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  #1  
Old 10-02-2003, 01:48 PM
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Need help with electrical to injectors. Where's the ECU?

I have a 74 450 SL with sudden onset of erratic power. The engine will stop running on 4 of its cylinders (numbers 1, 4, 5, 6).
I've pulled the spark plugs, they are good and are getting a spark. I pulled the electrical connectors to the fuel injectors and found that there is no power going to the four dead cylinders when it acts up. During the times it fixes itself, there is electrical current flowing. The Haynes manual says the ECU is near the battery but it isn't. Another source says it is on the firewall beneath or by the passenger floor mat but a cursory look couldn't find it. Is it there? Could this be where the problem is or should I look elsewhere? I don't see a common thread between these four cylinders in the electrical schematics.
Thanks for any input.
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  #2  
Old 10-04-2003, 11:47 PM
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I don't know about a '74 450sl but on my 1980 450 sl the ECU is under the floor mat close to the firewall on the passenger side of the vehicle. It is under a trap door that is held on with one screw.

Good luck. Hope your ECU isn't a windows based one (crash!)
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  #3  
Old 10-05-2003, 07:18 PM
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Thanks, I found the ECU. Now to try and open it to see if the problem lies within.
Thanks again.
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  #4  
Old 10-05-2003, 07:33 PM
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Quote:
I pulled the electrical connectors to the fuel injectors
This is a CIS injection system. It isn't sequential fuel injection. There is no electrical connector on the individual injectors, just a fuel pipe. Except the one cold-start enrichment injector.

Gilly
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  #5  
Old 10-07-2003, 02:46 AM
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Gilly,

A '74 SL would be D-Jetronic, not K-Jetronic. CIS wasnt used till about '76 or therabouts. OB Joe has enough to keep him concerned without confusing him further


Joe,

Bad connection would be my guess Joe - that big wide sucker back on to the ECU main board would be where I would start with some contact cleaner. If its like all three of my D-Jet cars there will be oxidisation all over the contacts. Theres no intermediate connections between injectors and ECU so if it isnt there then its within the ECU - best of luck.
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  #6  
Old 10-07-2003, 07:31 AM
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I would also look at the trigger points in the dist. for oil contamination.

Gunny
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  #7  
Old 10-07-2003, 05:46 PM
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Actually thinking about it the trigger points are the most likely cause for you to have 50% of your injectors failing at one time.

Start stripping your dizzy ......
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'71 250CE (sold)
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  #8  
Old 10-07-2003, 09:20 PM
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I did look as best I could at the injector contacts but could not really pull the whole tray out all the way. Do you think I need to pull the entire distributor out to make sure?
And yes, every one of my injectors has not only a fuel line but also an electrical input.
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  #9  
Old 10-09-2003, 03:56 AM
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OB Joe,

If the o ring on your dizzy drive shaft is shot then oil will be contaminating the trigger points for the EFI. This should be self evident due to the quantity of oil in the dizzy. If its clean then thats not your problem.

Other possibilities:

Trigger points worn out/ failed - check with MB for replacement cost
Trigger point connector (just outside dizzy housing - small white flat connector with 5-6 pins) has a bad/ intermittant contact
ECU connections as mentioned above
Dry joint on the ECU motherboard ... a sod of a thing to find unless you're some kind of electronics propellor head.

As you say its affecting 50% of your injectors at the same time everything points to the trigger points.

Keep us informed of what you discover & best of luck.
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'73 350SL manual
'98 528i (yeah I know but at least its German)

'71 250CE (sold)
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  #10  
Old 10-10-2003, 05:22 PM
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Well I opened the injector trigger tray as best I could without dismantling the distributor. It did not look oily wet. I sprayed it all down with electrical part cleaner (Lektra-Motive by CRC), hit it with compressed air and let it sit for about an hour before putting it back together. At first it coughed and hesitated but eventually started firing on all 8. This morning it was sluggish with poor power but after a few miles perked up and has been running fine ever since, not a hic-cough. I hope it lasts or else I will likely end up stripping it down and replacing the contacts.
Thanks again for all your help.
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Need help with electrical to injectors. Where's the ECU?-p1010152.jpg  

Last edited by OB Joe; 10-10-2003 at 05:33 PM.
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  #11  
Old 10-24-2003, 03:03 PM
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ECU, ICU, Tune up parts

Hi Joe

I recently replaced the fuel injectors on my 120,000 mile 500sl (Euro) and they are not electronic injectors. I would like to see a photo because in 1985 they were not electronic. Check the ICU in the drivers front fender well and see it it is hot. I have the manual on CD and it helps a lot. You can buy it here on fast lane.
Best luck,
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  #12  
Old 10-28-2003, 08:53 PM
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Hey Steve,

The ECU does not get hot. Here is picture of the injectors and you should be able to see the wiring coming in on the right side. One of them has the rubber boot on, the other has the boot pulled back.
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Need help with electrical to injectors. Where's the ECU?-injectors.jpg  
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