Idle Speed Air/Control/Slide Valve Confidence Check

[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