560SL Idle Speed Control Unit Test Part 1

[BabyBlueBenz]

My '86 has been experiencing the Treaded High-Idle Syndrome (I'll call it THIS), which has been discussed at length on this and other forums. Based on what I learned on the forums and a cursory reading of the MB service manual documents related to the subject, I determined that my Throttle Valve Switch (TVS) had gone bad. As indicated by my in-dash tachometer, my idle was previously around 1500 rpm, then dropped to about 1100 after the TVS was replaced. I also replaced the Idle Speed Air Valve (ISAV) on principal, since it is a key component affecting the engine's idle.

Not satisfied, I continue to scrutinize all factors related to THIS...and there are a lot of potential sources of trouble.

First, I must stress to anyone interested in fixing their own MB vehicle that although these enthusiast's forum are a great asset, YOU MUST INVEST IN A SERVICE MANUAL. Believe me it's worth it getting one. The folks on these forums are a helpful lot, but they're not always going to be there to answer your questions and sometimes they answer incorrectly. The R/C107 series was produced for 17 years (the longest of any MB model) and given all its variations, it's difficult for even the experts to know every detail. The manual itself can be confusing, as the accretion of references builds up as you research a subject. To avoid more confusion, I will attempt to use the terminology as presented by MB in its documents.

That being said, here is my (currently incomplete) experience with testing the heart of the 107's idle management system, the Idle Speed Control Unit (ISCU). I am attempting to perform every task within the manual's procedure with just minimal resources that a shadetree mechanic has (although I've been buying new tools recently) .

[BabyBlueBenz]

For my car equipped with the M117.968 V-8 engine, the stated ideal idle speed is between 600 and 750 rpm. For the test procedure you will need (and know how to use) a decent multimeter with voltage, resistance, and amperage measuring functions. Naturally, automotive-related tools will be used. Since portions of the test involve a running engine, you must exercise extreme caution when working near moving parts. I know the danger personally, having lost two (almost three) fingertips to a motorcycle chain. An improvisational mindset is also needed to overcome the obstacles that will occur in pursuit of desired results.

Hidden at the beginning of the service manual document 07.3-112 titled Testing electronic idle speed control among the shop-related requirements is mention that the ISAV needs to be performing properly. See the thread Idle Speed Air/Control/Slide Valve Confidence Check at Idle Speed Air/Control/Slide Valve Confidence Check for a detailed discussion and photos on this device. To triple check, I unplugged my ISAV and ran jumper wires to the two male pins from nearby electrical ground and 12 volt sources under the hood. This is done with the engine off, and with a brief (less than 5 second) application of current, the ISAV should make a click noise. Reverse the jumpers to confirm the same result.

The engine needs to be warmed up before testing the ISCU, and the manual suggests that the engine-driven accessories NOT be active during running tests (although earlier it says contrary...I think they mean after all the tests are done). It is very helpful to have a hardcopy of the electrical diagram matching your car's year and model. For me and mine, pages 2 & 3 of 07.3-128 were applicable. Ya gotta know the identities of the gizmos involved. If you're not sure where they are located, peruse through the Electrical Troubleshooting Manual (ETM), which provides component locations. Granted, some of the images are poor in quality. When in doubt, ask the forum! Take a moment to review the How to Use this Manual preface to the ETM, as it describes the graphical representations used throughout the manual and demonstrates how to connect your metering device for the desired property.

The manual also assumes that there are no leaks in the intake system (it is critical that any vacuum leaks are detected and addressed beforehand), idle contact is correct (I don't know if they mean physical linkage or the TVS...which is testing within this procedure), coolant temperature sensor (another chicken-and-egg dilemma) is working right, and EGR is operating properly.

Here's the drawing showing the connector pin assignments:

[BabyBlueBenz]

The first test is to confirm that the ISCU (identified as "N8") is grounded properly. This is done by removing the ISCU from its perch behind the glove box. This in itself can be a challenge, since there are numerous items in the way. You will need to pull the carpeting out on the passenger side and the under-dash footwell panels. On my car, I have been deleting systems that I no longer desire, such as the climate control ducting and vacuum control pods. I have removed several unused control units, so don't be alarmed by the absence of in-dash equipment in the photos that follow.



I found it easiest to unbolt the bracket that holds the cluster of control boxes behind the glove box, for insertion of the metering probes into the female connectors that are snapped within the bracket. You can separate the individual connectors from the bracket, but it's tough to do from underneath. On previous testing events, I tried to pierce the appropriate wire with my probes, but the wire jackets are nearly plastic-hard on my car. Whatever it takes, just take your time and don't get too aggressive with the pulling and tugging of wiring.



Here I am testing the ISCU's ground by measuring the voltage from the connector's socket number 11 (reading the molded numbers on the connectors is another challenge) to a nearby source of 12 volts. I found an open lug terminal above the fuse box, among the forest of plug-in relays. I measured 12.5 volts. For reference, I also measured the voltage across the battery posts, which was also 12.5 volts (engine is off at this time). The target value is between 11 and 14 volts.

[BabyBlueBenz]

With the N8 ISCU still disconnected, measurement of voltage between sockets 11 and 9 is called for. This requires turning the ignition key On.



I measured 12.2 volts. This confirms that the K1 Overvoltage Protection Relay (OVP) is present and its fuse not blown. Again, the target value is between 11 and 14 volts.

[BabyBlueBenz]

Not having the manual-mandated special test cable, I improvised this method for testing the ISAV. Also, since this test requires running the engine, I made my meter connections in advance of starting to make sure the wires were not in harm's way. Having alligator clips for your meter's probes is most helpful in this situation. I unplugged the connector from the ISAV, reoriented the connector vertically, and then plugged it back so that only one ISAV pin was connected to the plug.



Here I am measuring the current flowing between the two male pins on the backside of the ISAV. I'm not sure if this is a practical method, but I did read a value in the target range of 700 to 1000 milliAmps (the meter's scale is set at 2A).

Next is a measurement of the resistance within the ISAV with the engine/ignition Off, the target value desired between 3.5 and 5.5 Ohms. I got 4.5 Ohms.



So far so good. Now we check the wires between the ISAV and the ISCU. Hopefully, your meter has long enough probe leads or you'll need to improvise extensions...mine just made the reach between pin 1 on the ISAV (reference the numbers molded on the connector plug) and socket number 6 belonging to the ISCU.



I didn't get a great value (the target is <1 Ohm), but I considered it acceptable. To follow-up, the manual said to check continuity between pin 2 of the ISAV and socket 6 at the ISCU. When I got no reading, I checked the probe connections twice. Then, I thought that I had discovered another cause of my THIS. Then, a clever synapse went off in my brain that questioned the circuit being measured. The manual is wrong! The ISAV's pin 2 should connect to the ISCU's socket 7, not 6! Moving the probe, I obtained another acceptable value.



That's the first time I encountered a typo in the service manual, and it demonstrates that even MB can sometimes make a mistake.

[BabyBlueBenz]

Now it starts to get scary, as the manual needs you to disconnect the N3 CIS-E Control Unit located under the passenger footwell deck, as well as the "electronic ignition control" which I understand to be the N1/2 EZL unit hiding under the hood at the front of the driver's-side fender well. That done, I measured the continuity across the ISCU's sockets 11 and 4...it should be less than 1 Ohm.



Close enough. Now you must press on the accelerator pedal (remember engine is still Off).



The meter indicates Infinity, meaning an open circuit, meaning that the Throttle Valve Switch is working correctly. It better be working, because it's brand new!

Next, the manual has you checking the wiring between the TVS and the ISCU. I got lazy and blew off this test, since my meter probe leads could not reach that far. Besides, I have tested my TVS wiring before by disconnecting the plug identified as S29/2x (also known as X56 in other service manual documents) while the engine is running. The resulting increase in idle demonstrates that the TVS and its wiring are working. So there.

I did do the last task in this series, which is to confirm the continuity of the middle pin (#2) on connector plug S29/2x to ground.



I didn't bother locating the specified ground point called W11. I just found a convenient lump of engine metal. Before posting this, I looked for W11 in the ETM, and it's located at "top left rear of engine" and their photo places back near the EHA. Oh well, close enough. As said before, I know the TVS is working fine.

[BabyBlueBenz]

Now this is were I found a failure on my first attempt testing the ISCU. I previously did not perform each item leading up to this one, which is why I'm being so thorough this time around. I also believe I probed the wrong sockets on the first try...which is easy because you can get absorbed in testing an item, then assume the next test addresses the same item. For instance, at this point I realized that I need to restore those things disconnected in the previous step. Like, the CIS-E Control Unit and EZL cables unplugged earlier. This test requires unplugging the N16/4 Fuel Pump Relay which lives right behind the N8 ISCU. But in reading a few steps ahead, I realized that the ISCU will be disconnected again. Which means that it should now be plugged back in for this task. The manual doesn't tell you when to plug stuff back together...you need to remember to reverse your tracks with each test step taken.

So, with the CIS-E, EZL, ISCU and whatever else you've touched all plugged back in, this task has you run a jumper wire between the N16/4's sockets 7 & 8. Understand that it isn't written in the manual to do that, but is indicated by the graphic symbol that denotes the test connection. I did not understand this setup on my first, second and third readings of this document. Then I reviewed the ETM's How To... and discovered what this "bridging" symbol meant. So, armed with a short wire crimped at each end with a bullet-shaped terminator, I bridged the sockets. When this is done, the fuel pumps will start running, which is heard as a whine from the rear of the car. The engine is now started, and a voltage measurement made between sockets 11 and 2 of N16/4.



You can barely see the jumper wire in the photo. My meter is displaying 0.79 Volts, which is far short of the desired range between 3.5 to 5.0 Volts. Something is wrong here, and I've duplicated this test a few times to make sure that I'm doing it right. The manual states that the possible cause is either the N3 CIS-E, the N8 ISCU or the wiring. I'm hoping its just wiring and not an expensive control unit. But, as the sun had set on today's adventure, I had to stop my tests at this point. I will resume tomorrow and post the outcome here as Part 2 at 560SL Idle Speed Control Unit Test Part 2.

During previous test sessions, I researched that this test is only confirming the TF Signal as it is relayed by the N8 CIS-E Control Unit. It does not confirm the functionality of the actual temperature sender, which is identified as B11/2. As a sidebar to this thread, I will post another relating my experiments in testing B11/2 (found at 560SL Temperature Sensor Test).

As usual, comments and corrections are welcome.