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  #1  
Old 08-12-2016, 07:01 PM
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CIS ECU for 1986 300e

How interchangeable are the ECU's for the M103 300E's ? Looks like mine is failing and I need to find another, but I'm not sure whether I need to look for an exact match or if there is a degree of interchangeability.

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1986 250D 5-speed, 210,000
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1986 300E 112,000
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  #2  
Old 08-12-2016, 07:23 PM
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They're relatively robust modules, what make you think yours is failing?
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  #3  
Old 08-12-2016, 08:00 PM
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This assessment comes from my mechanic. I've had some warm shut down issues -- car would stall out at a light and not restart until much later. After a crank sensor replacement didn't help, the mechanic (who just does MB and has been pretty good in my experience) noted that the engine is flooding and tried another ECU from one of his cars which eliminated the flooding issue.

I have to say, I love my diesel -- many fewer gremlins than this 300E

Geoffrey
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1986 250D 5-speed, 210,000
http://images.spritmonitor.de/407216_21.png
1986 300E 112,000
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  #4  
Old 08-12-2016, 08:13 PM
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Get rid of that mechanic and give your car a full ignition system tune up with quality parts.
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  #5  
Old 08-12-2016, 08:43 PM
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Yes but, the cap, rotor, plugs, and coil have 4000 miles and the plugs about 2000. All bosch (except the plugs which are non-resistor denso 5031.

I admit ignition was my first thought.
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1986 250D 5-speed, 210,000
http://images.spritmonitor.de/407216_21.png
1986 300E 112,000
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  #6  
Old 08-13-2016, 09:25 AM
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Quote:
Originally Posted by geoffreymbrown View Post
Yes but, the cap, rotor, plugs, and coil have 4000 miles and the plugs about 2000. All bosch (except the plugs which are non-resistor denso 5031.

I admit ignition was my first thought.
How about wires? Also don't rule out that one of your parts could have been installed incorrectly and is now broken again.
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  #7  
Old 08-13-2016, 09:29 AM
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Or even better, have that mechanic buy that $1200 module, tell him that f it doesn't solve your problems 100% that you are not paying for it. If the mechanic is going to guess at his job, instead of careful diagnosis, let him guess with his money, not yours.
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  #8  
Old 08-13-2016, 11:32 AM
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Thank you.
Geoffrey
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1986 250D 5-speed, 210,000
http://images.spritmonitor.de/407216_21.png
1986 300E 112,000
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  #9  
Old 08-13-2016, 11:54 AM
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Just to be clear. I think the root cause of my problem is likely to lie elsewhere, and I'm only considering trying out a used ECU. I do think it's likely that the existing ECU has issues. The previous owners (in a different location and different mechanic) also had stall/non-restart issues from time to time. At some point their mechanics (who I definitely think were crooks) sent the ECU for "repair". Not sure what this means, but it leaves me wondering about the reliability of the existing ECU. It is also the case that my car has been running very rich and reportedly running with another ECU resolved that issue.

I do trust my mechanic, but I also ask a lot of questions. With all intermittent issues (of which I've seen many in my career as an electrical engineer), debugging is difficult -- if you can't reliably reproduce a problem, then it's hard to find the root cause.

I agree there is a chance that the cap and rotor are an issue, but internal inspection didn't reveal an obvious problem. The CPS was a reasonable thing to replace -- the mechanic did test the other one and found it sub-standard.

Geoffrey
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1986 250D 5-speed, 210,000
http://images.spritmonitor.de/407216_21.png
1986 300E 112,000
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  #10  
Old 08-13-2016, 12:04 PM
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As an electrical engineer, you will be competent to open up the fuel pump relay and OVP relay and resolder any questionable solder joints. There are usually quite a few which can lead to intermittents as you know.

I deleted my previous post as to the interchangability of the modules. I was thinking of the ignition module, not the engine control module.

Last edited by Rick76; 08-13-2016 at 12:15 PM.
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  #11  
Old 08-13-2016, 12:36 PM
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Quote:
Originally Posted by geoffreymbrown View Post
How interchangeable are the ECU's for the M103 300E's ? Looks like mine is failing and I need to find another, but I'm not sure whether I need to look for an exact match or if there is a degree of interchangeability.
To answer the original question - that of interchangeability - look through the FSM to see the variations of the same theme.

Around 1988 and 1989 (if I remember correctly) the sensors and wiring changed a little bit. So I wouldn't get anything from 1988 onwards it is unlikely to be a plug and play solution.

The addition of a sensor on the back of the speedometer (road speed sensor) was something that happened towards the end of the CIS engines: If your car was newer I'd say that that might be worth checking but I don't think it was fitted to 1986 models. (Still thumb through the FSM and check)

So long story short - if you really want to swap ECUs it is probably best you get the same Bosch part number - this would limit the cocking about

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