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1973 450 SL
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Discussion Starter #61
This oscilloscope picture was taken of the pulse generator channels of my engine running at idle. From top to bottom - 13, 22, 21, 14. I have not figured out how the ECU pushes the 5V carrier signal but you can see the result of the points - 5V square wave. The cams and positioning of the points cause the 90 degree phase to occur. Any work with Norbert's tool affects the period of the opening / closing (horizontal parts) of these traces. Bouncing that Graham and the Porsche guys refer to would be evident as disruptions to the pattern seen below. The spikes seen at the 4th peak of trace 13 and the 3rd and 5th peaks of trace 22 are noise - the oscilloscope used is way more sensitive than required for automotive electronics.

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I know this is not much use to those of us who don't have an oscilloscope (I had to borrow one from a buddy). I post this simply to enhance understanding of what is physically going on with the pulse generator and the 700 series components in the ECU. The ECU uses the rise to 5V of each channel to set timing for the start of injection pulses. I've highlighted the PG terminals at the ECU and the area of the main PCB that handles these signals.

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The flip flop circuit aligns these signals to the injector pairs at ECU terminals 3, 4, 5 & 6. Disruptions to the square waves in the first picture cause problems in cycling of injector groups (1 group = 2 injector pairs). The ECU does this work in the 800 series components . . . starting to go beyond the scope of this thread.

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Bottom line . . . Norbert's tool keeps this part of the ECU happy on a worn set of trigger points. This is the foundation for the basic injection pulse. ECU biases for temperature, operator demand and engine load depend on this part being in spec.
 

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'72 450SL, 107.044-12-000422
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The first graphic in CaptainSLC’s ECU schematic shows the output from the ecu to each of the points. I can’t calculate the theoretical signal voltage because I’m not sure when that Z diode breaks. Cush measured 5v on the scope, cjhols is measuring 2.6v on 2 of his four outputs.

Point is, there should be some voltage from the ecu while cranking on all four points. So, seems like bad continuity in the harness/connectors or a bad ECU.

I think........


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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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Hi,
measuring the Voltage at the working trigger points is a testing method Bosch describes in the special D-Jetronic manual describing the testing procedure using the EFAW228-D-Jetronic tester.
@MBGraham, I think I sent this to you some years ago?
Regards
Norbert
What I posted above about voltage at trigger connections was from memory. I knew I had read it somewhere. As it turns out, it is in the early Bosch Djet Training manual. I have uploaded thatto BW several times, but the links keep getting broken. It is now on BW member oncebitter's site if you don't have it: https://www.tonk.ca/misc/djet-efi-manual.pdf This is Page 30.
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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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Sorry to be a pain,
I put the distributor back on and noticed that it was only running on 4 cylinders 2 pairs (1 and 5 and 4 and 8 are NOT working).
How you making out Peter? Your problem seems a bit lost among the Tech stuff :)
 

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1973 SLC 450
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Hi all,
Just an update I went out and "mapped" the ignition points on the distributor to the opening/closing of the trigger points ( I don't think it has anything to do with my present problem) but all the ignition points (bar the green #14) are exactly the same as the closing (impulse) of the trigger contacts. I thought they had to be roughly 15° before spark??? I am wondering, and I'm not sure if this is possible, whether the guy who refurbished my distributor (and again I don't even know if it comes apart) may have split the shaft and put it back together out of its original alignment.???? I also checked the corresponding injector leads for voltage and there was none. I'm sorry about the quality of my drawing and the colours on it match the coloured wires at the trigger points. Someone replaced the original. I really am grateful for all the help you guys give and enjoy working on the old girl - bit like a 1000 piece jigsaw puzzle -sometimes with 1 piece missing :unsure:.
Thanks again
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Discussion Starter #66
Injectors fire approximately 180 degrees prior to ignition of the lead cylinder in the injector pair. Injectors 6&3 open near TDC for cylinder 1.

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Hi just clarifying, doesn't #21 trigger point connect to cylinders 1 and 5 ? If so I was going off Oltimer.tips article... And isn't this crankshaft rotation not distributor rotation as in my mind (and I'm probably wrong) but 360° by 8 cylinders = ignition every 45 °...
 

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Peter,
On the ignition points, I don’t think it’s too important that spark be perfectly timed with injector pulses. The injectors fire into the manifold, so the fuel/air mixture sort of “hangs around” there until the intake valve opens and piston sucks it in. I’ve actually accidentally swapped injector plugs before and saw no noticeable difference in running!

Regarding trigger points, were you able to check continuity of the the four wires? I would disconnect from ECU and check between there and the trigger points.


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Hi just clarifying, doesn't #21 trigger point connect to cylinders 1 and 5 ?
This is a question, not an answer :)

Trigger point #21 (and 22,13,14) connect to the ECU via the harness. The ECU then (after a lot of AndOr-ing, Flip-Flopping etc provides output via pins 3,4,5 & 6 which the harness carries to the injector pairs.
What happens in between is too complicated for this Chemical Engineer!

Brad or anyone who knows - Which trigger point pins are associated with which injector groups? Is that shown somewhere?
 

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Discussion Starter #70
cjhols,

Ignition in a 4 stroke engine occurs once for every 720 degrees of rotation of the crankshaft.

Graham,

I had this wrong for years - Volker set me straight on it . . . refer to the diagram above in post #66 and the table on this page 7. Trigger Contacts

Group III - 22 - 6&3 - open at TDC
Group IV - 14 - 7&2 - open at 180 degrees ATDC
Group I - 21 - 1&5 - open at 360 degrees ATDC
Group II - 13 - 4&8 - open at 540 degrees ATDC

Note that when you group the injectors in order, the M117 firing order is made apparent [1-5-4-8-6-3-7-2]. Also note that Bosch and MB technical references keep 6&3, 7&2 semantically reversed because of the firing order.

The ECU toggles power between 2 groups of injector pairs during the engine cycle. Groups I/III and Groups II/IV are separate channels. I'm getting all this from @CaptainSLC 's recent work on the ECU schematic.
 

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cjhols,

Ignition in a 4 stroke engine occurs once for every 720 degrees of rotation of the crankshaft.

Graham,

I had this wrong for years - Volker set me straight on it . . . refer to the diagram above in post #66 and the table on this page 7. Trigger Contacts
Thanks Brad. I looked at Volker's site, but somehow missed that! Not really important to me, but when I looked at 4-cyl ECU wiring diagram , it wasn't apparent which trigger contact led to which injector group. The lines seem to Criss-Cross :)
 

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Hi all,
I think I found the issue. The Yellow/Red wire in the connector from the ECU had broken off # 21 and I assume this effects its partner Yellow/Blue #22 as well ??? I haven't fixed it yet but I'm hoping all is good??? I'm wondering whether I shouldn't change the connector whilst I have it off??? Thanks you all for your input - although I don't understand every detail I like the learning experience.
Thanks
Peter

PS. All good except I found that #1 cylinder is not firing. Is it possible to have a faulty injector that still shows a resistance? O'h and I'll check the compression. leads and plug tomorrow. Enough for 1 day.
Thanks again.
 

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Excellent!

Yes, as Norbert points out in #60, one bad trigger point circuit will prevent its pair from signaling. Replacing the connector depends entirely on its condition.

As to the number one injector, are you checking continuity from the ecu or at the injector? It is possible to have a clogged injector that shows continuity. You could verify operation by removing the injector but leaving it connected, crank the engine and check for fuel spray.


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Hi all,
Success. Apart from the gremlin in the wiring and after testing the injector and compression I checked the spark plug ( I should have done this first but as these had been fairly recently replaced I didn't suspect them). Sure enough, very weak, erratic spark. Replaced the plug and she runs beautifully... Thanks again...
Peter
 

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Great News...
but one question remains: You told before, the distributor has been revised, and you found oily trigger-points afterwards?
I hope they didn't clean it as they should have done.
It's not easy to replace the Shaft seals...

Regards
Norbert
 

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Hi Norbert,
I had the distributor refurbished as it had slight movement at the top. The person took it completely apart and did a great job of getting it back into its original shape - bead blasted and silver cladded the housing, replaced the points and tested, cleaned and readjusted the vacuum unit assembly. He also had a machine which he tested the unit - unfortunately he was not concerned about the movement which would have saved me a lot of money as this was the reason I sent it to him. Whilst this was happening I also ordered new trigger points (these were very short) as this was another reason I thought the car was running rough - However these were never oily. He did put some high temp grease on the cam shaft but I wiped most of this off as I've read it can effect the trigger point contacts. I also found a good quality second hand T/C which I also fitted, new distributor cap and rotor button. The car is now running very nicely ( but I do wonder whether it was really the issue with the broken wire at the Trigger Contact connector and the spark plug which was the real reason I had issues).

I do have a question though, what is the reason that these earlier cars have the timing set at 5 degrees ATDC ?
Also whist the car is running beautifully at speed at idle it does have a slight gurgle - like it has a mild cam in it - is this normal?

I have to say that even though it must be quite frustrating trying to educate the inexperienced I am really thankful for the input that all of those that contribute to the forums.
Regards
Peter
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The 5 degree ATDC is an attempt to reduce emissions during idle. It is not neccessary to set the timing that way. In fact it is turned off whenever a/c is on or if coolant is over 100C.
Your car will run better if you set timing at 5 to 8 BTDC with the vacuum line to distributor disconnected and plugged. Reconnect vac after setting. At idle, you will now be at tdc - 5 BTDC Timing should then advance to about 27 BTDC when you rev to 3000rpm. If it doesn't, then there is a problem with centrifugal advance (internal weights)
 

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Excellent - He did test the advance and said it was working but I'll change the timing and check. Any thoughts on the slight burble at idle?
Thanks
Peter
 

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I don't know what a burble is, but you can fine tune idle mixture using the knob on ECU. Set it in middle of range for a start. Then try moving it one way or another 1 or2 notches.rev engine in between and let it settle. Find the sweet spot. This is just fine idle tuning - you cant mess anything up.
I would adjust timing first. That might also even out idle.
 

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Thanks - I've used the idle mixture knob before. It's like it has a mild cam in it. Just a little burble at idle... Not really worried about it I was just wondering if this was normal?
Peter
 
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