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.

[BabyBlueBenz]

Thanks for the reality check Strife. It's too easy to read other people's planned remedies...and then make them your own.

Originally Posted by 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.

Reseating didn't help me.

Originally Posted by Strife

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.

No ABS warning lamp anomalies. It comes on when the key is turned to Run, then goes off when the engine is started.

Originally Posted by Strife

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.

I've remove the fuse and it looked good (no gap). I scuffed the blade contacts (& the OVP pins) with a bit of emory cloth. No change detected.

Originally Posted by Strife

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.

That's a good tip, and I will put a voltmeter on the alternator ASAP. I have an aftermarket volt gauge and if I remember right, it displayed a healthy charge rate. Am I correct in correcting an overcharging alternator by replacing the voltage regulator?

[BabyBlueBenz]

I checked these and additional items today in further diagnostic pursuit of my high idle:

Originally Posted by Strife

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.

As before, ABS light comes on only briefly after engine start.

Originally Posted by Strife

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.

I pulled my OVP relay and its piggybacking 10 Amp fuse off. The fuse looked good and past a continuity test. I wirebrushed the relay's pins and reinserted it. No change in idle resulted.

Originally Posted by Arthur Dalton

While it is running, tap on the OVP w/screwdriver handle..look for changes..
If yes, pull OVP and clean the pins/sockets and retry...if still yes, replace the OVP.

I tapped the OVP relay while idling, but no change occurred in the idle speed. I feel that my OVP is working correctly.

Originally Posted by Strife

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.

While idling a 1000 rpm, my voltmeter read 14.09 volts across the battery terminals. I think the alternator and its voltage regulator are working OK.

Originally Posted by rocky raccoon

Look closely at the vacuum fitting that comes out of the intake manifold at the rear (firewall side) of the engine on the passenger side. That fitting has a soft rubber attachment to the hard vacuum line. It is vulnerable to heat and oil and deteriorates before any other.

The existing connector had no detectable faults, but I exchanged it with a new one anyway. I am confident that there are no leaks in my vacuum control system. But this doesn't mean that "false air" is not entering from other areas (my intake manifold risers are hard and therefore suspect, and I need to reinspect my Air Guide (aka throttle boot). I still need to replace the guide-to-throttle hose clamp that I recently broke.

I attempted to perform the "TF" signal check per service manual document 07.3-112 Testing electronic idle speed control, but didn't understand the procedure. I do now, so I'll try it again asap. This test checks whether the coolant temperature signal from sensor B11/2 that is digested by the N/3 CIS-E Control Unit and then sent on to the N8 Idle Speed Control Unit and N16/4 Fuel Pump Relay is working properly. The test requires disconnecting the Fuel Pump Relay and then bridging its connector jack a certain way. I didn't understand the service manual's symbology for the bridge, and couldn't fathom how the car would idle without the Fuel Pump Relay. I measured the voltage at N16/4's pin no. 2, but it was far short of the spec of 3.5 to 5V, and I suspected a bad B11/2. But, I did the test wrong so I'll be trying it again soon.

I messed around with the N8 Idle Speed Control Unit as suggested by Tom Hundt's High Idle Hell (http://www.fly.net/~thundt/mercedes/high_idle_hell.htm), but found that my 86's ISCU has been modernized compared to Tom's photos. Mine is a much smaller unit with several IC chips and daughter board upon it. I tried a little flexing of the circuit boards, but no idle speed changes were noted.



I also played with B11/2 Coolant Temp Sensor for CIS-E. This sender has two wires connecting it to the CIS-E and EZL control units. Unplugging the EZL lead resulted in a faster idle and replugging it dropped the idle to its previous state. Unplugging the CIS-E lead did nothing. Switching the connections did nothing. This seemed suspicious, but I will wait to complete the proper TF signal test before considering a replacement sensor. UPDATE 3/8: In performing additional tests today, I discovered that the short extension wires between B11/2 and the wiring harness were crossed. While functionally harmless, this reversed the observation made earlier when I unplugged the EZL lead from B11/2. I was in fact unplugging the CIS-E lead, and it resulted in a favorable idle speed change. I've have also confirmed in reading the manual about the EZL's usage of B11/2 is indicate when the engine temperature is over 95 degrees C. So, when unplugged at lesser temperatures, B11/2's lead for the EZL will have no affect. This is good news, but I may still replace B11/2 in the quest for idle speed perfection.

P.S. For those just tuning in, I've already replaced my Idle Speed Air Valve (old one was cleaned and probably working fine) and the Throttle Valve Switch (which failed testing parameters). Idle was as high as 1500, but the TVS replacement has brought it down to about 1100.

Any ideas and/or suggestions are welcome.

[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.

[BabyBlueBenz]

I retested the signal indicating temperature to the Idle Speed Control Unit (ISCU). The service manual has you unplug your Fuel Pump Relay (N16/4) located behind the glove box. Then you run a jumper wire between jacks 7 & 8 in the connector that this relay plugged into. This "hot-wires" the fuel pumps into operation. With the car started and idling, you are to measure the voltage across jack 2 (coolant temp "TF" as produced by the CIS-E unit) and 11 (ground) with a target between 3.5 and 5 Volts. BTW, port 2 of the Fuel Pump Relay and ISCU are connected in the harness.

An early result, with my digital coolant temp gauge displaying 106 degrees F, I measured 1.1V. As the temperature rose, the voltage dropped. At 138 degrees F and 1000 rpm, I measured 0.6V. For ****zingiggles, I unplugged the B11/2 leads one at a time. Unplugging the CIS-E feed did nothing to the idle, or the voltage being measured. Unplugging the EZL feed resulted in an increased idle speed. Swapping the leads had no effect.

The manual sez that possible causes of failure are either the CIS-E control unit or the Idle Speed Control Unit. EDIT: I couldn't understand how a failing TF test would mean the ISCU was bad (this signal is an input). I think MB is suggesting causes for both results, i.e. a Fail indicates a bad CIS-E, whereas a Pass suggests that the ISCU is bad because the TF signal is correct.

Did I do this right?
Is measuring in VDC correct, or is this another Duty Cycle vagary?

I discovered the lengthy service manual procedure for testing the CIS-E, with details on testing individual sensors. I keep meaning to check my car for the presence of the X92 diagnostic port. I would build a blink code reader so fast if I knew that the car would tell me what is at fault. Regardless, I feel the need to try and test the various CIS-E input sensors, starting with my B11/2 and the handy table of resistance values. A cursory comparison of its values and some measurements I took yesterday (matching in a positive way) are suggesting to me that the CIS-E is receiving coolant temp input fine, but that some other factor is skewing the coolant temp output to the ISCU. Could it be another sensor that feeds the CIS-E control unit? O2? Air Flow Sensor?

[Strife]

If your car is not a 560 and not a later one from California, I don't think it has the pre-OBD diagnostic system.

[BabyBlueBenz]

'Tis true.

Good thread on the subject at 560SL diagnostic port/tool.

Alas, my '86 560SL is not equipped with the advanced blinkernator technology.



I will have to determine the proper function of my CIS-E input sensors the hard way.