ECU harness) was correct. The test on pins 6 & 14 showed open. (says it should be between 0 and 1K Ohm) I picked up a used ECU and swapped it with the original, no change. I swapped the air mass sensor from an 89 240 GLE.. not exactly same but thought I would try it, no change. Now, I suspect the the ECU will NOT fire the injectors if the air mass sensor does not send it the correct information. I also suspect that the pin 6 & 14 shows the health of the embedded components in that sensor module. Now, it is very difficult to determine what effect incorrect test readings will have on the output of the ECU. I can not find any information on the input/output functions of the ECU (Jetronic 2.2 ECU). I know it is an electronic control unit but really, I don't think that it contains Dilithium Xtls and a matter/anti-matter proton reducer valve system !! "no user serviceable components inside" ! There is a lot of information on how things are supposed to work, but little information on proof positive elimination of components or their possible effect on the ECU. Has anyone figured out this prob. yet ? I see this starting problem popping up a lot, with this particular vehicle. Any food for though would be gratefully digested !! Thanx I'MAOf.

 

Injectors clogged? have another set to test?
ground leads and wires good to the injectors? that would prevent car from starting.
bad fuel and sitting aren't a good combo. Any rust in tank?
I don't remember which year started the crank position sensor, but that will prevent car from starting.
The car can run without the MAF. it will just idle like crap.
Does this car have the fuel relay and radio relay in the engine bay? try swapping them.
Im not sure what else to check. wait for the experts.

 

an 89 should be LH 2.4, and will have the crank position sensor, the OBD-I diagnostic box, and yes, a radio suppression relay. the fuel injection and pump relay should be in the main relay panel behind the ashtray in the center console.

the injectors are powered directly off that radio suppression relay so that might just be the next thing to check. unplug an injector, and switch the ignition on, you should see +12V on ONE of the two pins in the plug that fits onto the injector. if you do, the radio relay is probably good, if not, its time to check that (I'd probably unplug the relay and probe its socket, verifying its getting power to the right places...

radio suppression relay pins...
1 - black wire to ground
2 - blue yellow wire from fuel injection main relay, switches the RSR on
3 - green wire to the fuel injectors
4 - red wire direct from battery, should have +12V at all times. jumping 4-3 will power the injectors

the other side of the injectors goes to another green wire that goes to pin 18 of the ECU... the ECU pulses this to ground to fire the injectors (all 4 fire together)

oh. the radio suppression relay (confusing name, its really the 'fuel injector power relay'), is just forward of the left suspension tower, behind/near the power steering reservoir and ABS.

 

la RAP. belay that last. an 89 TURBO is *NOT* LH 2.4, its still LH 2.2. it took them another year to come out with LH2.4 on the turbo.

solly about that. so, NO crank position sensor, instead there's a hall sensor in the distributor. no OBD-II either. but the radio relay is there as is the fuel pump relay in the center console.

 

You guys are great ! Been lost on this one for a while. You have given me new hope. I have changed the fuel pump relay but the radio suppression relay... that's a new one to me. They used to just use a choke/chap. filter for RF... bet they have gone and changed Ohm's law on me too !! Yes, it appears to be the LH 2.2 from the system used for the mass air sensor. Manual I have is a little difficult to weed out what is and is not relating to my car. It's a one book fixes all type. Anyway, more things to try, and yes the term "radio suppression relay" is a little off the mark ! Thanks again, will try checking your suggestions and will post back with results in a few days.

 

the deal on the 'radio suppression' relay is like this...

on 240's, the fuel injector power was supplied via the main fuel injection/pump relay, which was located behind the glovebox. on 240's, relays were scattered all over behind the dashboard.

on 740's, they centralized all the relays and fuses in a new panel under the dashboard behind the ashtray. when the injectors are pulsed, they draw heavy square wave current, this induced a rather nasty buzz in the radio which was mounted directly above this new relay panel. so they added another relay in the engine compartment to provide injector power (as well as powering the ignition coil in several versions) so the current path to these noise makers was kept shorter. they called this extra relay the Radio Suppression Relay. In 1994 or so, on the last versions of the 940, they realized this extra relay could be used to power the ECU too instead of the expensive special dual relay they'd been using for the fuel injection + fuel pump since the early 80s, so they redid that, these cars have a different fuel pump relay, this has caused a fair amount of confusion.