Following the number of Motronic no starts of late relating to E28 535i and 525e first gen Motronic cars i though it about time i pulled a few common tests and problems together in one post. please flee free to add comments below!

The following information has been gleaned from my experience, trial and error and others but mostly from the Bentley Manual, Haynes Engine management manual and the Haynes manuals for the e28 520 &525e and the 3 & 5 series manual. I cannot stress how useful these books can be especially the Bentley manual!

The advice below is offered in good faith and is implemented at your own risk. The Neither the author nor BMW5 can take any responsibility for damage caused accidentally or otherwise from following the procedures below.

Warnings by no means exhaustive!!!

The shock from HT leads can cause nerve damage ALWAYS use necessary precautions when dealing with the HT circuit.

Always have the ignition off when making or breaking electrical connections

Always take care to avoid ‘shorts’

Only use a test meter with a high impedance – other types may cause incorrect results or damage.

Switch meter ranges only when disconnected

Do not connect instruments with a 12V supply to terminal 15 of the coil – this can damage the ECU.

So to the diagnosis!


The system is not desperately complex but one must be methodical and check everything to ensure the obvious is not over looked. Do not discount new parts either!

Test equipment needed is a digital multi-meter and some leads with crocodile clips – nothing too fancy but we need to be able to measure volts (both AC and DC), continuity and Ohms.

So your symptom is that the engine turns over but will not fire? We are not looking here at running issues – for instance a duff AFM should allow it top fire if not run above idle.

Do you have fuel? Obvious but often over looked!

Was the car running fine before? If so could it be flooded? Try cranking over with a wide-open throttle for a good few seconds to see if it wants to catch. If so continue trying to start it and it should cough into life but you need a good battery for this.

Is the battery good? Poor batteries have lead to no start scenarios however good you think they are!

So to actual faults or failings:

Do you have a spark at any of the spark plugs?

Test by removing a plug and holding the plug against the block with insulated pliers and have an assistant turn the engine over whilst you watch for a spark. You are looking for a strong blue spark.

If there is no spark (or very weak yellow) then connect a ‘plug straight to the lead (king lead) coming from the coil and repeat the test.

If you now have a strong spark then a new rotor arm/ distributor cap or plug leads are required.

If there is still no spark, swap over the king lead (from coil to distributor) with another plug lead and repeat the test.

If there is still no spark we can assume for now that the plug leads, distributor cap and rotor arm and in serviceable condition.

If none of the above has yielded success we need to test the coil. Break out the multi-meter and set to DC 0-20V scale. With the ignition switched on we should have 12V between the +ve (15) terminal of the coil and ground (eg. The engine block or –ve terminal of the battery.

If not the coil is not receiving its power supply from the ecu. This leads us to suspect either the ECU or the continuity of the supply wire from the ECU and you should refer to the ECU tests described later.

If 12V are seen at the +ve terminal (15) then we need to test the coil.

With the ignition switched of and the multi-meter set to Ohms we can test the coil:

Between 0 & 15 (-ve and +ve) we should have 0.5Ω
Between 15 and the tower (where the king lead connects) we should have 6000Ω

Anything wildly different could suggest the coil is at fault and a substitution should be tried.

If the coil checks out ok we need to look for causes as to why the ECU is not sending the spark signal.

We must now confirm that the DME relay (Digital Motor Electronics or main engine relay) is functioning.

This is the 5 pin relay located on the left hand side of the fuse box, externally at the front of the fuse box. (BMW part Nº. 61361729004)

To test remove this relay and switch on the ignition then gently replace relay – it should be hear to click. If so we can eliminate the OBC security code, ignition switch and supply to the relay.

To confirm this we should check for the following readings at the DME socket:

With ignition on we should see:

+12v @ tab 30 (battery) red wire
0v @ tab 85 (earth) brown wire
+12v @ tab 86 (switched +ve) green wire
+12v @ tabs 87 and 87 (injector & ECU output) red/white and red/black wires

With ignition off:

0v @ 87 & 87

This relay feeds the fuel pump relay (located next to the DME relay) so it makes sense to substitute this relay next and test the fuel pump. The fuel pump relay is a standard type and a matching relay can be pulled from the fuse box for substitution.

The fuel pump should briefly hum when the ignition is switched on if not we can confirm the pump is ok by getting under the car (stands please!) and applying 12V direct to the pump terminals.

So if still no joy let us eliminate the crank position sensors there are two located in the gearbox bell housing and their plugs are located side by side at the back of the engine in a metal channel.

We can test these as follows:

Pull each plug in turn at the bulkhead and check for the following:

Each plug has 3 terminals we shall call them 1,2&3 from left to right. Looking at the plug we assume that the single locating key/ tang is upwards and the tow are down wards.

Between 1&2 we should see 960Ω +/- 96Ω
Between 1&3 and 2 & 3 we should see 100,000Ω minimum.

Switching to our AC volts scale on the multi-meter we should see between terminals 1& 2 nominally 0.7 and 1.4v AC being generated on cranking if not substitution of these components is required if still no figures given we need to consider that thetrigger pin has dropped off the fly wheel…..

Best follow the other tests to ensure that there can be no other possibilities before assuming the worst!

Make sure you put the leads back on correctly – with the leads swapped the car may run and misfire or it may not run at all! Lets not add to our problems!


Assuming the CPS sensors check out ok we need to look at the ECU we can eliminate this by substituting for a known good one but first we must ensure the ECU is seeing the inputs that it needs.

To do this we need to release the ECU from the glove box to allow access to the multi-plug to allow us to test the inputs the ECU sees. Please carry out all these checks regardless of where you think the fault lies – often it’s the combination of faults that gives us the clues!

The ECU multi plug is numbered as follows:

(1-18 is 18 pins across the top and 19-36 17 pins below)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

1. Voltage supply to ECU: With the ignition on we should see 12v (battery voltage) between terminals 18 & ground and 35 & ground.

2. Main Grounds/ earths: With the ignition off we should see continuity between terminals 5 & ground, 16 & ground, 17 & ground and 19 & ground.

3. Fuel pump relay control: With the ignition on grounding terminal 20 should cause the fuel pump to run.

4. Starter input: On actuating the starter we should see 8v DC minimum between terminals 4 & ground.

5. Throttle switch (manual): Ignition off we should see continuity between terminal 2 & ground, on opening throttle no continuity and at full throttle continuity between terminal 3 and ground.

6. Idle position switch (auto): Ignition off we should see continuity between terminal 2 & ground, on opening throttle no continuity.

7. Idle stabilizer valve (535i only): 40Ω between terminals 33 & 34.

8. Engine speed sensor (CPS): With the ignition off we should see 960Ω +/- 96Ω between terminals 8 & 27

9. Reference Sensor (CPS): With the ignition off we should see 960Ω +/- 96Ω between terminals 25 & 26.

10. Coolant temp sensor: With the ignition off the resistance should vary depending on temperature (notionally 2000Ω cold and 300Ω hot)

11. Fuel injectors 1-3: With the ignition on when terminal 14 is grounded the injectors should click.

12. Fuel injectors 4-6: With the ignition on when terminal 15 is grounded the injectors should click.

13. AFM: resistance read between terminals 7 & 9 must fluctuate when AFM flap is moved by hand.

14. Coil: continuity between terminal 1 & coil –ve terminal

These tests should either identify you a suspect component or a combination of faults could lead you to a common connector plug or earth point.

In simple terms check:

Fuel
Good battery – perhaps try jump starting the car
Rotor arm/ distributor cap is intact
DME relay is working
Fuel pump is working

The above tend to point to the most common no starts!

Obviously the above does not take into account dodgy alarms/ immobilisers, which are a whole new headache!

I'm almost tempted to buy an E28 just to give all this a go - absolutely cracking write-up, Sir.

I now have no excuse not to make a real effort to get my dead 525e running again, although I don't think SWMBO will welcome a second E28 on the premises

thanks for putting this together

Excellent write up Duncan, jsut hope I never need it

What have you got against shorts though ?

After seeing you wearing a pair at Gaydon..quite a bit I would imagine

Top write up Duncan!

I knew that would be mis-read ( i got bored typing) and besides you know what i mean!

great write up boss

should put up my one on brake conversions as well

Nice one Duncan- this is bloody brilliant!

Dan

This site might have been linked before on here, but whenI checked my ECU (see my thread) I had the exact problem shown. It might be woth a look for people with non starting cars or intermittent non starting.

http://e28-535i.com/technical/e28-ECU_Repair.php

Nice addition, thanks.

cracking write up duncan, I must say

D reg 528i: motronic or not motronic?

Cheers Dave, i believe the 528i is non but the principles would be the same.