1261 code mystery despite aux grnd of Fuel Relay

[Gene in NC]

1261 code with Fuel Relay ground... toggle controlled.

'89 525a has had 1261 codes and engine intermittent "stumble" for years. Seems worse in winter, but is active this summer. Documentation for 1261 suggests that ground to fuel relay (FR) is interrupted, and symptom feels like that. Have dynamically tested and found that fuel pressure drops and power to pump is interrupted.

HOWEVER, toggle switch rigged to provide full time ground for the FR as a test device, does not prevent "stumbles". Toggle works because when "on", the pump runs continuously with ignition on, but engine not running.

SO, if 1261 condition still causes engine "stumble" when the FR ground, which is controlled by the ECM, has been "jumpered" to full time on, does it also affect ignition?

Ocasionaly, at highway speed, several stumbles in rapid succession will cause the tach to drop like a stone. Doesn't seem like engine rpm related because when stumbles stop, the tach jumps back to its "normal" indication.

[whiskychaser]

1261 code with Fuel Relay ground... toggle controlled.

'89 525a has had 1261 codes and engine intermittent "stumble" for years. Seems worse in winter, but is active this summer. Documentation for 1261 suggests that ground to fuel relay (FR) is interrupted, and symptom feels like that. Have dynamically tested and found that fuel pressure drops and power to pump is interrupted.

HOWEVER, toggle switch rigged to provide full time ground for the FR as a test device, does not prevent "stumbles". Toggle works because when "on", the pump runs continuously with ignition on, but engine not running.

SO, if 1261 condition still causes engine "stumble" when the FR ground, which is controlled by the ECM, has been "jumpered" to full time on, does it also affect ignition?

Ocasionaly, at highway speed, several stumbles in rapid succession will cause the tach to drop like a stone. Doesn't seem like engine rpm related because when stumbles stop, the tach jumps back to its "normal" indication.

If grounding the relay doesnt stop the fault, could it be on the positive side of the fuel pump relay-say the main relay? That powers the fuel pump and ECM doesnt it? The tacho signal comes from pin 6 of the ECU so if thats ditched the engine will die and the needle will drop. HTH

[Gene in NC]

Response below from long time associate, a top notch Electrical Engineer.

"The tach falling is a clue that the ECM might be involved in the problems. This might be an intermitent ground or voltage supply to the ECM. This problem may be internal to the ECM case or external in the wiring harness. Internal causes might be a corroded screw or connector providing voltage or ground to the circuit board. Older circuit boards sometimes exibit cracks in the copper lands and sometimes in the board itself. It may require a good look under a magnifing glass. The voltage and ground are common to most of the ECM functions so should need a good look. Also look for cracked solder joints. Swapping the ECM would be a good diagnostic tool if available."

[Gene in NC]

Another clue? If air conditioning on, shifting to neutral frequently causes engine to stall.

[bubba966]

Sounds like an E34's fusible link problems. Have you checked your fusible link? I don't recall where it is in the earlier E34's, so someone else will have to chime in with that info.