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

Test switch at throttle plate thats usually the cause,i dont remember the values resistance ? or continuity?

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

OVP

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.

Just swapped the OVP from my 300 real quick. No dice. So we can positively rule out ovp.

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

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.

Thanks for the specs. The switch seems to meaasure just fine. I guess we are looking at bad ICU or wiring?

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

O2 sensor?

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.