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  #1  
Old 07-21-2012, 09:15 PM
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Please help me out of a real pickle - '88 560SEL High Idle

Hi everyone,

I know I haven't been here in a while, but leave it to a 126 to send me back to my old friends for help.

I am in a really bad situation. I've been planning a road trip for months (and paid tons of money for reservations) and now one day before I am to leave for 3 weeks, my idle has shot up to 1500-1800 RPM.

I need a fix FAST!

Here's what I've ruled out:

1) vacuum leaks. All the rubber, inlcuding the intake boot, etc. are new. Vacuuum is good even at 1800rpm.
2) stuck throttle plate
3) OVP relay

Here are some tests I've performed

1) removing idle air wires has no eeffect. Applying a 9V battery to the IAC will cause a near immediate stall. The IAC mooves freely with a 9V battery, and is clean inside.
2) temp sensors sensors seem to measure within spec.
3) voltage to the IAC is 7.6 DC (though I neglected to look for any AC component or duty cycle to the signal) when disconnected from the IAC. When conneceted, it is near zero (measured in parallel).
4) shunting the signal wire from the throttle position sensor had no effect on idle speed.


Can anyone help me shed light on this or suggest some further tests? I am in a real jam here.


Thanks so much,


TP

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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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  #2  
Old 07-21-2012, 09:21 PM
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Test switch at throttle plate thats usually the cause,i dont remember the values resistance ? or continuity?
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  #3  
Old 07-21-2012, 09:30 PM
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If anyone knows the specs, please post them. I havent tested that yet. I didn't even see it, its buried down there. Looks like a real pain to replace, but the terminals are accesible. I think i mistakenly tested the fuel meter position sensor thinking it was the TPS. So please disregard the comment that the TPS was tested at all in my previous post.


TP
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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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  #4  
Old 07-21-2012, 10:19 PM
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OVP
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600SEL '91
300E 4Matic '88
240D '83
280SE '77
350SE '73
The most complex systems can fail in the simplest way.
Contra verbosus noli contendere verbis, sermo datur cunctis, animi sapientia paucis.

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  #5  
Old 07-21-2012, 10:27 PM
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Thanks for the reply, but I don't think so. SRS light comes on when ovp is out, I verified this by unplugging the OVP. No change to idle, but the SRS light cam on immediately and went away when I plugged it back in. I'll swap the one from my 300 real quick and see, just so im sure, but I'm already pretty sure.

Please keep the ideas coming!


Thanks.
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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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  #6  
Old 07-21-2012, 10:39 PM
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Join Date: Jul 2008
Posts: 1,236
Just swapped the OVP from my 300 real quick. No dice. So we can positively rule out ovp.
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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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  #7  
Old 07-22-2012, 09:12 AM
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Join Date: Feb 2007
Location: los angeles
Posts: 832
to test throttle switch , test ohms at pin 1-2 =idle 0 ohms full load infinity,test at pin 2-3 idle infinity ,full load measures 0 ohms, remove plug at/near cold start injector , test with 3 pole plug unplugged x56- to s29 switch
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  #8  
Old 07-22-2012, 09:40 AM
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While you're at the throttle linkage checking throttle microswitch (to cut fuel and bring back idle to spec), might as well check for binding or rusted components/joints. Lube per manual.
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  #9  
Old 07-22-2012, 12:55 PM
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Thanks for the specs. The switch seems to meaasure just fine. I guess we are looking at bad ICU or wiring?
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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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  #10  
Old 07-22-2012, 06:59 PM
Starstruck
 
Join Date: Mar 2009
Location: Iceland
Posts: 423
Quote:
Originally Posted by tinypanzer View Post
Just swapped the OVP from my 300 real quick. No dice. So we can positively rule out ovp.
They're not the same so what did you hope to achieve by switching?

560SEL = A1265404745 replaced by A2015400845

300SE = A2015403245 replaced by A2015403045 replaced by A2015403745
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600SEL '91
300E 4Matic '88
240D '83
280SE '77
350SE '73
The most complex systems can fail in the simplest way.
Contra verbosus noli contendere verbis, sermo datur cunctis, animi sapientia paucis.

i don't believe in the lord! He's never bought me a Mercedes Benz.

Last edited by daddi; 07-22-2012 at 07:18 PM.
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  #11  
Old 07-23-2012, 03:49 AM
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O2 sensor?
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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

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  #12  
Old 07-24-2012, 02:58 PM
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Who's flying this thing ?
 
Join Date: May 2006
Location: N. California./ N. Nevada
Posts: 3,611
The dreaded 560 egine high idle problem

I am almost certain the Idle control valve unit needs to be replaced.

Doing this (in my case, repairing it, not buying an expensive new ICV) --- completely cured the same problem in mine back in around 2002 and it never recurred.

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