On a good note (420SEL "mostly" fixed) - Mercedes-Benz Forum
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post #1 of 13 (permalink) Old 08-13-2019, 05:01 PM Thread Starter
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Date registered: Sep 2010
Vehicle: 1986 Aust. spec 420SEL, U1700L Unimog ex.ADF
Location: Canberra, Australia
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On a good note (420SEL "mostly" fixed)

Hi all,

Been having a bit of a hell of a time with my newly acquired 86' 420SEL, anyway with a commited attitude and wanting to fix the problems, I've slowly been working away at it.
The car had a bunch of problems, of which the previous owner had masked and having only owned a diesel W124 previously, I didn't know what the symptoms were I should be looking for, anyway we live and learn I guess.

When I purchased the car, it was night time (after work), in Sydney, Australia where it's hot (I live in Canberra, where it's cold) beginning of May this year.
It idled pretty well, even though the idle was a tad high 850rpm hot (but didn't actually notice that till after the purchase).
Ran well, didn't sputter etc. issue with left mirror, power antenna and right rear window moving.
Had a couple of suspension knocks which I thought were some bushings that needed doing (it's an old car and expected).

So, after purchase:
I pulled apart the mirror and fixed the motors.
Pulled apart the rear door and fixed the broken sliding shoe.
Pulled apart the power antenna and fixed "everything in there".
Car had a leak in the boot, was the tail light, fixed that too.
Knock in the front was the right outer ball joint, swapped it and fixed.
Knock in the back is the strut top bushings I believe (they just arrived so I need to do them soon)
Then onto the seemingly high idle I had... ohhh and terrible cold start

This got interesting and involved wayyy more learning then I would cared to have learnt, this is basically the very short version of how events took place.

1) Found OVP fuse blown
2) Replaced it, car would die when hot due to low revs at idle.
3) OVP replaced, no change.
4) ICV pulled, noted it had been gutted and a necked down rubber hose inserted.
5) Non-genuine ICV purchased and fitted, problem made significantly worse (1600rpm idle).
6) Idle Control Module (ICM?) opened and resoldered, no change.
7) ICM changed, no change in problems.
8) Many MANY things tested, temp sensor swapped.
9) Brand new ICV tested, wouldn't close with 5v, spring adjusted, still no dice.
10) Over-priced used OEM (new in 2016) ICV purchased, fitted, idle fixed instantly (550-600rpm in gear, 850-900 neutral) - which I still think at this time is a little high.
11) Strong emmissions smell from car noticed under ANY conditions, idling or at speed (idling would have visible "fog" from the tailpipe) and slightly "bumpy" idle when hot.
12) Injectors replaced with new bosch, no changes noticed... except seemingly by coincidence...
13) Hard cold start condition returned (was resolved after the OVP fuse was initially replaced).
14) Many MANY things checked/tested again (inc. CSV, no voltage was getting to it)
15) Fuel relay replaced, no change.
16) Pins to fuel pump relay tested, instead of 3-5v, 2v was read.
17) Random - seatbelt light begun staying on (constant flashing), globes removed to "fix".
18) ECU replaced with 2nd hand unit after I resoldered the pins.
19) Cold start fixed, bumpy idle fixed, idle in neutral lowered even further (800rpm) AND most notably emmissions smell/fog fixed!

It would seem my issues where casued by a bad ICV which was replaced with a NEW BAD ICV.... AND a bad ECU.

Who would've though a bad ECU could cause such failures!
Idled okay, controlled the ICV correctly, seemingly was working the CSV for "a period of time"...

My old ECU has visible burnt connections, a split open resistor and burn smell (only on the board once removed from the case).

So, while I understand there are many things that cause a blown OVP fuse, it seems another one can be... blown ECU!

Now just onto fixing my strut bushings, a dodgy dash light (have globes), odometer that's broken (have new gears) and spongy seats lol.


It would seem these machines can still be fixed, but by god there is some complications... I like my mechanical diesels much more any day!
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post #2 of 13 (permalink) Old 08-13-2019, 05:03 PM Thread Starter
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Date registered: Sep 2010
Vehicle: 1986 Aust. spec 420SEL, U1700L Unimog ex.ADF
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Ohh and replace the fan belt idler pulley that rattles (new one sitting on my desk) and fit the accumulators that are in my garage.
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post #3 of 13 (permalink) Old 08-13-2019, 07:14 PM
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Wow. You've been busy! Glad to see you're sticking with it.
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post #4 of 13 (permalink) Old 08-13-2019, 07:24 PM
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I also had some of the same issues you describe, and with a lot of help here, I've been able to sort it out, and now have a very good running car for the most part.
Would I do it again? oh yeah!

One symptom at a time.
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post #5 of 13 (permalink) Old 08-14-2019, 02:09 AM
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Went through with most of the things you mentioned and then some. Had all the intake gaskets changed. 8 o rings, intake boot,( throttle body intake boot ). Injector seals, lower intake gaskets. Bunch of vacuum lines.

Itís great to know what is wrong if your intake system is not in good shape. This can get expensive.

Overall these cars are not as simple if you donít know what you are doing. But there is a lot of information on this forum with people to help at any given time.

Again, not as simple as say my 92 Lincoln LSC, where any idle issues are either a throttle position sensor or idle valve, or 1 or 2 vacuum lines.


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post #6 of 13 (permalink) Old 08-14-2019, 03:11 AM
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Spongy Seats can be fixed with a $3 foam pool noodle for swimming pools
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post #7 of 13 (permalink) Old 08-14-2019, 01:29 PM Thread Starter
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Odometer and instrument cluster light now fixed also (I think, need to drive it first!)
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post #8 of 13 (permalink) Old 08-14-2019, 04:55 PM Thread Starter
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Date registered: Sep 2010
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Quote:
Originally Posted by STOIE View Post
Odometer and instrument cluster light now fixed also (I think, need to drive it first!)
I have to add though, never have I seen a car with soo much deception - which ultimately has made it so hard to resolve issues with it.
The person who sold it to me, I wouldn't rate as someone I care to know.

Turned out the check engine and abs lights had been pulled from the cluster...

I know the reason, thinking now...

Going back, when I bought the car it had a blown fuse in the OVP - I know now that it was placed there intentionally.
Used to mask that the ICV had been screwed with and ultimately rendered unusable.
And then the ABS light pulled to again mask the fact about the OVP.

Had I have gotten in the car and seen the ABS light on, I would've walked away... but the bastard had pulled the wool over my eyes.

I get that buying cars privately (actually in general) is allways a risk and I've bought many, but it's sad when you meet someone that'll happily lie to your face to make a dollar.
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post #9 of 13 (permalink) Old 08-14-2019, 05:20 PM
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Your experience is exactly why I still search for a pristine 300SD/L or a 2005-2006 CDI 320.

My 90 SEC generally starts right away with only an occasional stumble, particularly after its been sitting. Every once in awhile the idle wanders away from straight and level but that is generally resolved once I have driven it a bit. This car hates not being used. But, having read most of this forum since 2012 I know that I'm lucky. And since I am not the master tech, I have trepidation about what potentially lies ahead. I can hold my own with brakes, bolts and such. Electrons? Not so much. I'm like a dog watching TV when I have a multimeter in my hand.

Conversely, my 85 300CD can be put away wet and ignored for months. Yet, it has already started by the time I even think about releasing the key. I have learned to just bump the starter once the GP light is out. Boom. Yes, it sounds like a toolbox in a dryer for a bit, but it's as reliable as tomorrows sunrise and makes me smile like an idiot with its simplicity, clatter, and overwhelming desire to combust.

Thanks for posting. This is the type of repair info that makes this forum a gold mine for me.
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post #10 of 13 (permalink) Old Yesterday, 12:36 AM
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Quote:
Originally Posted by oldmako View Post
I'm like a dog watching TV when I have a multimeter in my hand.
Funniest quote I've seen in a while 😂😂

'86 420SE
'90 420SEC


Daily transport 9spd Giant DH Comp downhill bike
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