High Idle on my '85 380sl

[benzthr]

Just started having a high Idle upon startup.Drive around and even after warmup ,its still high 1500rpm range.Ive never had this happen before.It did this once a couple of weeks ago then was normal,now its back.Anyone know where I should start?
Thanks in advance..........

[txstinger]

This is a problem with the idle speed controller/adjuster. There are many post on this. Best, to do a search on high idle. If you have any questions I will be happy to assist.

[Pili380SL]

You might check by making sure the engine is warmed up and then unplugging the oil temperature switch located down on the right side at the front of the engine, just above the oil filter can. If the idle goes down unplugging the switch, the swith is defective. The terminal on the switch should have continuity to ground cold and be open when warmed (above 16C) up. Your 1985 may have a pressure step switch, a round valve mounted near the coolant tank, I believe. You might try unplugging it and see if the idle drops. This switch is for enrichment while accelerating and may be stuck. If neither "unplugging" changes the idle, check for voltage at the idle adjuster connector, a two prong connector just to the left of the distributor cap when standing in front of the car. If you have approx 5 volts, the idle adjuster may be bad; if you have no voltage, the idle controller (behind glove box, to the left side of the box as you are sitting in the passenger seat) may be bad , or the connector may have fallen off, as happened on mine. Good luck!

[BabyBlueBenz]

Originally Posted by Pili380SL

You might check by making sure the engine is warmed up and then unplugging the oil temperature switch located down on the right side at the front of the engine, just above the oil filter can. If the idle goes down unplugging the switch, the swith is defective. The terminal on the switch should have continuity to ground cold and be open when warmed (above 16C) up. Your 1985 may have a pressure step switch, a round valve mounted near the coolant tank, I believe. You might try unplugging it and see if the idle drops. This switch is for enrichment while accelerating and may be stuck. If neither "unplugging" changes the idle, check for voltage at the idle adjuster connector, a two prong connector just to the left of the distributor cap when standing in front of the car. If you have approx 5 volts, the idle adjuster may be bad; if you have no voltage, the idle controller (behind glove box, to the left side of the box as you are sitting in the passenger seat) may be bad , or the connector may have fallen off, as happened on mine. Good luck!

This helpful old post prompts me to ask:
Is the item highlighted in the photo of my '86 560SL below the 16 deg C Oil Temperature Sender?



Also, does the 560SL have a Pressure Step Switch as mentioned above...and where would it reside?

UPDATE: Never mind, I think that's the Oil Pressure Sensor B5.

Based on researching the electrical diagrams, an oil temp sender (if said even exists on an M117) does not appear to be involved with the CIS-E control. Coolant Temp Sensor B11/2 provides the input to the CIS-E controller, which in turn outputs the coolant temp to the Idle Speed Control Unit N8. This sender's location is by the text as at "left rear of engine" and by a photo caption as at "top right side of engine". Whatever. I'm gonna test this thing's "TF signal" as mandated by 07.3-112 Testing electronic idle speed control.

, these cars are complicated!

[450slcguy]

Someones been studying their documentation very closely.

[BabyBlueBenz]

Originally Posted by 450slcguy

Someones been studying their documentation very closely.

I'm putting together a shopping list of items to replace during my upcoming high-idle-hell intake manifold R&R:

• Based on previously mentioned test results, maybe a new B11/2 coolant temp sender
• Many cans of carburetor cleaner
• Either my original repaired throttle-to-air-guide hose clamp or a new replacement (hoping ctaylor can provide part number soon)
• EGR gaskets at both ends of the tube (for the plates that I'll fab to replace that ugly POS)
• 8 manifold seal rings (the reason for this project)
• intake manifold gaskets, left & right
• another throttle housing gasket (only $2)
• valve cover gaskets and crush washers (already on hand, part of timing chain inspection)
• fuel injector insulators (7 on hand, 2 more coming)
• fuel injector seals (8 on hand)
• idle air distributor hose (the Y-portion, revised design)
• new spark plugs (since removing old to hand-turn crank during timing chain inspection)
• new set of belts (since removing viscofan to hand-turn crank)
• 1.5 gallons of Zerex G-05 + 2 gallons Distilled Water + Wetting Agent (since partially draining coolant is prerequisite to intake manifold removal)

Can anyone think of anything I might do "while I'm in there"?

[Strife]

My $0.02, the first place to look is the OVP relay; the entire electronic engine management system won't work if it has failed. "high idle" is the analog equivalent of the modern "limp home" mode. In my case, just removing the OVP and reseating it worked.

If you have ABS, if your ABS light comes on immediately without the car moving (and this just problem started at the same time as high idle), the problem is certainly the OVP.

If the fuse is blown on top of the OVP, you might get lucky and replace it with a like value fuse. But more often than not, this won't help and it will fail again.

Before replacing an OVP, check to see that the alternator is putting out ~14 volts at about 1000 RPM. More than 14.5 volts, bad.

[donbryce]

Originally Posted by BabyBlueBenz

Coolant Temp Sensor B11/2 provides the input to the CIS-E controller, which in turn outputs the coolant temp to the Idle Speed Control Unit N8. This sender's location is by the text as at "left rear of engine" and by a photo caption as at "top right side of engine". Whatever. I'm gonna test this thing's "TF signal" as mandated by 07.3-112 Testing electronic idle speed control.

I'm trying to pin-point which coolant temperature sensor is the B11/2 on my '86 560SL, and from your advice above, I found the picture on page 204 of my trouble shooting manual. Thanks much. Agree the captions appear contradictory. I think this is the one at the rear right of the engine, driver's seat perspective?

Since I may need to order a new one, I'll await your test results, which I'll try to duplicate to confirm if I need one. Sure would appreciate a simple description of how to do the test with a multimeter, or whatever, for us less expert DIYer's.

Meantime, to get pricing on new, could you tell me what the Fastlane part number is? It seems like there are 3 coolant/temperature sensors: B11/2, B13, and S25/5. The picture of B11/2 is dark, can't tell what the contact looks like.
I think the B13 and S25/5 are at the front by the ISAV, and this puppy is at the rear of the engine?

(BTW, car is in storage now, so I can't readily pop the hood and look for myself. I'd like to be prepared though for spring...if it ever arrives!)

Would one of these be the one?

http://catalog.peachparts.com/ShopByVehicle.epc?q=1986-Mercedes--benz-560sl-Cooling--System&yearid=1986%40%401986&makeid=63%40%40MERCEDES%2DBENZ%40%4063%40%40MERCEDES%2DBENZ%40%40X&modelid=6313%3AMBC%7C1548%3AED%7C10000059%40%40560SL&catid=240909%40%40Cooling+System&subcatid=241011@@Engine+Temp%2E+Sensor&mode=PA
http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1986%40%401986&makeid=63%40%40MERCEDES%2DBENZ%40%4063%40%40MERCEDES%2DBENZ%40%40X&modelid=6313%3AMBC%7C1548%3AED%7C10000059%40%40560SL&keyword=sensor&subcatid=P:240675@@Water+Temp%2E+Sensor&mode=PA
http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1986%40%401986&makeid=63%40%40MERCEDES%2DBENZ%40%4063%40%40MERCEDES%2DBENZ%40%40X&modelid=6313%3AMBC%7C1548%3AED%7C10000059%40%40560SL&keyword=temperature&subcatid=P:242071@@Water+Temperature+Sender&mode=PA

[ctaylor738]

I think you want this one for CIS control:

http://catalog.peachparts.com/ShopByVehicle.epc?q=1986-Mercedes--benz-560sl-Fuel--Injection&yearid=1986%40%401986&makeid=63%40%40MERCEDES%2DBENZ%40%4063&modelid=6313%3AMBC%7C1548%3AED%7C10000059%40%40560SL&catid=240621%40%40Fuel+Injection&subcatid=240675@@Water+Temp%2E+Sensor&mode=PA

IIRC, it's right behind the ICV.

[BabyBlueBenz]

I agree with Sir Chuck Taylor. This drawing from service manual document 07.3-121 'Testing electrical componenets of CIS-E injection system' confirms the location of B11/2 at the front of the motor, between the distributor and air meter.



Here's a picture of my motor, with B11/2 Coolant Temperature Switch enboxed (new word?):



The two wires connected to it match up with the electrical schematics showing B11/2. The Green with Black striped wire leads to the EZL unit. The Green with Red striped wire leads to the CIS-E control unit. So they say, since I was unable to trace these wires beyond where they enter major loomage.

The B13 and S25/5 donbryce mentioned are located on the water pump inlet connection. This is probably the drawing he saw:



Now the contradiction presented by the service manual is on page 207 of the Electrical Troubleshooting Manual, which states that B11/2 is located "left rear of engine". And here's the crappy photo it references:



On my engine, there is a sensor with one Brown with Black striped wire leading to it. The photo below is oriented to match the previous (as in leaning over the passenger fender).



So, I'd like to know: what does the unidentified sensor do on my '86? I scanned through the electrical diagrams for a Brown with Black striped wire and found only one: it connected to the black wire leading from the Oil Level Sensor...I don't think this mystery sensor is that.

The last two pages of 07.3-121 reveal that B11-2 has been moved around over the years. Before 12/85 it was at the rear of the engine (their diagram shows it with two wires, over by the throttle bellcrank). As of 01/86, B11/2 migrated to the location donbryce and I know it at. In '88, California (then Federal in '90) got upgraded to a 4-pin equipped sensor at the same spot.