m104 Misfire
 

My '94 E320 just started to misfire.
I checked the HT from each of the 3 coils using a clip on timing gun and found that there is no output from the rear coil to plug 1.
Swapped the coils around, fault stays with whichever coil is at the back.
Same with the HT leads (1 & 5).
So, I think the may be a fault with the feed to the rear (6) coil.
How can I check this?
The feed, presumably, come from the ECU. Is there any way to trace this fault back?
Or, have I missed something simple?

Change the plug connectors under each coil ...all 3 of them

..and check the wire harness for flaking insulation..

Archieves search

http://catalog.eautopartscatalog.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=1D00YU6RY1JX0Q0NKY&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1994&product=F1010-84561&application=000357356

Thanks for the reply.
I swapped the coils with the connectors.
The wiring harness is not flaking (I'm aware of wiring harness problems).

I can only think that there is no input to this coil but I'm not sure how to check this or trace it further back.

Coil is hot 12v all the time ..ecu is a switched ground.
First try would be to change the plugs on 1 and 6 . A bad plug will do the same as a bad connector.. and see if you have 12v to that coil.

I don't understand your comment asbout 'Same with HT leads 1 and 5
???

Hi Arthur,
I think we're getting somewhere now. The Input to the front two coils is 12v but the rear one is only 6v. Measured with a very crude analogue voltmeter, coils disconnected.
Any idea why the rear one would measure lower?

Seen that before
Cause was bad connection in harness where the three wires branched off the main feed to each coil.

Stripped the loom as far as the 12v splits into 3 but all looks okay.
If I measure the rear coil using the +/- on the connector I get approx 6v but if I use the +ve and any -ve on the other coil plugs (or any earth point) I get 12v.
Looks like the fault might be to do with that coil's -ve supply? Any idea where that goes?

Do not use coil connector plug for ground for this test
Test the pos of the connector and pick up the neg at engine ground .. all should read 12v. [ coil feed connectorunplugged]

In that case all the coil feeds are 12v.

OK
now test each connector from engine ground to connerctor neg side
[ unplugged , key off] with an ohm reading

All three read about 2 Ohms (again with cheap analogue meter).

The neg connector goes back to the ecu , Pin 21 [ blk] for coil T/3 [ plug 1/6]
T1 is pin 8 , T2 is pin 9.
You may have a bad connection there .
You have the early FM system and the ecu was changed for the 94/95 versions . I remember seeing a service sheet here somewhere that stated these had a problem was switching transistor heat sink dis-connect for 1 and 6 circuit.
Let me see if I can find that and I will get up schematic
This may also show up on pin #8 of diagnostic socket as a code 21 for T3 output.
Simple led tool will get that info..

Hi,
I checked the wires back to the ECU. T3 is pin 65, T2=pin 54, T1=pin 53.
Therefore the problem seems to be with the ECU.
Can I just get one from a breaker or are they set up differently with the eprom for every car?

<<I checked the wires back to the ECU. T3 is pin 65, T2=pin 54, T1=pin 53.
>>

Those should be the # on the ecu, the connector # should be t1-9,t2-10, t3-21. these line up with your numbers.

color codes should be
t1-blk/yl
t2-blk/wt
t3-blk

couple of quick things to try first.
compare omh readings from blk at ecu end back to coil connector with the other two.
In other words ,check resistance of the wire from coil back to ecu for each coil at both ends for resistance.. and also check each to ground.
A scope would be nice , but I would also go into the pin 8 of the diagnostic box with a code tool and see if you have a code 21 .
Can you build this tool ?

http://www.peachparts.com/shopforum/showthread.php?t=107630

Ahh, I see, my numbers were from the ECU, yours from the loom.

The resistance of the wires is much the same for all three. (0.2-0.3).
So I think we can rule out a wire problem.

I built the LED checker and connected it up to pin 8. The LED was on when the ignition was off but off when the ignition was switched on (if you see what I mean). No amount of button pressing would get the LED to fire up with the ignition on.
I wonder if my car is different because it was first registered in Singapore?
There is no connection to pin 14 on the 16 pin connector.

I'm in the UK and I've found a breaker who has one for a W124 E320 4 speed so I'll order it tomorrow. The catch is it's £245 (about $500US). :eek:

If that doesn't cure it I'm going to cry!