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starting problem

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Printed on: Oct 20 2019


Topic author: Rob-M545i
Subject: starting problem
Posted on : Oct 18 2004 13:53:43

my 745i '86 with motronic has a starting problem,
is start fine, but when the ignition key goes from position 3 (start)
to position 2 the engine suddenly stops. this can be repeted over and over. the car start fine but won't keep running.

i checked if the motronic has power to pins 18 and 35.

what can be the problem?

greetz from a Dutch 745i driver


Reply author: Jim
Replied on: Oct 18 2004 15:02:04

Fuel related? Have you tested fuel pressure? Fuel pump(s) supposed to run while engine is running and while starter cranking. Almost sounds like fuel pumps are only engerized while starter is cranking.


Reply author: marc79euro635
Replied on: Oct 18 2004 18:15:41

I don't know the motronics as well as the Ljet but it sounds to me like you have a wire that should be on in the second position,thats only on in the 3rd position. check your wiring, coil should be hot in 2nd position,main relay & fuel pump relay should be on in 2nd with afm door opened, green wire on three flat connector by ecu should be hot in 2nd(this is obc circuit). was the car running before?
good luck

Reply author: tihuovin
Replied on: Oct 19 2004 00:30:28

It sounds like the problem Scottie had in his "645". Lets wait also Scottie's response, but I try to help as well...

Have you verified the condition of the fuel pump and fuel pump relay? Motronic ECU is controlling fuel pump via controlling the fuel pump relay. The fuel pump relay is located nearby the fuse box, attached outside the fuse box (i.e. not under the lid). Motronic ECU is "grounding" the relay pin 85. When this pin is in the ground, the relay is "pulling" and fuel pump is getting its supply voltage and runs.

The other pins of the fuel pump relay are as follows:
* Pin 30, +12V, connected directly to battery + pole.
* Pin 86, +12V, connected to same "spot" as the fuel injector's. That "spot" gets it's supply voltage when main relay is "pulling" (via main relay pin 87).
* Pin 87, mains (+12V supply voltage) to fuel pump

You should be monitor fuel relay pin 85 while you crank, you should see how it goes to ground and back to "infinite" using multimeter (measuring the resistance of that pin).

Main relay is also an issue (located under the hood, co-driver side firewall). It feeds the supply voltage to many many places. Including the motronic ECU. Main relay pin 85 is always directly grounded. The realy pulling coil (i.e. pin 86) got its mains (+12V supply voltage) via ignition lock and its pin 15. When you turn the ignition key to position II, ignition lock pin 15 has +12V. It goes to 3 pole flat connector (nearby motronic ECU and its 35 pin connector) and via that to main realy and its pin 86. Main relay pin 30 (as the fuel pump relay), is directly connected to battery + pole.

Via main relay and its pin's 87 (two of them), the mains are connected to:
- Fuel injectors
- Motronic ECU
- Air Flow Meter
- Knocking ECU
- Transmission

Check these two relays, that the wirings are OK and the relays are OK.

One possibility is the OBC relay ("anti-theft system"). You have a possibility to "feed" the code to OBC, and the car won't start if the same code is not fed when the car is started next time. But I doubt that the reason is OBC relay, because the car WILL start. but won't stay on.

Also, there are few relays that connects the mains OFF from several devices while the car is started. So blower motor, A/C, windscreen wiper etc. are not working while you start the engine. I have to study this issue, is there something which cause this problem.

Download the E23 schematics from the file library. It contains also engine control schematics and might be helpful while troubleshooting this problem.


Reply author: scottie
Replied on: Oct 19 2004 08:45:51

That problem is on a 745i that I have bought from Ohio, but can't ship them because of that problem. The owner doesn't know cars very well and we are trying to troubleshoot that same issue so I can get the car towed. We've already bypassed the main relay. No change. Certainly, it is either OBC or fuel pump power related, eh?

Reply author: Bob745_Dallas
Replied on: Oct 19 2004 12:24:06

If the main relay is by-passed then it should not be OBC related. A malfunctioning OBC relay will prevent 12v from getting to terminal 86 on the main relay (same thing the code function would do if the wrong code is entered). What about the unloader relays K5 and K7. Do the mirrors, wipers, etc. work with the key in the run position? Try switching out the unloader relays. Is there 12v on the primary at the coil with the key in the run position? The ignition swich itself could also be suspect.

The speed sensor could also be suspect. The reference sensor is needed to start the car, but the speed sensor is needed to keep it running. The speed sensor takes it's reading from the teeth of the flywheel as it rotates.

Reply author: qwikxr
Replied on: Oct 19 2004 15:11:14

Originally posted by Bob745_Dallas

Is there 12v on the primary at the coil with the key in the run position? The ignition swich itself could also be suspect.

Ah, yes. I had this with the infamous VW ignition switch.. But all would work in the run position, except the one "keep alive" wire for the ignition coil.
And on a 1982 Scirocco, It wouldnt run, but would try to start - problem was a corroded ground from the motor to car chassis..

Just some more ideas to look at..

Reply author: tihuovin
Replied on: Oct 20 2004 00:07:25

Sorry Rob-M545i, I did not read your original message accurate enough. If the motronic ECU has +12V in pin's 18 and 35, it means that main relay is OK.

But check the fuel pump relay, is that OK? Check also the wires coming to fuel pump relay connector/socket, are those OK? Check the grounding signal from motronic, is that OK?

As Bob suggested, check also those two underloader relays K5 and K7(called also as power saving relay's). The function of those is to prevent several devices NOT to work while you crank. Devices like power window, blower motor, A/C, rear window defogger etc. Those relays can be found in the fuse fox.

That problem is quite a strange in engine management point of view, because the engine should and will start if all the mains are connected (motronic ECU, knocking ECU, fuel injectors, etc.) and all the sensors are OK and the starter gets its start signal. That start signal comes from ignition lock, via its pin 50.

Have you tried to start the engine using a "jump wire"? So that you
- put the ignition key to position II (mains are ON)
- put the gear to neutral or P (automatic transmission) or neutral (manual transmission)
- remove the starter motor solenoid wire
- take the "jump wire" ("female abiko connector" on the other end), connect the other end ("abiko" connector end) to starter solenoid (pin 50) and the other end to battery + pole.

Does it crank? Does it start?

Basically, by doing so, the situation should be exactly the same than you turn the ignition key to position III, except that the power saving relays K5 and K7 do not work (i.e. blower motor, A/C, etc. are functioning during the crank).


Reply author: Rob-M545i
Replied on: Oct 22 2004 08:25:40

i checked all relais, wires, sensors. everything seems to work fine.

i noticed that the ignition shuts down when the key is turned from
position 3 to position 2.

a further thing i tried is to use a normal M30 3.5 motronic ECU. now the engine starts and keeps running. its runs very bad but it is a start.

does this mean the problem is in the ECU itself, or is something wrong with sensors the turbo ECU uses, wich the normal 3.5 ECU doesn't use??

Reply author: scottie
Replied on: Oct 22 2004 09:06:06

Sounds like your ignition switch is bad?

Reply author: Rob-M545i
Replied on: Oct 22 2004 09:17:16

if the ignition switch is bad, how can it be that with a normal 3.5 ECU the engine keeps running? what part of the ignition switch is causing the problem?

could it be that the 4hp22EH part op the motronic is the problem, since the normal 3.5 ECU doesn't have this?

Reply author: scottie
Replied on: Oct 22 2004 14:43:17

hmm. good point. i'm stumped. (I'm also stumped by the same problem on my own car!)

Reply author: Rob-M545i
Replied on: Oct 24 2004 02:08:53

here is an update to the problem:

i first used a motronic from a M535i '86. the engine started and kept running but very bad. with the smallest touch on the gas the engine stalled.

i now use a motronic from a E23 735i '83. and the engine runs verry smooth, and revs up good when i touch the gas.

there seems to be a big difference between the E23 3.5 M30 engines en ande E28 3.5 M30 engines. i don't know yet what it is.

does the normal 3.5 motronic use the knock/overpressure ECU that is in the glovecompartment?.

the only difference i know of now is that the 745i motronic has a additional ECU program for the 4hp22EH. Could the automatic gearbox give of wrog information to the ECU so that it won't run in posiotion 2? in position 3 (starting) the ECU isn't getting any info from the 4hp22 EH or from the knock controlbox?

i just keep on searching...

Reply author: scottie
Replied on: Oct 24 2004 10:10:17

There is no difference between the e23 and e28 version as far as I know.

The M535 used the euro motor, high compression, no 02 sensor. The motor should run fine with this ecu. In fact, you can pretty much use any 3.5l Ecu and the car should run, albeit not ideal.

The symptom of dying when you put your foot on it is indicative of a bad AFM, or wiring to the AFM.

Reply author: Rob-M545i
Replied on: Oct 24 2004 13:39:06

the engine will start with the E28 ECU, but runs realy bad. with the E23 ECU it runs actualy very good. the 2 motronics have different part numbers, so the could acualy be a difference between them. the E28 and E23 3.5 use different injectors. this could be because the E23 has an 4hp22 and the E28 has a manual transmission.

Now I got the 745i running with the 745i motronic!!! the problem of starting and running in key position 3, and shutting down in position 2 is bypassed. I don't now yet what causes the problem but this is how i got the motronic working (sounds a bit strange maybe):

-I pulled the black/yellow wire from the starter
-plugged a loose wire onto the black/yellow wire
-plugged a losse wire onto the starter were the black/yellow wire was
- put the ignition key in position 2
- put the 2 loose wires verry shortly onto the battery
- after the engine started i pulled the 2 loose wire of the battery and the engine kept running!

i don't know yet why it only keeps running this way and not with the black/yellow wire connected correctly.

the engine isn't running very smooth. it looks like its running on only 3 cilinders. the car hadn't run for 3 years so some parts could be not working. i put new oil and spark plugs in it, they al work .
fine. when the engine runs fine the building of my M545i can begin!

thanks everybody for the help so far!

Reply author: tihuovin
Replied on: Oct 27 2004 03:57:25

The only difference between those two ECU must be the eeprom (and the code inside it). The fuel and ignition maps must be different. The reason might be, the other is for US market, the other for EURO market. Because the compression ratio is different, the ignition/fuel maps must be different too. OR, the other motronic is slightly broken.

The black/yellow wire is the wire from ignition lock and its pin 50 (black wire from ignition lock). Maybe you have something trouble with the ignition lock, hence the strange wiring.

When the ignition key is turned to position III, it feeds +12V to starter (to its pin 50, via black/yellow wire). But simultaneously, two "power saving" relays (K5 and K7 in the schematics) start to work too, taking the mains OFF from few devices. So during the crank; blower motor, rear window defogger, A/C etc. devices do not work, so the starter gets all the power possible.

When you did the "wire trick", you basically did the same thing than using the ignition key position III except all the electrical devices were working during the crank.

It is funny though, because if I do the same and do not connect the original black/yellow wire where it supposed to be, the engine idle's badly. When I do connect it back, the idle is smooth again.

I had something troubles with my previous E23 (the starter "bush bearing's" were leaking the current to ground). So few times I was forced to "jump wire" my car, in order to get it running. I had a piece of "jump wire" under the hood, the other end has the "abiko" connector (to starter), the other one did not had it (to battery + pole). And always the case was clear, the starter original wire on its original place and the engine started to run better, MUCH better.

But of course, BE EXTRA CAREFUL when you "jump wire" your car, make sure that the gear is OFF (either P or N), otherwise your car will rush forward and possibly over your foot (instep) and capture you (please, do not ask how I know [:((])


Reply author: Rob-M545i
Replied on: Jun 26 2005 06:51:24

After a few other projects I now finished building the M106 into my 86'M535i.

i changed several parts, checked the engine and everything seemed OK.
but I still got the same problem!!??!!

with the 745i motronic the engine starts, but dies emediately, but only ignition, the injection stays on. when i do the wire trick the engine wil keep running, but very bad. there is no reaction if the TPS and AFM connectors are taken off. my gues is that the motronic things its still in start-mode sow doesn,t care about these sensors.

with a 735i 83, motronic its starts and runs fine! but the knock control doesn't work, it can be disconnected without any affect.

sow i thing the problem is in the 745i ECU, the knock control box, or in a sensor that only the 745i ECU or knock control uses.
unfortunately i do not have a spare 745i on which i can try which sensors are needed to keep the engine running.

therefore my question:

does the M106 with 745i ECU run with the cempressionvalve disconnected, the knock sensors disconnected, the AFM disconnected or the TPS disconnected? and are the temperature sensors needed to keep the engine running?

P.S.: the difference between 735i 83'motronic and 86'535i motronic has become very obvious. i blew up a 86'ECU by using 535i injectors. wich are i beleve Los impendance. also AFM is verry different, 0-5V instead of 0-10V. other thing is dat '86 535i uses automatic idle control, whereas the 735i uses a bypas valve wich can be adjusted. so don't mich te compontents of those two...[2]

Reply author: Rob-M545i
Replied on: Jul 20 2005 12:11:19

problem solved!!!

swapped the ECU with one from the junkjard. same number etc. swapped the knockcontrol and ECU. now it runs, keeps running, and runs beautifull!! nice smooth and silent.

i will try to find out wether it wat the ECU or the Knock control. and than look a bit closer to the cause of this and try to fix it, ECU aren't cheap even not on the junkjard..

only when i rev the engine for approx. 1 sec (idle to 3000-4000 rpm). black smoke comes frome the exaust. is this normal for a turbo engine?, i saw some tuned turbos running a little rich with the same "problem". Or does the 745i standart engine produce no smoke what so ever?

Reply author: RaceMe
Replied on: Oct 09 2005 17:23:57

HI,i have a 1984 745i when i start it in the morning it runs fine when it is cold when i drive it for couple minutes the car start running rough and alot of black smoke comes out of the exhaust.when i shut it off and try to start it up again the car wont start.Any idea?

Reply author: ra0507
Replied on: Oct 10 2005 09:59:14

As you know, black smoke is a fuel issue -- running too rich. Do you have an air/fuel gauge. I am not the most qualified but you may need to get advice on setting up the AFM spring.


Reply author: aaiello48
Replied on: Oct 11 2005 16:16:52

Sounds to me like your motronic has gone bad...
your are right if it were the switch it would not start with the "059" motronic
I have a 745 that a previous owner had a older transmission( non EH) replacement installed for my purposes the other half of my "013" motronic is just a dead leg, however my car runs fine,despite half the ECU is not being used!
you are on the right track.. the 745 will just barely start and idle with the "059" motronic if given any gas will stall and die...
meaning to me you will need a 745i "013" motronic,sorry
They are as rare as a hens tooth.
Try asking one of the members
Good Luck

Reply author: dnliii
Replied on: Oct 11 2005 18:21:25

If you own a 745i you should have a spare ECU.... there is one on ebay- price is 15.50 with one day to go:

Good luck,

Reply author: dnliii
Replied on: Oct 11 2005 18:47:39

OK- I just put in a bid so do not bid against me... let me know how high you want to go if you want the part and I will bid it for you. Most of the guys that have 745i's know my ebay initials and won't bid against me... Be prepared to spend about 250.00.

Reply author: Rob-M545i
Replied on: Oct 11 2005 23:15:53

if your car run's oke when cold, and starts having problems when warm your blue temp sensor (for motronic) could be bad. its on the motorside of the thermostat.

you could measure if it is ok by checking resistance of the sensor. can't remember the resistance it needs when cold and warm but they can be found by searching other toppics, library file or perhaps can be supplied by one with good memory.

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