420SEL, 1988

[Please understand that I spent at least four hours searching and reading about 150 prior posts, and did not find a solution to my problem, before asking for MB Tech help.]

SYMPTOMS:

1. Will not hold idle RPM. Starts great, but letting up on the throttle after starting, the RPM drops quickly to zero and, of course, it quits.

2. Worse when cold (RPM drops faster to zero), but the problem does NOT resolve after engine reaches normal operating temperature.

3. Starts immediately, and accelerates smoothly, so long as driver holds the accelerator in far enough so that RPMs cannot drop to the point of engine quitting.

4. Develops full power at highway speeds; not indicative of a fuel pump or injection problem.

5. The problem occurred suddenly, not over a period of time. [Had been idling great earlier in the day. Wife says she started car, and it briefly idled. Then, she thinks she heard a “pop”, followed immediately by a brief faint smell of something burning. And the car died. The sound and smell could have been her imagination, or from something other than the event that began the idle problem.]

[NOTE: In August, a 30-year MB dealership tech did a valve job and renewed timing chain and guides. Car idled perfectly after that work, and for first 500 miles thereafter. At 1200 RPM and above, the engine still sounds so smooth and strong that there is no suspicion of a valve, timing, or timing chain problem. BTW, that MB pro is 300 miles away, so I cannot simply take the car to him for this idle problem.]

DIAGNOSTIC STEPS TAKEN:

1. Overload Relay: (a/k/a OVP) Both 10-amp fuses intact. Put new spare in system to troubleshoot. No correction of idle problem. No ABS light appears, as would happen if OVP failed. Concluded OVP is good.

2. Idle (Speed) Control Unit (ICU): Seems to check okay. To test, I rigged the throttle linkage to hold 1800 RPM. Then I unplugged the ICU. The speed increased to 2400 RPM. Reinstalled ICU, and speed decreased to 1800 RPM. I do not have a bench test to evaluate all ICU outputs, but this simple test leads one to conclude that the ICU has power to it, and that it is controlling speed. Also, I keep a spare ICU for troubleshooting. Tried the spare and it acted exactly the same as the primary unit. [p/n 004 545 1032, located inside cabin, at R/S foot-well] Of course, they BOTH might be faulty in one or more important functions.

When I rigged the throttle linkage to hold 1800 RPM, then turned on the A/C, the additional drag on the engine took the RPM down slightly, but in seconds, it came back up to 1800. The same was true when activating the defrost function of the ACC. [I understand from other posts that these two tests are at least partial indicators of a good ICU.]

-- QUESTION FOR MB TECH:
-- With all of the indicators I have of a good ICU, is it likely that my ICU is the problem?

3. Throttle Linkage: No difference from before idle problem began. All appears intact with proper tension.

4. Throttle Plate: Thoroughly cleaned throttle plate (lower one, inside throttle air chamber, connected to throttle linkage).

5. Idle Air Control Valve (IACV): Removed and soaked in carb cleaner. Kept rinsing until no more particles of carbon came out with the fluid. Depressed valve with small tool to assure it was moving freely. Lubed with WD-40. [p/n 000 141 1625]

6. Power to Idle Air Control Valve:

a. With engine running at 1500 RPM, removal of the power connector causes increase in speed to 2000 RPM. Replacement of power connector causes speed decrease to 1500 RPM. Also, put 12v directly from battery to the valve. Got the distinctive “click” of the solenoid activating. [Leads one to conclude that the solenoid valve is reacting to power.]

-- QUESTIONS FOR MB TECH:
-- Is this a valid test of the IACV?
-- Is this telling me that the valve defaults to open, thus allowing more air to bypass the throttle plate?

b. With engine running at 1500 RPM, voltage measured at the IACV power connector cycled from 0.9V, to 1.4V, to 2.4V, then back to 0.9V [If I understand this valve correctly, it uses variable voltage (dithering) to vibrate the valve piston so it does not bind against its cylinder.] But, this voltage does not look high enough, to me, to activate the solenoid, and there was not “click”.

-- QUESTIONS FOR MB TECH:
-- Are the voltages stated above in proper range?
-- I have the W126 tech data on CD-ROM, but have not found a reference for the correct IACV voltages. Would you have them?

c. DISCREPANCY WITH PRIOR POSTS: Someone reported measuring 12V from the hot contact of the IACV power connection, when the “common” test probe was put to ground. I do not get 12V for that test. Further, with the other (common) wire on this connector, I cannot get continuity directly to ground. It appears to me that this “common” is isolated from ground. The valve is not grounded to the engine block, and it is held in a rubber-lined bracket.

-- QUESTION FOR MB TECH:
-- Should the hot lead to the IACV be 12V?

d. The IACV does not buzz or vibrate. [I believed I have seen them buzz or vibrate (dithering) during operation. However, the one on my 380SL is also perfectly quiet, and that car idles fine.]

-- QUESTION FOR MB TECH:
-- Should the IACV buzz or vibrate when working correctly?

e. With power disconnected from the IACV, resistance across the two posts of the IAC is 5.4 ohms. This is consistent with my spare IACV. [However, the IACV on my 380SL (p/n 000 141 1225) reads 9.4 ohms, and I have a never-used spare for the 380SL that reads 9.4 ohms. Could be just a different type solenoid in the different part numbers.]

-- QUESTIONS FOR MB TECH:
-- Is the resistance on the IACV critical to its operation?
-- What is the acceptable range?

7. Vacuum: Tests strong from manifold. Economy gauge needle pegs to far left at 1400 RPM and above (as it should). A vacuum test instrument measured 18” Hg directly off the manifold (port at aft end of engine, near firewall).

-- QUESTION FOR MB TECH:
-- Is there a particular vacuum line which, if defective, would cause the RPM not to hold at idle speed?

8. Vacuum lines: Checked for breaks, cracks, and disconnects, including Brake Booster lines. Renewed a number of rubber connectors that were getting hard. All now appear okay. With the engine running at 1500 RPM, sprayed WD-40 over all vacuum connections and lines. No change in RPM.

9. Vacuum Line to Thermo Valve: [Component #79 on the official MB diagram for the vacuum system.] This valve had one of the nipples broken off, but I believe it has been that way since well before my idle problem appeared. I installed a new Thermo Valve. No improvement to the idle problem.

-- QUESTION FOR MB TECH:
-- Must this valve be functioning to achieve proper idle RPMs?
-- Or, is it solely for EGR?

10. Ignition Control Module:

a. With engine running at 1600 RPM, disconnected the vacuum line. Speed fell to 700 RPM, and engine continued to run. Economy gauge moved to about ¼” from left (likely due to open vacuum line).

b. Reconnected vacuum line: Speed rose to 1600 RPM.

c. No other negative indications with this module, but I have no way to bench test it further. Seems important for starting, but I cannot see how it would affect idle speed.

-- QUESTION FOR MB TECH:
-- Is there a function of the ignition control module that affects idle speed control, which, if failed, causes loss of idle?

11. Switchover Valve, Air Injection: Vacuum lines intact. With speed at 1600 RPM, disconnecting the power connector has no effect on RPM.

-- QUESTION FOR MB TECH:
-- Does this valve have anything to do with idle speed control?
-- Or, is it solely for exhaust gas recirculation?

12. Alternator: Output reading 13.5 volts.

13. Air Pump: Disconnecting the power connector has no effect on idle.

14. Cruise Control Actuator: Disconnected linkage that connects to the throttle linkage to rule out possibility that it was preventing movement of throttle. No cure for the idle problem.

15. Hall Effect Sensor: (a/k/a Speed Sensor) I am hoping to rule this out before pulling the instrument cluster to check it. I have a spare W126 instrument cluster from a 1985 car, and I don’t see anything attached to the back of the speedometer. But, in this case, I don’t know what I am looking for. [An earlier post says it is located on the speedometer on pre-June 1988 cars.]

[The interesting thing about this component is that it is inside the cabin, and fits the story my wife related after she tried to use the car, started it, and could not keep it idling. She thought she heard a “pop”, and then smelled something resembling a hot electrical component.]

-- Does a 420SEL have a Hall Effect Sensor?
-- Does the HSE have a capacitor or other component that would blow, making a “pop” and burning smell?

16. Rubber vacuum lines that run under the air chamber and connect, I think, to the intake manifold:

While I cannot see all connections, there seems to be nothing disconnected. And, the manifold vacuum measures 18 in. Hg.

Also, I sprayed WD-40 into the cavern where these hoses reside, and nothing changed.

-- QUESTIONS FOR MB TECH:
-- If one or more of these rubber lines had come off, or failed, would I still get 18 in. Hg vacuum?

The bottom-line question for anyone reading is this:

-- With all of the information given above, can you identify the likely cause for the failure to hold idle?

Well, sorry for the long post. But, I wanted to give all the information to avoid back and forth questions.

And thanks in advance for any help you might provide.

Steve Guinter