Choke Absolute Hell is it ECU or Actautor(S)? Help & Ide

[Fred555]

BMW 316 Auto Coupe 1987 - been in the family since 3 years old always running with only the occasional brief mechanical battle - until now.javascript:emoticon

One morning car suddenly would not start from cold - had a look found that the choke actuator (replaces carburettor accelerator pump system & gives cold running enrichment) was not moving at all.javascript:emoticon

However the Pierburg 2BE Carburettor Throttle Actuator (controls & varies idle speed) & Distributor Ignition Advance Vacuum Changeover Solenoid were still successfully controlled by the Carburettor ECU & unplugging the blue inlet manifold temperature sensor while the engine is running (simulates very cold running) speeded up the idle speed but still no action at all from the Choke Actuator (even though it moves freely by hand) - car does eventually start & once warmed up is more or less driveable (in the current cool climate) & keeps running - anyway a brand new original BMW Choke Actuator was ordered.

Choke Actuator arrived & was fitted - car fired up and runs perfectly - in fact even better than before.javascript:emoticon

Two weeks later - without any warning - the fault suddenly comes back!

Car was driven most of that day - totally perfect - but then in just the last hour of driving first a hesitation was felt when pulling away when warm and then it started hesitating severely or cutting out when pulling away from stationary a couple of times!

Take another look and now the NEW choke actuator is not moving at all - just like its predecessor - but everything is still free and easy when moved by hand.

A SECOND brand new original BMW Choke Actuator is ordered.

It is fitted to the car but this second new one doesn't move at all - nothing from the very start and so I am back to the beginning with the same starting/running problems.javascript:emoticon

Any ideas?

It is noted on the wiring plan for this ECU that two wires run straight from pins 14&15 of the ECU Plug to the two pin plug on the Choke Actuator & connect to nothing else.

Also the resistance of the Choke Actuator can be successfully measured from the ECU plug end which is both good to know and an absolutely sickening development as it (in my mind) makes me suspect the ECU output that drives the Choke Actuator might have failed.

Which gives the horrific thought that either I have THREE (two new & one original from circa 1987) Choke Actuators that all in fact work but are not being driven by the ECU output.

OR

The ECU has blown all three with overvoltage? Or overdriven till Actuator burnt out - is that even possible? Does anyone know what the Choke supply voltage is meant to be? - I cannot find out this data anywhere.

OR

BMW have supplied faulty "New Old" stock from a suspicious old batch of Actuators and there is nothing wrong with the ECU - in which case where do I get a "good" Choke Actuator? javascript:emoticon

And how do you test a Choke Actuator away from the car (without blowing or damaging it electrically) beyond just the resistance tests?

[pacerpete]

In 25 years of working on E30s I have NEVER known a 316 ECU to die BUT yours could be the first !
The 2BE is overly complex rubbish and the aggro and cost of keeping it working has made Weber rich . I;E all the 2BE Weber replacements they have sold over the years !

I have a good ECU if you need one.

[Fred555]

If only the ECU was definitely dead!

Instead it is working everything else output wise except the Choke Actuator where something strange is going on - ECU is even responding to the temperature sensor input being unplugged correctly except for the choke actuator output being suspect as no choke plate activity.

Equally strange are the two replacement choke actuators - one failed two weeks after fitting the other never ever worked but all three pass resistance tests - but resistance (unless open circuit or dead short instead of the particular resistance reading you are looking for) is not a particularly reliable test.

Do you (by any chance) have a choke actuator that is known good?

I might still need the ECU - what do you want for the ECU?

[Fred555]

UPDATE:- ITS THE ECU

More poking around with a multi-meter discovered that if you unplug (simulates very cold running) the ECU Temperature sensor while at the choke actuator connector 12Volts appears and its frequency / duty cycle signal to the choke actuator changes between 30 to 50 (from plugging in & unplugging the temperature sensor) but if you apply the load of the choke actuator by reconnecting it to the loom plug the voltage disappears by failing under amperage load of the connected choke actuator.

Right (despite being warned not to) so now we can test the actuators without the risk of blowing them on the direct battery voltage through an ammeter.

Results? ALL THREE ACTUATORS WORK!

The two new actuators draw 6.5Amps peak dropping to just below 6Amps - the original actuator draws 6.75 Amps peak dropping to a bit less than 6Amps faster than the other two new ones - is also perhaps slightly less free to rotate output shaft by hand.

The voltage to choke actuator tests repeat identically at the ECU Wiring end so looks like an ECU fault as wiring tested fine again but ECU has direct connection to actuator.

Also noted that the ECU is slow to or does not always turn off the engine controls supply relay that the ECU controls the coil of - but this only happens if you start the car or turn the ignition on then off again - so if you disconnect then reconnect the battery the relay stays off correctly until you turn the ignition on - the relay then correctly comes on - but turn ignition back off again and the relay (along with all the engine mounted actuators it supplies power to) does not want to switch off - looks like the ECU again.

Seems to me that only the final output stage of the ECU (probably a power transistor or other amplifier chip) has failed as a very weak signal that is probably the correct type is detectable and varies with hot & cold running input signals from the temperature sensor - while the relay coil output from the ECU could also be a similar kind of fault or something as daft as a blocker diode is shorted within the ECU so is allowing power to reach the relay coil output terminal by flowing through the diode in either direction when the Diode is meant to be one way voltage flow only .

Who is good at repairing & rebuilding these ECU's?