560SL Idle Speed Control Unit Test Part 2

[BabyBlueBenz]

Pre-empted by the desire to determine whether my car's CIS-E had the Fault Detection feature and whether it would tell me what ails it, I determined It Does, but It Didn't. See 1986 560SL Duty Cycle Madness for details on that experience.

In the afternoon, I returned to diagnosing my Idle Speed Control Unit (ISCU) per service manual 07.3-112 Testing electronic idle speed control. Previous test results were posted in Part 1 at 560SL Idle Speed Control Unit Test Part 1.

[BabyBlueBenz]

To recreate the fault I detected on previous occassions, I got the car warmed up and then recreated the necessary conditions called for. The N16/4 Fuel Pump Relay was removed and a jumper wire inserted between its connector sockets 7 and 8...which effectively "hot-wired" the fuel pumps into operation. The engine was restarted and a voltage measurement taken betwen sockets 11 (ground) and 2 (TF Signal) on N16/4's connector. The ISCU remained plugged-in out of necessity, but it and the Fuel Pump Relay share the same TF Signal wire coming from the CIS-E Control Unit. The initial value read was 0.72V. As the engine temp escalated, another measurement read 0.48V. The service manual specified a desired range between 3.5 and 5.0V, so this test failed yet again.

Pressing onward, the next test checks the wiring between the CIS-E and the ISCU. With the engine off, the large cable was disconnected from the CIS-E to expose the connector pins within. Referring to the pin assignments diagram for the CIS-E connector page 20 of 07.3-121, the continuity between the origin of the TF Signal on the CIS-E's cable and the receiving socket at the disconnected ISCU's socket was measured at 0.4 Ohms. This was considered a Pass.



The next subtest checked the short connection between the ISCU and Fuel Pump Relay via the number 2 socket on their respective connectors. Proper continuity was confirmed here as well.

[BabyBlueBenz]

With the previous group of tests completed, the service manual moved on to confirmations of the transmission gear selector's position and the air conditioning relay's signals. But I felt that there were still more wires involved with the B11/2 Coolant Temperature Sensor that were not proven...namely those from the sensor leading into the CIS-E control unit.

Back in the engine bay, I measured the continuity of the short extension wires connecting the B11/2 sensor and the wiring harness. This extension was likely added when B11/2 was relocated from the rear of the engine to its current position. In fact, a plug fitting can be seen at the sensor's previous position on the intake manifold. Both the Green with Red striped extension wire and the Green with Black striped extension wire were continuous electrically.

To complete the wiring query, with the multimeter's probes stretched between the engine bay and the passenger footwell, I measured continuity from the Green with Red striped wire's connector in the engine bay all the way to the CIS-E's unplugged cable connector at pin 21. It FAILED! Unsure, I checked and rechecked the multimeter's probes to make sure they were seated and in contact as desired. Again, no continuity. I took a break to contemplate the situation and its ramifications...if the wire was broken somewhere in the harness, this would be a preferrable repair versus replacing a defective CIS-E control unit. I reviewed the electrical diagram and quickly realized that I had connected the wrong color wire in the engine bay. I ran out the door to correct my mistake.

At this time I realized that the extension wires were not connected to their matching-colored brethren on the harness (easy enough to do considering the faded colors and grime). Although functionally harmless (since B11/2's connectors are interchangeable), the incorrect connection of the Green with Black stripe wire from the sensor to the Green with Red stripe wire in the harness led me to believe in earlier diagnostics that unplugging the Green with Red strip wire at B11/2 had no affect on the engine's idle speed...when in fact it did! So, the B11/2 was being detected by the CIS-E and subsequently the ISCU, which has been reacting accordingly. With a quick probe swap to the Green with Red striped wire, and I confirmed continuity between B11/2 and the CIS-E (with and without the short extension).

Unfortunately, I hadn't digested all this revised input into my diagnosis and I continued testing on the assumption that the weak signal voltage being sent by the CIS-E to the ISCU must by the fault of the CIS-E. I reasoned that while not perfect, the B11/2 sensor was providing information to the CIS-E. The problem seemed to be in the output of the CIS-E's TF Signal.

[BabyBlueBenz]

Already disconnected, I removed the CIS-E control unit from the car. I unscrewed the black case covering the innards. Inside on my hobby bench, I examined the double circuit board arrangement to determine which board could be freed to allow examination of the components upon them. The lighter green board populated by integrated circuits was carefully separated by removing two screws and unlocking several plastic bayonet post mounts. I carefully unfolded the two boards on their shared ribbon-cabled hinge. Using the inspection methods recommended for the ISCU went it becomes internally suspect, I looked at the components for any signs of overheating or bad solder joints. A few questionable solder welds were reheated with an small-tipped iron. Overall, the stuff inside my CIS-E looked good.

With the sounding continuity feature enabled, I attempted to trace the circuitry from the connector pin assigned to connect with B11/2. The related components looked good, and so did there solder welds. I did the same thing with the pin that outputs the TF Signal to the ISCU. Again, nothing signficant was noted.



Back in the car, I reconnected the naked and unfolded CIS-E. I successfully measured continuity from the B11/2 sensor wire plug to the backside of the CIS-E's mounted connector. With this additional access to the connections within the CIS-E, I confirmed and performed a few more electrical checks from the previous test groups. Nothing remarkable was found. I duplicated the previously failing TF Signal test and got two measurements of 0.50V and 0.57V.

As the engine continued to warm-up, I repeatedly measured the voltage input from B11/2 at the CIS-E's pin 21 and the output from pin 9. When the temperature stabilized at 159 degrees F (70 deg C) the two values converged at 0.46V. At that time, I realized that the manual's specification (3.5 - 5.0V) and my measurement were off by an order of magnitude. Could it be that the manual contains another (potentially the third for me in two days) typographical error? Can anyone confirm the correct output voltage of pin 9 for a 1986 560SL's CIS-E?

I realize on hindsight, that the engine was not warm enough the tests I performed today. I will be repeating several tests again tomorrow, and definitely double-checking if B11/2 is affecting the idle speed.

[tecqboy]

Great methotical troubleshooting. I think you've eliminated the temp sensor and the associated wiring from the problem. You are at the point where you need an electical diagram of the control unit. Does alldata have this? Anyone?

[BabyBlueBenz]

I retested the electrical properties of Coolant Temperature Sensor B11/2. Here's a chart depicting its Resistance during engine warm-up:



It would appear that my B11/2 is a bit off.

And B11/2's voltage as measured at the CIS-E Control Unit's Pin 21:



These values suggest that the sensor is borderline. Realize that these plots are dependent on the accuracy of the aftermarket digital temperature gauge that I installed last year. If the gauge is off by only 5 degrees, the observed values within the specs.

I think my B11/2 is good enough. But, what would you do?

I continued with other diagnostic tests today, targeting the CIS-E Control Unit. Besides the previously mentioned faulty TF Signal, I found that the Oxygen Sensor's measurements were way out of spec. I will be investigating this further by checking this sensor's failure indicator failure (aka Why the The Idiot Light hasn't come on if it's bad).

[Gurunutkins]

very nice bit of testing the sensor, from the graphical output, looks like it is good (assume you mean K ohms on the first?). There is very little left that you havent tested by now so hope its the O2 sensor! just a stupid question but you have tested the wiring and bulb to the idiot light to make sure some Previous Owner hasn't cut it?
cheers
barri

[BabyBlueBenz]

Originally Posted by Gurunutkins

...assume you mean K ohms on the first?.

Oops, did that before. Yes, K Ohms.

Originally Posted by Gurunutkins

There is very little left that you havent tested by now so hope its the O2 sensor! just a stupid question but you have tested the wiring and bulb to the idiot light to make sure some Previous Owner hasn't cut it?

I haven't checked yet, but will, per Service Manual 14-116 "Checking oxygen sensor failure indicator". After that, its 07.3-258 "Renewing oxygen sensor" and/or Maintenance Manual Job 1480 "Replacing oxygen sensor".

[BabyBlueBenz]

I investigated the proper operation of my O2 Sensor's "idiot light" and found it to illuminate as expected during the initial turn of the key. I've learned that this light is controlled by the CIS-E Control Unit (later years getting a purpose-built control module named N44/1)...except that for my year, the pin on the CIS-E that is supposed to do this is "Not Assigned". Whatever, that's probably just an anomoly in the kludgey service manual.

So my understanding of it is that the idiot light does not indicate a fault in the sensor, but merely acts as a reminder to replace it at 60,000 miles.

Regardless, I'm going to be replacing my suspect sensor with a Bosch 13926, which is OE. Job 1480 basically states:

• disconnect lead wires
• unscrew old
• apply lube paste to new sensor
• screw in new
• torque to 50-60 Nm

Here's what I'm facing:



It looks pretty gnarly (and wet?).

If I spray some PB Blaster on it and let it soak, will it come off without protest with a big crescent wrench or Vise-Grip?

What's the lubricating paste supposed to do? Can I substitute anti-seize?

[BigFish246]

What was the end result of this?
Was this fixed?