No start, was intermittent, now all the time

[ian m635uk]

My car is talking to me, after 15 years ownership i have recently put it up for sale and now it will not start...

Was first an intermittent problem, car just completely cut out at 50mph. Then it would have problems maintaining idle, now it has developed into a persistent no start, this has helped me with diagnosis.

Spent all day on it doing diagnosis but looking for further advice.

Car cranks fine And i have spark at the spark plugs.
Fuel pump relayed jumped - still no start
Main relayed jumped - still no start.
Engine will fire but only for 1 second and then not run. Suspect cold start valve is operating correctly.
Injectors have 12v to them and wait to be grounded by ecu but they do not fire on start up. No clicking on any of them, plugs are dry as well.
Checked both reference sensors. Resistance ok. I get ac volts under crank test. I'll check again today to get actual volts, i recall 0.4v ac but reading up on this one should show 4.0v ac.
Cts checked ok
Tps checked ok
Ecu is new 6 years ago. Inspected for dry joints and could not see anything.

Questions
Will ecu provide spark if one of the reference sensors is bad? If no then my problem lies in injector firing as the crank sensors would be ok.
Could it be a ground problem for the injectors? If so where do i look for this as it seems the ecu is grounded on 3 pins from its connector, but where do these actually get grounded.

I will replace both relays and both sensors next week anyway

Any help appreciated

Ian

[Brucey]

I'd suggest trying a substitute ECU as a test in the first instance.

It sounds like it might be a bad feed to the injectors, (I had the exact same symptoms with a bad main relay), a bad ground like you say, or a bad injector drive stage in the ECU.

I don't think you would get spark if the cranks sensors were no good.

Spark and CSI function on cranking will cause it to fire on cranking, but if the main injectors don't fire then the engine will quit as soon as the key is turned to 'run'. If the feed or ground are bad, this happens instantly (the extra load kills the spark too), but if (say) the drive stage of the ECU is bad, you still get spark and the engine will splutter on for another second or so. It isn't easy to tell the difference.

cheers

[ian m635uk]

Thanks Brucey
I might pull the ECU again and look for any solder issues. Can you advise where the output transistors (injector drives) are once opened up, or what I would be looking for? Is there two? One per bank of 3 injectors?

Thanks

Ian

[ian m635uk]

Tested reference sensors at ECU plug on crank.

I get 0.3v ac for one and 2.1v ac for the other one.

Resistances
940 ohms
950 ohms

Seems reasonable apart from the 2.1v maybe. Should I be expected higher at around 4.0v?

Ian

[ian m635uk]

Checked ecu. Absolutely no issues with soldered joints. Not surprised as it is relatively new. I think ill change sensors and relays anyway and take things from there. Very frustrating. Ian

[ian m635uk]

Also checked grounds to ecu and they were all perfect. Looking more and more like an ecu fault. Why would it be intermittent though. I would have expected the ecu to fail completely, not be hit and miss. It has gradually got worse.

Ian

[hornhospital]

In spite of the solder joints looking perfect, I'll betcha there a crack in the traces somewhere. Changes in temperature can trigger a failure when the traces cool and shrink, opening any cracks in the circuits.

[Brucey]

I would suggest that unless you are looking under a binocular microscope (with a ringlight fitted), you won't reliably see a solder joint that has gone bad (via fatigue).

If you google 'bad solder joint' images you can see a fair number of images; this one (from a Honda/Acura part) is typical;




Some solder joints look a bit like that from new; it really isn't obvious when they are bad enough to start going intermittent! If in doubt, reflow the lot on the power transistors.

Also 'newness' is no guarantee of quality here; probably the exact reverse in fact. A new ECU unit will be made with lead free solder; this is (IME) more likely to cause problems in service, unless the joints are all designed for lead free solder.

BTW the low voltage output on the 'ref' sensor is because there is only one peg I expect. Ideally you need to check the output waveform on a 'scope.

cheers

[wattsmonkey]

This will sound like sucking eggs, but my M535i did exactly as you're describing. It had compression, spark and fuel, but would NOT start.

The problem was as simple as a worn-out distributor cap.

It took me a very long time to work this out as I thought there would be no spark at all with worn components like this.

Other thought: I take it you've checked there is fuel at the rail whilst you're cranking?

I dare say you've already checked the above, but this does sound familiar.

Good luck,

Rob

[Chris Wright]

Thanks Brucey
I might pull the ECU again and look for any solder issues. Can you advise where the output transistors (injector drives) are once opened up, or what I would be looking for? Is there two? One per bank of 3 injectors?

Thanks

Ian

The three control transistors are on this heat sink as shown below. I believe the round transistor in the middle is for the ignition coil and, on either side, the other two are for the injectors, one for each bank of 3 injectors.




ECU - Re-flowing the solder Joints (With photos):
viewtopic.php?t=15771&postdays=0&postorder=asc&
http://e28-535i.com/technical/e28-ECU_Repair.php
http://www.teamdfl.com/bmw/e28/ecu_repair/index.html
http://forums.bimmerforums.com/forum/sh ... st16490553

[ian m635uk]

This will sound like sucking eggs, but my M535i did exactly as you're describing. It had compression, spark and fuel, but would NOT start.

The problem was as simple as a worn-out distributor cap.

It took me a very long time to work this out as I thought there would be no spark at all with worn components like this.

Other thought: I take it you've checked there is fuel at the rail whilst you're cranking?

I dare say you've already checked the above, but this does sound familiar.

Good luck,

Rob

Hi, yes checked this. It does occassionally run, for 15 minutes last time and then just instantly died. I also jumped the fuel relay and can here the pump running. Strange that you mention the distributor cap, i took this and the rotor arm off and thoroughly cleaned and checked both. Both were fine. I do get spark as well at the plugs. Ian.

[ian m635uk]

I would suggest that unless you are looking under a binocular microscope (with a ringlight fitted), you won't reliably see a solder joint that has gone bad (via fatigue).

If you google 'bad solder joint' images you can see a fair number of images; this one (from a Honda/Acura part) is typical;




Some solder joints look a bit like that from new; it really isn't obvious when they are bad enough to start going intermittent! If in doubt, reflow the lot on the power transistors.

Also 'newness' is no guarantee of quality here; probably the exact reverse in fact. A new ECU unit will be made with lead free solder; this is (IME) more likely to cause problems in service, unless the joints are all designed for lead free solder.

BTW the low voltage output on the 'ref' sensor is because there is only one peg I expect. Ideally you need to check the output waveform on a 'scope.

cheers

I see what you mean from the photo. As i get none of the injectors firing i suspect it might be an issue further back into the ecu before the two transistor circuits i.e. The firing signal itself or the chip that determines this. I have found a company quite close to me. www.ecutesting.com that might be able to at least test the ecu if not repair it also.

Ian

[ian m635uk]

Thanks Brucey
I might pull the ECU again and look for any solder issues. Can you advise where the output transistors (injector drives) are once opened up, or what I would be looking for? Is there two? One per bank of 3 injectors?

Thanks

Ian

The three control transistors are on this heat sink as shown below. I believe the round transistor in the middle is for the ignition coil and, on either side, the other two are for the injectors, one for each bank of 3 injectors.




ECU - Re-flowing the solder Joints (With photos):
viewtopic.php?t=15771&postdays=0&postorder=asc&
http://e28-535i.com/technical/e28-ECU_Repair.php
http://www.teamdfl.com/bmw/e28/ecu_repair/index.html
http://forums.bimmerforums.com/forum/sh ... st16490553

Thanks for this and the links, very helpful.

I think i will get the ecu tested by www.ecutesting.com. They are about 1 hour from me. Hopefully then can diagnose the fault and correct it.
I also have a contact with a main dealer for Bosch so i might be able to get a new ecu at cost price.
Ian

[ian m635uk]

Also i have ordered new main , fuel pump relays and two new sensors. These should come wed so i will fit these first to make sure before testing the ecu.
Ian

[Brucey]

CW's pic shows the last version of the ECU used on 6ers which fires the injectors in two batches, I think.

Earlier versions all fire in a single batch and (some at least) use a single transistor to fire the injectors in that single batch. The failure of the solder joints on this transistor causes the exact symptoms that you have at present.

IME the failure is invariably where the 'leg' joins onto the bard itself, despite the springy strain relief bits.

cheers

[ian m635uk]

CW's pic shows the last version of the ECU used on 6ers which fires the injectors in two batches, I think.

Earlier versions all fire in a single batch and (some at least) use a single transistor to fire the injectors in that single batch. The failure of the solder joints on this transistor causes the exact symptoms that you have at present.

IME the failure is invariably where the 'leg' joins onto the bard itself, despite the springy strain relief bits.

cheers

Hi mine is the one in CW's photo, i struggle a bit for time with working away from home so I might just drop it off at ECUTESTING and get them to sort it out. I have a potential buyer for the car but obviosuly need to get it sorted before it can go!
At least a know A LOT more about Bosch motronic now than I did two days ago...
Thanks for your help

Ian

[Pod]

I'm following this thread with interest - please let us know how you get on?

[ian m635uk]

Will do. I have to solve this anyway as the car is practically sold but will not be picked up for a few weeks.
Ian

[ian m635uk]

Ok update below.

Changed out main and fuel pump relay for new
Changed both crank sensors for new. Checked volts on crank for sensors and get 0.39 v ac and 7.4 volts ac. The 7.4 seems high
Checked 12v supply to injectors. I have 12v with ignition on.
Checked fuel delivery on start up via cold start valve feed , all ok. Pumping well
Next checked coil, 12 v supply ok, resistance checks ok.
Then checked for spark on number 1 plug on crank. NO SPARK
Checked spark from coil main lead remote end. NO SPARK.
injectors not working either as plugs are dry when pulled.

Removed ecu, re soldered all connections to injector drives (2off) and injector drive.

Tried again. NO SPARK. NO INJECTORS.

Dont think there is anything else to check, so ill get the ecu tested next and take things from there. Previously i did have spark on crank! But no injectors! Frustrating to say the least.

OBC inhibiting ignition and fuelling? Is this done through the motronic ecu itself or does it just cur feed to ignition and fuel relays?

Ian

[ian m635uk]

Just tested feed to terminal 86 of main relay from OBC. I have 12 v at this terminal so OBC is not inhibiting feeds. (Found procedure on the web)
Ian

[Brucey]

if you fit a jumper to the main relay then this already eliminates an OBC fault as the sole problem, since it bypasses the OBC circuit.

BTW when you test for 'a 12V feed' it is important that you use a load that is representative of that which is normally used on that circuit (choose carefully, a load that is too large is bad too) .

If you just use a voltmeter it proves relatively little; you can still have a bad connection that will only allow a few mA to be drawn through the circuit; this is enough to register '12V' on a meter but not enough to do anything useful in many cases.

cheers

[ian m635uk]

if you fit a jumper to the main relay then this already eliminates an OBC fault as the sole problem, since it bypasses the OBC circuit.

BTW when you test for 'a 12V feed' it is important that you use a load that is representative of that which is normally used on that circuit (choose carefully, a load that is too large is bad too) .

If you just use a voltmeter it proves relatively little; you can still have a bad connection that will only allow a few mA to be drawn through the circuit; this is enough to register '12V' on a meter but not enough to do anything useful in many cases.

cheers

Thanks Brucey
I was using the voltmeter to actual confirm I have a feed to the injectors and the coil to start off with.
For the speed sensors I followed the advice to interogate the ac volts or milivolts from each one.
Good that I have proven it is nothing to do with the main or fuel relay or power to ECU, or OBC inhibit etc.
It looks highly likely to me now that its an ECU fault.
Will find out when the ECU is tested early next week.

Ian

[Chris Wright]

It sounds like you have had the ECU in and out several times. Check to make sure that none of the connector pins have been pushed back and aren't making contact.

Do you have the Bentley manual? You might check the voltages at the ECU connector to make sure the wiring is Ok? Page 7-33, Table "M"

[ian m635uk]

It sounds like you have had the ECU in and out several times. Check to make sure that none of the connector pins have been pushed back and aren't making contact.

Do you have the Bentley manual? You might check the voltages at the ECU connector to make sure the wiring is Ok? Page 7-33, Table "M"

With the fuel pump running on crank i have assumed that this would only be possible if the ECU was powered up as it controls the fuel pump relay? Also the ecu is seeing the crank sensor inputs to continue to allow the fuel pump to run on cranking? Or am i wrong here.
I can check feed to ecu but i am away from the car for a few days now.
Ian

[ron]

Ian, Do you want to borrow a known "good" ECU?