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  #1  
Old 09-02-2010, 11:42 AM
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Location: Falls Church, VA
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1991 300SL Idle Speed Puzzle

I am consulting with a good local shop that has run out of time and ideas on this car. It is a 1991 300SL five-speed with appox. 78K miles. Generally good condition. The car came in with a variety of hard starting, poor acceleration, and other running problems, all of which have been fixed.

The following parts have been replaced:

- fuel filter and pump
- coolant temp sensor
- throttle position switch
- decel micro switch
- cold start valve
- idle air tubes
- plugs, wires, rotor, cap
- EHA
- air sensor potentiometer
- OVP relay

The remaining problems are that when the car is started hot or cold, it will idle at 1100 -1200 rpm until you press and release the throttle, at with point the idle drops to around 650 RPM. If you turn on the AC, the idle goes very low and if you turn the wheel, it almost dies.

When the fast idle occurs, the voltage to the idle control valve is around 7 V. When the idle drops, there is no voltage. You can unplug the connector and there is no difference in the idle.

The ECU was swapped for a known good unit. A functioning idle valve from another car was tried, but made no difference.

I have checked the verified the operation of the decel and throttle position switches at the ECU connector. I drove the car extensively with my meter connected to the EHA, and it is functioning perfectly.

So, any ideas why the ECU refuses to control the idle?

Thanks,
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Last edited by ctaylor738; 09-09-2010 at 12:10 PM.
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  #2  
Old 09-03-2010, 08:49 PM
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Posts: 4,327
Sounds like a bad idle valve. If you disconnect the valve it should default to wide open giving a very high idle. If it doesn't then that is an indication of it sticking mechanicaly.
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  #3  
Old 09-03-2010, 09:24 PM
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Just forget it and sell the car to me.

:-)
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  #4  
Old 09-04-2010, 09:43 AM
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I asked the shop about the idle valve. They said that they tried another one, but it made no difference.

You raise a good point. I assumed that on this model, with no power the valve was closed and that's the way it was designed. I will look into this some more.

[Added 9/9: per Bob Terry and David Poole, and the MB documentation, with no power, the valve is closed]
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Last edited by ctaylor738; 09-09-2010 at 03:34 PM.
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  #5  
Old 09-04-2010, 09:56 AM
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Additional Information

I took the car back to the shop yesterday because they had a similar car, but an automatic, which means the ECU is different. We swapped the MAS modules with no difference. The MAS module receives tach signal from the ignition module and compressor on and passes them to the ECU. So there went that theory.

The we dug around in the cabinets and found the MAS testing harness and a breakout box. So I was able to test a couple of things with the engine running:

Voltage from ignition module (tach) to MAS: 6.4V, spec is 7.5-9V.
Voltage from MAS to ECU (tach): 6.6V, spec is 5-7V.
AC signal from MAS to ECU 9.5V, spec is 5-12V.

I also tested the vehicle speed input from the speedo. At stop it was .77V. It went to 3.3V when the car started to move, then gradually increased with speed, like 4V at 50 mph. Spec is >1V at 13 mph.

So it looks to me like the ECU is getting good input on tach and speed. I do wonder about the low reading from the ignition unit to the MAS, though.

Thoughts ???
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #6  
Old 09-05-2010, 05:37 PM
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Found the diagnostic connector ...

This car actually has a 16 pin diagnostic connector, cleverly concealed under a cover on the right side of the firewall with the hood light switch sticking out of it. So I dug out my impulse counter and pulled the following codes:

CIS:

10 - Idle speed and throttle switch implausibel
4 - Air flow pot current implausible
18 - Current to idle speed valve implausible

Ignition

17 - Crankshaft position sensor defective

I cleared all codes and drove the car. The only one that returned was the 17. Not sure what this would have to do with the idle problem, but it's possible that this has screwed up the RPM signal.
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #7  
Old 09-10-2010, 10:52 AM
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Location: Falls Church, VA
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Solved

Per Steve Brotherton, if the voltage from the air sensor potentiometer at idle is out of spec, the ECU will not understand that the car is at idle and thus decide to go into a sort of limp mode, cutting the power to the idle valve and retarding the timing.

The spec is .5-1.2V output at idle measured across pins 1 and 2 at the pot. The easiest way to measure is to remove the back of the connector. I tested at the pot and found .07V. Adjusted it up to .8V by rotating the pot very slightly, maybe .1 mm. Now have perfect idle including AC compressor signal - no more drop when the AC comes on.

I tested the Key On values at the ECU harness, but did not test at idle because of the good readings from the EHA current. Lesson learned.
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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