Idle Speed Air/Control/Slide Valve Confidence Check

[BabyBlueBenz]

As a recent acquirer of a 1986 560SL, I too have recently begun to experience the aptly described "high idle hell". I've read the related Service Manual pages and the many threads on this forum and others. I did some preliminary diagnoses this afternoon...specifically, a few tests of the Idle Speed Air Valve [ISAV] (a.k.a. Idle Control Valve, Idle Air Valve, Idle Slide Valve). I understand that there are many prerequisite items to check in resolving this dilemma, and I've done some already with more to follow in the days ahead. The attached photo shows a close-up of the ISAV (MB component ID of "Y6") currently in my car. And Yes, the 50-degree Thermovalve (labeled 7 in the photo) has been temporarily "loop-capped" as part of my EGR-removal task in progress. And Yes, I capped the vacuum fitting at the rear of the engine that lead to this Thermovalve. I will be double-checking those hidden-in-the-rear and buried-in-the-V vacuum fittings again.

As suggested at Fast Idle Issues

Originally Posted by ctaylor738

One thing to check. When you get the high idle, slide the connector off the pins on the valve a little ways, and use a meter - carefully - to measure the voltage going to the valve. If it shows somewhere around 4 volts, the relay is trying, so replace the valve. If no voltage, replace the relay.

The plug and pins mentioned are labeled "2" in the photo. With the engine warm and churning away at 1500 rpm, I measured the voltage across the pins at 3.3 volts. Does this pass? I heard this component described as a "frequency valve"...is that another way of saying it is variable control? I never heard the term used before.

Per http://www.benzworld.org/forums/r-c107-sl-slc-class/1305243-560sl-fast-idle-speed-once-engine.html:

Check the ICV's coil resistance to see if it hasn't gone open circuit first. Often they gum up and won't close fully giving you a high idle. If you lucky you can clean out the inside with alot of carby cleaner (try soaking it in it) and it may work again. An ultrasonic bath may also be a good idea.

Shutting down the engine, I removed the ISAV from its rubber-lined loop clamp along with the short rubber hoses (labeled 3 & 4) and the interconnecting rigid vacuum line (labeled 5). I cleaned these four parts with the brake cleaner "Brakleen". The interior of the ISAV and hoses were dark with soot, but cleaned up quickly. I followed the cleaning of the valve with a rinsing spray of WD-40. Once dried off, I measured the electrical resistance between the two pins and obtained 4.5 Ohms. The Service Manual specified a target range between 3.5 and 5.5, so I considered the test a pass.

Per the second page of Service Manual document 07.3-112 "Testing electronic idle speed control", I applied 12 volts to the two pins and heard a slight click. When reversed, the current yielded another click. Tom Hundt's excellent tutorial on the high idle problem at http://www.fly.net/~thundt/mercedes/high_idle_hell.htm describes the result of this test as a "loud clack" with a spring-induced click upon removal of the voltage. I did not perceive this to happen with mine. Also, the ISAV rattles when I shake it. That's my primary question here: Does the lack of spring-return and the rattle suggest a bad ISAV to you? It does to me, but I'm a newbie to these electro-mechanical systems.

For extra credit, opine on this factoid:
The last page of Service Manual 07.3-112 "Testing electronic idle speed control" contains a note regarding the "Distributor line for idle air". It details the revision of two parts related to the idle speed control system (the hose is labeled 6 in my photo). My engine number happens to fall before the implementation was made. Does anyone think these revised parts are worth pursuing?

Thanks for any and all input.

[Strife]

1. A "click" is OK.
2. Rattling is good. NOT rattling is BAD.

[BabyBlueBenz]

Originally Posted by Strife

1. A "click" is OK.
2. Rattling is good. NOT rattling is BAD.

Thank you for the helpful answer.

I will proceed to look elsewhere for the cause of my high idling.

[donbryce]

I went through much the same diagnosis on my '86 560SL last year. I actually swapped the IACV from my '85 380SE, and it made no difference (idle on the 380SE is correct BTW). I bought a replacement control module with the right part number off EBay, noting that the one in my car was not the correct part number, obviously installed by someone else at some point. This did make a difference, and at last time the car was on the road (Oct '08), my idle was perfect on start-up, running high for a minute or less, then settling down nicely to about 750. But, it still seemed to be a tad high after the engine had warmed up to operating temperature, not 'settling down' like it did after a cold start. In the spring, I'll try changing the temperature sender to see if it is sending a false signal to this control module. Perhaps this might be something to look at in your case..I'm not sure which sender to check either..

[ctaylor738]

I would say if you only measured 3.3 volts at the valve, then you need a new control unit. The most reasonable source of dependables units is the ProgRama rebuilts sold by a variety of online vendors.

The idle valve is a frequency valve that regulates the amount of air to the engine, and hence the idle speed, by closing more or less of the time. The control unit sends ground signals to the valve to close it. For a reason that escapes me this can be measured as voltage. The higher the voltage, the higher percentage of time the control unit is sending the ground signal. So at 1500 RPM, a good control unit would be close to 5 volts trying to slow the idle down.

You might also make sure that your throttle valve is closing completely. Unhook the throttle and cruise control linkages, and see if that improves the idle. It's not unheard of for the linkage to bind and give the engine just enough "false air" to raise the idle. I suggest this because 1500 RPM is quite high, even if the idle system is completely non-functional.

[Strife]

A caution about reading a pulse with voltmeter: GENERALLY, analog meters will go up and down linearly with pulse width. So, with a 5V signal at 50% duty cycle (50% 5 volts, 50% 0 volts, and note that the actual low signal is likely to be above zero volts), you should read AROUND 2.5 volts, but this is dependent on the qualities of the meter movement.

HOWEVER, digital meters may not react this way, depending on how their internal sampling and averaging algorithms work. Generally, a more expensive meter is going to behave better than a cheap one in this area. Setting the meter to "AC" generally won't work because non-true RMS voltmeters are designed to treat every AC voltage as a sine wave, and adjust downwards accordingly. Real "true RMS" voltmeters, still not cheap, actually try to calculate the area under the curve of the sine wave of utility AC, or the area under the rectangle (a pulse).

[BabyBlueBenz]

Thanks for the useful information Gents. I will be checking the other potential contributors of high idle, before revisiting the ISAV's potential contribution:

- as ctaylor mentioned, making sure the throttle linkage is free of binds,
- checking for sneaky vacuum leaks (visually, with a gauge, and strategic carb cleaner spraying) like the inside of rubber elbows and the fittings at the back of the engine.
- checking the throttle valve switch "S29/2" pinouts for continuity
- and applying some RTV to one loose-fitting end of the rigid vacuum line that interconnects the ISAV hoses (labeled 5 in my original post's photo)

After that, I may try a little "bendy-twisty" action on the Idle Speed Control Unit circuit board to see if that gets a reaction and indicates a poor solder connection.

[ragtopper]

I too have been suffering from occasional high idle. Recently cleaned IC valve, replaced some vacuum hoses and things returned to normal...temporarily.
Now, high idle occurs about every other time I drive car. Seems that if I slide connector off pins for a few seconds, idle drops to normal.
I'll then reattach connector, and everything's fine until next time I drive - when high idle returns.

Bad connection? Thx.

Ragtopper
380SL

[donbryce]

I have a virtually brand new ICV on the way from a California seller who had offered it at $75.00, couldn't get a bid, so relisted at $25.00 - and I snagged it. Never mind the $35.00 postage and exchange brought it to $75.00CDN, it's supposedly only been on the car briefly (wrong part apparantly) and came from the dealer (VDO). I can hardly wait for spring to see if it cures my erratic idle. As stated above, the idle is great on cold start, but on hot restart, tends to go high and stay there.

[ragtopper]

Sounds like good deal.
Meanwhile, I'm gonna try again......
I too have been suffering from occasional high idle. Recently cleaned IC valve, replaced some vacuum hoses and things returned to normal...temporarily.
Now, high idle occurs about every other time I drive car. Seems that if I slide connector off pins for a few seconds, idle drops to normal.
I'll then reattach connector, and everything's fine until next time I drive - when high idle returns.

Bad connection? Thx.

Anybody have any experience w/this? Does this seem more like an electrical issue vs. the actual valve?

Ragtopper
380SL

[dpetryk]

I have had all of the issues discussed here and I have found the solution. But first a little history.

One of my cars had the high idle syndrome. I checked all the systems involved with the idle control and everything was working. I ended up replacing the idle control valve with a new unit and it solved the problem. I was confused because I had checked the original ICV and it was working but it could not control the idle. So now I measured the actual performance of the original against the new one I had just installed. I measured valve movement against current applied to the coil. I could not find any difference between the two. Why did one work and not the other? I needed to find out.

After a lot of time I found out that the valve is "adjustable" and making that adjustment solved my high idle speed problems. The adjustment is set at the time they are made. You can change this adjustment and reduce your idle to the right level.

I ended up adjusting my original valve and everything is good with the world again.

[dpetryk]

Here is a copy of another thread I wrote on the same subject from 2001.

I have investigated this phenomeon very closely. I had a high idle condition on my 87 420sel. Applying a variable voltage to the idle control valve allowed me to control the idle. But when it was connected to the idle speed controller in the car it would not bring the idle low enough. It seemed like the valve did not have enough authority to do the job. The idle speed controller was putting out the maximum current that it was designed to do. It was just that the valve would not close down enough.

I installed a new one and the problem was solved. However before I installed the new one compared it to the old one on the bench and did several measurements to find the differences between them. I measured current verses stroke and all sorts of other electronic measurements. I could not find any differences!. They performed the same on the bench. I cant explain why one worked and the other did not. I assume it was because of some mechanical wear related to the sealing surfaces or something like that.

In one case I tried to clean the valve using brake cleaner (a good solvent) but it didnt do any good. I ended up buying a new valve and the problem was fixed. However, these valves are way expensive. So on the other car I spent a lot of time cleaning the valve. This time I used carburator / choke cleaner. It did a better job of disolving the carbon junk. I kept working on it until I could not get any more junk out. I was surprised how much stuff came out. I spent almost 2 hours soaking, spraying, and repeating the process. To my surprise, it did fix the problem. I also replaced both of the hoses since they were hard. On hind sight, I should have spent more time on the first one and I would not have had to spend the $ to buy a new one. Live and learn.

However even if the electronics is OK the valve seems to have an authority problem. Let me explain what I mean. It started with my 87. The idle was always hi. About 1200 RPM. I could not get it to come down. When disconnecting the valve connector - the idle went up. When applying 12 volts to the valve - the engine died. I connected a variable voltage source to the valve and was able to regulate the idle speed quite nicely. I was then convinced the problem was in the control module since the valve seemed to work. NOT. The valve had the ability to control the idle, I demonstrated that. So why didnt it work?

The control module was fine. It seemed that it was not able to supply enough current to the valve to bring the idle speed down far enough. Thats what I mean when I say it didnt have enough "authority" to control the idle. It was trying but it did not have enough strength. I bought a new valve and installed it and low and behold everything was fine. So now I was determined to understand what the problem was.

I took the "bad valve" apart - I mean completely apart. I could find nothing. I checked the stroke vs current & voltage signature from the new valve and checked it against the bad one. No observable difference. The bad valve had no observable difference to the new one except it was not shiny. So I accepted the fact that there was still so something I did not understand and I let it go at that until my 91 had the same problem.

This time I reasoned that there was something physical in the valve portion causing the problem. Thats when I got real agressive on cleaning the thing. I worked on that thing for hours until I could get no more carbon out of it. And it worked.

There is another phenomom used to drive these valves. The control module modulates the current applied to the valve with something called "dither". Dither is needed to overcome something called "stickshion" . Stickshion is best described as something like sticky friction. Its where the valve does not want to move shoothly and porportionally with the applied current. Because things get sticky it wants to jump rather than slide smoothly. So the electronics applies a low level rapidly varying current to it to constantly wiggle it and keep it free so it wont stick in one place. You can feel the dither when the engine is running. It feels like the valve is buzzing. This buzzing effect makes the valve move smoothly even though it is full of gunk and other sticky stuff.

My conclusion is that over time these valves get crudded up and wear mechanically and the result is that it takes more current to drive the valve shut. After a while the control electronics cannot supply enough current to bring the idle down and hence the idle creeps up. Cleaning it can cure it if it is not worn out. Dither makes the valve move thousands of times per minute and the resulting wear changes the stroke / current characteristic of the valve to something beyond what the electronics can compensate for. So try cleaning it really well and if that dont work, grab your wallet.

[dpetryk]

Since that thread was written in 2001 I have learned that a simple adjustment can set the valve to work as desired. I thought I had taken some photos of how to do the adjustment but I could not locate them. I thought I had written up the procedure before but I could not locate that either.

The prerequisite is that everything is working and you don't have any vacuum leaks. If the idle is too high, which seems always to be the case you make the adjustment on the ICV and then re-install it and check the idle speed. You have to make small movements in the adjustment until you get it to idle set at the correct RPM. Move it too far and the idle speed will be too low. The adjustment cannot be done while the hoses are connected so you have to remove it , make an adjustment and then test it. Repeat until the desired idle speed it achieved.

[ragtopper]

Wow. Thanks for the great, detailed explanation!
My fear is that since my high-idle is intermittent, it's gonna be tougher to determine if/when it's fixed.

I'll start the "lather, rinse, repeat" process - and see what happens.

Ragtopper
380SL

[BabyBlueBenz]

Thank you dpetryk!

After I check the prerequisite items I mentioned back on 1/24 (it's gotten COLD here!), I too will be revisiting my ISAV with an eye towards cleaning it better and confirming its "authority" to control the idle speed of my ride.