'73 450 SL electronic fuel injection

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Isdyldan

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Hi - sorry but I made a mistake before. The strange connection on HT lead 1 actually goes under the distributer and up to a socket with a cap on it. This looks like a connection for servicing like resetting the service lights on a BMW. Still quite odd but probably a method for adjusting the timing.
 
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Hi, Yes OSC you are right about the extra wire on the throttle body. It does tell the ECU that it's at zero throttle and leans out, but also tells it that adjustment to the mixture via the knob on the ECU can be made. I've just had a brilliant session with Joe of SS on the car who expained all sorts of things. Unless the ECU knows the throttle it at zero it is not possible to adjust the tickover mixture. I now need to swap over the 4-pin throttle switch to the 2-pipe throttle body I have because it is simpler than running an extra wire to the ECU. Another thing Joe diagnosed was that the distributer vacuum unit is knackered and need exchanging - ie the engine is completely out of timing currently due to no automatic adjustment from this unit. There was also a vacuum leak from a missing pipe to the heater accuators. I now have a few tasks to sort out until my next lesson!
 
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Throttle switch and distributer sorted out now and all is running fine at last! Further investigation of the spark plug lead attachment shows that it is attached to a sensor down near the crank and also bach to the rev counter connections near the ballast resistors. Now the throttle switch tells the ecu when it is at tickover, the mixture can be set via the knob on the ecu. Tickover speed was adjusted using the large air screw at the front of the engine.
 

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Throttle switch and distributer sorted out now and all is running fine at last! Further investigation of the spark plug lead attachment shows that it is attached to a sensor down near the crank and also bach to the rev counter connections near the ballast resistors. Now the throttle switch tells the ecu when it is at tickover, the mixture can be set via the knob on the ecu. Tickover speed was adjusted using the large air screw at the front of the engine.

wow you've made my year! would love to come over at some point and hear her running.
 

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14,105 views this thread, with a happy ending as well :D
 
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Thanks, and yes OSC any time! There is still some tidying up to do, and fine tuning as it often sounds like there are only 7 firing. The ignition unit which controls the coil spark is not connected and I am still using the 'ebay' box. This is because the diagram in the Haynes manual seems to indicate that the +ve side of the coil (terminal 15) is connected to earth, which doesn't seem right to me. I tried copying the wiring in the silver car I bought only to find there is no ignition box and the coil and points are wired conventionaly. It looks like it has been like that for a while so I expect the ignition unit failed years ago. Could someone tell me how the ingnition unit is connected please?
 

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sorry i'm not getting the alerts anymore when the thread is updated. have you found the wiring for the iggy unit? i can dig it out if you still need it.
 
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Hi OSC, Thanks but only if you get a chance please - it's not urgent now as I've fixed the new ignition unit for now, but may swap back to the original in the future. Am now preparing for interior retrimming in early march and maybe a respray.
D
 
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Hi all - I have decided to wire in the proper ignition unit as without the wiring correct I have no rev counter. Car runs beautifuly now as we went over to see SS motors near Weybridge last week and Joe tuned to perfection.
I am following the wiring diagram in the Haynes manual for 1973 model and I just want to double check - it indicates that the possitive side of the coil (terminal 15) is connected to earth, and the negative (terminal 1) is connected via a 0.6Ohm resister to the ignition unit. Does anyone know if this is correct please? Seems odd to connect the possitive side to earth!
 

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That is incorrect, the + side of the coil is fed from the ignition switch, if a ballast is used it will be in series with this lead and the starter motor is used to short this ballast resistor out when starting. The - lead goes to the distributor where the contact points ground this lead to energise the coil and produce a magnetic field, it is when the points are opened that the induced magnetic field gives the spark to the plugs.

The correct capacitor over the points should be 2,2mfd for a 12v coil and 3.3 mfd for a 8 volt coil that is used with a ballast 500volt working DC gives 250 AC working.
 
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Ok thanks - so the Haynes diagram must be wrong then. I have tried to attach an image of a section of it - actually the 1974 model diagram, and on my coil terminal 15 is definately the + and 1 is the -, and 4 is the HT lead to the distributer. On the attached diagram:
49 coil
50 resistor
51 ignition unit
46 distributer
Clearly shows terminal 15 conected to earth, unless I am miss-reading this diagram somehow?
 

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OK this has transistorized ignition, this is rather different, that circuit is correct as the coil on a conventional is grounded via the points , the + on the coil must be grounded so that circuit is correct. The coil manufacture never ever changed the markings from the older days
 
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Thanks - I'll give it a go then and hope for the best!
 
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I know it took me a LONG time to get round to doing this, but didn't want to mess things up when the car was running so well. Anyway, I followed the Haynes wiring diagram and even though I was dubious about wiring the +ve terminal on the coil to earth, she started up straight away. And the rev counter worked! Brilliant - now just got to get the speedo working.....
 
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Ok - it's official - I own the worlds most annoying car. It is now only firing on 5 cylinders! It was running on 6 earlier with just cylinders 1 & 5 with no spark, but I swapped over leads and plugs 5 & 6 and now no spark at 6 either! Can only assume the ignition unit is on the way out, unless anyone has any ideas please? I saw that 1 and 5 are next to each other on the distributer, but checked that the points were opening as thought maybe rotation may be bent.
 
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Just on the off chance that anyone is still interested in this, it turned out to be simply spark plugs. I have no idea why as they were all quite new, but I replaced some and all running fine. I first swapped out an injector as believe it or not this was less hassle! I know nothing about spark plugs and why they fail sometimes. The ones I took out were Champions and they had a core resistance of about 70-80 Ohms, and the ones I put in were NGK which were about 10ish Ohms. Maybe I have a weak spark and the lower resistance is making all the difference.
 

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This would not make a difference I feel,,, perhaps the insulation is down and that most of the spark to ground before it ignited the fuel. Sparking plugs behave very different when sparking in free air to when they are under compression.

Good news though and nice to know, in the olden days we used to sandblast them and this took away any conductive film that can build up on the insulator
 

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I know it's a German car

but it needs an Italian tune-up.
I'm glad you've sorted it Dylan. those plugs are a pig of a job, and I'd rather do the 12 on my w220 than the 8 on the SL. Talk about double-jointed contortionism.

If it's any consolation, I haven't used Champion since my Dad;s day. I have always preferred Bosch or Beru or NGK. I have given up on Bosch, so am running NGK's in the SL since the last change. I had no trouble with them in about 150,000 miles in my old Beemer ( natch, I did change them a few times)
 
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Yes it is a bit of a fiddle - I had to remove the header tank to get at one them! Regarding the tune up, you're right, she's booked in to SS on Wednesday next week so very excited about that.
 

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