ICV Weirdness (among other things) - Mercedes-Benz Forum
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post #1 of 7 (permalink) Old 05-18-2019, 06:02 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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ICV Weirdness (among other things)

Hi all,

I recently bought my w126 1986 420SEL Australia model and I bought it in a hot climate so didn't realise the cold start issues it has. However, I did know it has a high idle in park and neutral (wish I had done this research more before buying the car). Anyway, it was cheap so I cannot complain too much.

So, I've been trying to track down the issues. And this is what I've found:

1) idle in park is at 650ish cold, 1150 hot. In drive 600ish forever.
2) has a strong fuel smell when idling for an extended period, like it's too rich.
3) the idle is a bit "bumpy" when stopped at traffic lights and it stutters while revving when you peg the throttle quickly (eg. no acceleration enrichment).
4) the OVP relay had a blown fuse in it (so, literally no computer or tuning, explains pretty much everything above - correct?).
5) replacing the OVP fuse causes too low an idle and the stalling condition on idle once warmer (even when driving down-hill etc, literally not drivable, so fuse removed again).
6) I tested the ICV in the car, gets a click with a 9v battery attached.
7) replaced the voltage regulator in the back of the alternator (brushes were pretty darn worn).
8) new OVP ordered (less than 2 weeks away from arriving)
9) pulled the idle controller from the passenger footwell, opened it up and noticed that two resistors were replaced with a single one in the middle of the board (not on the little daughter board part). But job looked good everything on the board appears fine (photo attached).
10) pulled the ICV out of the car, found as per the attached photos (cracked housing, hose stuffed down the guts, suspicions about the little crossover tube etc.)

My questions are:
1) do you think I'm on track/going the right way about this?
2) do the things I have encountered all add up?
3) with the ICV in the photos, why would have someone shoved a hose down into it?
4) is the little crossover tube that by passes the ICV meant to have an open end (aka so air can flow through it) or is mine correct with the closed end, as per the photo? (want to know if I need to replace that little tube too).
5) I'm going to be buying a new ICV, putting the new OVP in and provided my idle controller is okay, do you think this *should* fix my problems? (basically I'm asking does everything add up right)

1) I'm also thinking of changing all the injectors out for new
2) putting a new bosch fuel pump in
3) new fuel filter and accumulator
4) apparently the distributor and leads were done on it, but you know how can I trust that in any way (but, they don't seem to be a problem).

Be really keen to hear from people!

Appreciate any help!

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post #2 of 7 (permalink) Old 05-18-2019, 06:44 PM
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Vehicle: 1989 560SEC, 1989 560SEL, 1995 E420
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Wow. You've got so much going on here I'm going to suggest carefully following the troubleshooting guide in the manual (available in the 'stickies' of this site) step by step, confirming along the way that each component meets specs. Surely in this manner you're apt to find what's wrong. See section 073-121.

You'll find great info there.
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post #3 of 7 (permalink) Old 05-25-2019, 03:48 PM
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Vehicle: 1983 380-SEL, '67 220D (sold), '65 200D (sold), '75 L-206D (sold)
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I fixed an high idle by resoldering the Idle Control Unit. It should be a last resort step, because you may damage it. If the solder contacts look milky, then this may be the issue. In these cars in California and in Europe I have fixed AC controllers, cruise controls and idle control boards all by resoldering. Obviously this step is useful due to the cost of replacing the boards. If you have access to an alternate board, that would be a good way to test prior to attempting any solder fix.

Hope that helps.
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post #4 of 7 (permalink) Old 06-16-2019, 09:49 AM
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Have been trying to fix idle control too. Resoldering the controller helped a bit. Still idle is too high but there is some current to the valve. Next i have to check the capacitors and big transistors if they are ok. A notice on the troubleshooting guide. The TF voltages stated in the guide seem to be 10 times too high compared what they actually are. My car is model year 1990.

signal TF signal TD - PeachParts Mercedes-Benz Forum
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post #5 of 7 (permalink) Old 06-16-2019, 10:37 AM
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I believe a vacuum leak can also cause a high idle. There are vacuum tests. Maybe just pull the vacuum off the timing advance on your distributor to see if the idle gets worse. I assume you have a distributor on that model.

If someone has fiddled with the throttle connecting bars adjustments, that also can cause a high idle. It's a bit of a process to get the throttle bar setting back to correct, but luckily, most people never touch those bars. The idle is automatically regulated, from what I understand, so tweaking the throttle bar spacing is an incorrect fix.

Having the timing off would make the idle rough rather than too high. It might be a factor. I think your SEL still has settings that can be done with a timing light. I set my 380 SEL at 14 degrees advanced for regular gas.

My guesses are a perfect example of why troubleshooting is a much better step to do first. You can go broke and waste a lot of time, trying to fix everything that might be causing an issue.

Previous post about a similar issue. I mentioned my steps to get my 380 SEL to start and idle like new. Lots of other people also commented on their fixes. Maybe something in this post is helpful.
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post #6 of 7 (permalink) Old 06-16-2019, 11:08 AM
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On my car now controller outputs around 1000 mA but idle on neutral is still around 900 rpm. Guide defines current 700-1000 mA when engine is idling warm. 12 v shortly to ICV will stall the engine. Possibly there are vacuum leaks as the car is old but economy gauge goes all the way to left on idle. Anyhow some previous owner had partially plugged idle air passage at front of ICV to make the car idle somehow. It is possible that he has adjusted air flap stop too to get idle adjusted.Throttle bar is not under tension when idling. Idle switch gets activated when pedal is not pushed. I'll take a look how and where the air flap stop is adjusted.
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post #7 of 7 (permalink) Old 06-16-2019, 02:31 PM
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Nice video on a Mercedes R107 500SL
How to repair Mercedes High idle troubleshoot and cheap fix pt. 1

Part 2
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