Idle Air Control Voltage Drop Case Study

Brian Diagnostician Willoughby, Ohio Posted   Latest   Edited  
Case Study
Trained By Techs
1988 Mercedes-Benz 300SE 3.0L 4-spd (722.3)
Low Idle

Today, I had a 88' Mercedes 300SE come in with the customers concern of an intermittent stalling issue. The customer said it would restart every time and would run well for a short while and then occasionally stall thereafter. 

I was able to re-create the symptom without too much extensive driving. Within 10 minutes on my initial test drive, the vehicle would stall. Just like the customer said, it would restart just fine and you could continue on. 

I made it back to the shop and began to make my lab-scope connections to pinpoint the "funnel" or direction to head. My first connection was at ignition coil primary control circuit. This vehicle has one single coil (2 wire) with a distributor. Shortly after the connection was made, the vehicle stalled in the bay. GREAT! Broken cars are much easier to diagnose than ones that never act up! I attempted to restart the vehicle and it would not start without depressing and holding the gas pedal. 

Aha! I have a new funnel! Idle control seemed to be missing! 

My next connection was to the control and power circuits to the Idle Air control solenoid. This vehicle has a very large valve under the air cleaner with 2 hoses attached to control the air flow bypassing the throttle plate. 

In figure 1 you can see my first scope capture where Channel A in blue is my IAC ground control from the powertrain module. On channel B is the B+ circuit which is delivered from the "Over Voltage Protection Relay" and on Channel C is Ignition Coil Primary.

In this first capture, you can see the vehicle stalls when the green ignition trace drops out, however, whats more important, is what happens right before the vehicle stalls. You'll notice on channel A that the ground control to the IAC is present even thereafter the stall event which is proof that the powertrain module is attempting to control the Idle air solenoid. On Channel B, however, you will notice the voltage that is feeding the Idle air valve takes a rapid plunge right before the vehicle stalls. This is proof that this circuit cannot maintain the proper voltage and current level due to a voltage drop/high resistance condition. In Figure 2 you can see the capture zoomed in for more detail. Also, Notice that during every event of the duty cycle the solenoid is receiving, the B+ voltage is being pulled down dramatically. This is also hinting that the circuit may be compromised even prior to the stall event. Channel B voltage measurements were made and an average of 12.6 volts were found (lower than charging voltage) prior to the stall and an average of 9.4 volts were found during the stall event. 

The rapid plunge of voltage which ultimately led to the vehicle stalling was believe to be caused by another circuit that this relay provides power to. This Relay is a very sophisticated relay. It has 3 integral relays inside that are soldered to a large board with various chipsets etc. The relay measures at approx 3"x2" . It is quite the extravagant piece! When the unknown circuit was turned on, voltage plummeted and the idle air control solenoid no longer could maintain the proper air flow and the vehicle stalled. 

Further voltage drop testing was performed. In figure 3 you will see the wiring diagram that I annotated to show my connections. 9.4 Volts were found @ the IAC connector and also at the output of the relay. There was 13.6 volts measured at the B+ circuit from the battery that feeds this relay. This is proof that a 4.2v drop is present in the relay itself. 

In figure 4, you will see what a "known good" example should look like. This capture was taken after providing the compromised circuit a new path to B+ via a jumper wire and you will see the circuit is restored. Channel B voltage is elevated to charging voltage and does not show dramatic "pulls" while the IAC is being controlled. The circuit can now adequately provide enough current to maintain the proper voltage and the vehicle is happy again. 

We called the Mercedes dealership to find price and availability. The parts manager claimed the relay has been updated to a new design however is 286 dollars COST and 3 days out. What a relay!

Brin Diagnostician
Melbourne, Florida
Brin Default

A great example of a solid diagnostic approach. I have no critique. I appreciate your contribution. I enjoyed the read. I even felt like I was there harassing you for your music choices as you delved deep into the mystery of the intermittent stalling Benz.

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Brian Diagnostician
Willoughby, Ohio
Brian Default

:) Thanks Brin. Complex diagnostics are often complimented with the ambiance of soft jazz. I most likely was playing some classic rock during this one!

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Michael Mobile Technician
Clinton, Utah
Michael Default

Very nice write up. Just so I am clear, the relay is the culprit because it is not able to keep up with the demand for the IAC? Could there still be an issue with a shorted IAC and the relay is dropping the power as designed to protect other components?

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Brian Diagnostician
Willoughby, Ohio
Brian Default

That is correct. The voltage drop across the relay was the cause of the failure on this vehicle. I verified this by supplementing an alternate power source (jumper wire) to the B+ terminal on the IAC valve. When doing that, the system was restored. Thanks for the feedback and kind words!

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Leo Technician
Rhinebeck, New York
Leo Default

Excellent. That was a pleasure to read.

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James Owner/Technician
Pike, Pennsylvania
James Default

Very common problem with mid to late 80's E bodies overvoltage and fuel pump relays with cold solder joint issues. I hope your not waiting for a dealer ordered relay as WorldPac supplies both relays at $24.54 and $96.34 respectfully. Nice captures and hunting!

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Tomi Diagnostician
Chicago, Illinois
Tomi Default

Great write up Brian. Funny how sometimes the most complex issues can be resolved sticking to the basics. It was a great read looking forward to more!

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Brady Owner/Technician
Yakima, Washington
Brady Default

It's still early and I'm barely getting my coffee so I apologize in advance if I'm missing something :) I'm curious as to why the voltage on channel A doesn't drop with the voltage on channel B??? As far as I can tell it keeps cycling between approx 12 v and ground, shouldn't it drop down to 9 volts along with the power supply?? Great write up! Thank you

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Chris Diagnostician
Commack, New York
Chris Default

Great Brian!! Not enough people get into this depth of testing on those old Mercedes. I do love being able to see the actual issue graphically. Good job again.

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Brian Diagnostician
Willoughby, Ohio
Brian Default

Thanks Chris!

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Thomas Diagnostician
Saint Petersburg, Florida
Thomas Default

Great case study, Brian! The older Mercedes relays (as mentioned) would be opened up and least as a temporary repair. Hope to see you in Atlanta.

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