Idle Air Control Valve (IAC) question

[Michael_P]

I was just wondering if anyone knows how much input values vary from year and manufacturer on idle air control valves. The reason for asking is; I am working on a 1981 Mercedes 380SL. After testing the control unit (as well as over voltage relay) and voltage to the IAC pigtail as well as testing the IAC itself, I have determined I have a bad IAC. The part is about $400 new. Since I have a machine shop, I could build an interface from billet in order to run an IAC which is much cheaper. Building an adapter pigtail is no problem either. I have the values for the idle air control valve for the 380SL, but cant seem to find values for other idle air control valves. One thing all idle air control valves seem to have in common is; with no voltage to the idle air control valve, they are in full open position. This seems to hold true for pre and post OBDII vehicles. Regardless whether idle air control valves operetate using a solenoid or motor, the function seems to be the same.

I was just wondering who here would think building an adapter to run a cheaper idle air control valve (lets say a $40 IAC from a Ford) from a different car would be a colossal waste of time as well as a waste of money. And why? Is there a possibility it would work or no way would it ever work?

I know it sounds sort of outlandish, but it also goes beyond saving money. I want a part that could be widely available as opposed to having to wait 6 weeks for a part. I have heard of people changing pigtails on O2 sensors (provided they have the same number of wires) to use cheaper and more widely available sensors with no issues. But I dont know if anyone has ever tried using idle air control valves which were not specifically meant for a certain vehicle.

 

[LvR]

These things are normally stepper motor driven with overall valve response characteristics tailored to the size of the motor. My guess (electronic engineer and a lot of garage experience) is that the engine ECU needs fairly well defined actuators (read IACV as well as a few others) in order to successfully control any given parameter (read engine speed) ..................... my gut feeling is you are going to struggle to find/create an IACV with identical response characteristics to the old. You may very well end up with a run-away ECU that will be unable to stably control the engine speed in the normal closed loop control system fashion.

 

[spackard]

I've looked before for parts, either swimming upstream to the original vendor to the OEM or finding a part whose behavior is identical.

In California, any emissions part that isn't original in look and form will fail their test. So there's that. Otherwise, if you have the ability to put another part in there go for it.

 

[Wolf359]

Interesting that you mention Ford. They always had a history of going bad eventually too, but you could clean them and then they'd start working again. But people who would replace them with an aftermarket part used to end up with problems as it was usually the OEM one that worked well. Also at that age, can't you get one from a junkyard?

 

[rubberchicken]

Look on eBay, or perhaps a junk yard that specializes in Mercedes , possibly even in Germany.

 

[Falcon_LS]

Mercedes-Benz calls it an Idle Speed Control. IIRC in that generation, it used an engine oil temperature sensor to control the engine RPM though the ISC, which in turn sent a current to the oxygen sensor control unit. Did you check the engine oil temperature sensor to make sure it's functioning as it should? On a stone cold engine, you should be getting about 5 ohms KOEO. Alternatively, pull the connector off and fire up - if the idle drops to 500 rpm, the temperature sensor is faulty.

Is this a USDM vehicle with part number 000-141-12-25?

 

[Falcon_LS]

Originally Posted By: Wolf359
Interesting that you mention Ford. They always had a history of going bad eventually too, but you could clean them and then they'd start working again. But people who would replace them with an aftermarket part used to end up with problems as it was usually the OEM one that worked well. Also at that age, can't you get one from a junkyard?


I usually buy Motorcraft, but I've used Hitachi on two applications. They were made in Japan and work fantastic, one of them being four years old.

 

[Merkava_4]

It must be one heck of a special car to put up with prices that high.

$400 for an idle air control valve is ludicrous.

Just to give you some perspective, a brand new Delphi IAC for my Buick is $28.79

 

[Wolf359]

Originally Posted By: Merkava_4
It must be one heck of a special car to put up with prices that high.

$400 for an idle air control valve is ludicrous.

Just to give you some perspective, a brand new Delphi IAC for my Buick is $28.79


It's an SL, they were always expensive. A current model SL450 or SL550 ranges from 87k-110k. Mercedes parts are always ludicrous, usually at least 2-3x times what other companies charge. In this case, much more than that. Probably because it's an SL and maybe because it's so old. You gotta pay if you want to play.

 

[Falcon_LS]

At least new OE parts are available, unlike some manufacturers that stop producing parts after a certain time frame, or worse, second or third tier "value" parts (e.g. ACDelco's Professional and Advantage range vs. OEM or Motorcraft's Value range). That's one of the advantages of owning a German car I suppose.

If there is a limited production run, cost is going to be a lot more than a wear/tear item that's mass produced. I had to replace a cracked central high mount stop lamp housing on my GMC - $200 for a piece of plastic that probably doesn't cost $10 to produce. You gotta pay if you want to play is right!

 

[daman]

If they can afford the car and insurance they can afford the IAC, stop being cheap and just buy one.

 

[Wolf359]

Originally Posted By: daman
If they can afford the car and insurance they can afford the IAC, stop being cheap and just buy one.


When they get that old, they're not that much money, probably worth 15k+. I think with MB, you can get parts for a car no matter how old it is.

 

[Michael_P]

Quote:
If they can afford the car and insurance they can afford the IAC, stop being cheap and just buy one.


Truth be told, I am "they". I do have the capital to finish the car correctly, but I want a car I can keep for a long time with more widely available parts if possible. I will never enter this car in a show or try to sell it as an original. I just wanted a fun, unique car. If I can replace a part with something less expensive and more common, all the better. But not at the expense of reliability. Anything I do of this nature will be able to be undone with basic tools. Also, insurance is super cheap for this car. Surprisingly cheaper than most other convertibles. I picked up the car for less than what I paid for some of my bicycles and its pretty solid overall. In the week I have had it, I have diagnosed just about every issue with it and have fixed the rusty section of floor (the right way). My goal is to enjoy it this summer and fix it up as I drive it. Come fall, I would like to take it to the paint shop, have it done right and pick it up in the spring.

Thanks everyone for the advice, suggestions and information. I did snag a used MB idle control valve on eBay for $55. Also, I tested the oil temp sensor based on Falcons advice (thanks for the P/N too). I would still like to know if other idle control valves would work. It seems to be a common part for any KE Jetronic FI system and vary widely in price. The part can be made by VDO or Bosch and it looks the same whether it goes on a VW, Audi or BMW or Mercedes Benz of that era less the connector type. Even though they look the same, I know they could have different resistance values. At least this time, I can go used OE.

 

[Falcon_LS]

Originally Posted By: Michael_P
Thanks everyone for the advice, suggestions and information. I did snag a used MB idle control valve on eBay for $55. Also, I tested the oil temp sensor based on Falcons advice (thanks for the P/N too). I would still like to know if other idle control valves would work. It seems to be a common part for any KE Jetronic FI system and vary widely in price. The part can be made by VDO or Bosch and it looks the same whether it goes on a VW, Audi or BMW or Mercedes Benz of that era less the connector type. Even though they look the same, I know they could have different resistance values. At least this time, I can go used OE.


Glad you were able to source one.

Most of the ones I've seen for Mercedes-Benz are Siemens/VDO, although Bosch certainly makes them as well. That being said, a lot of these are tailored toward a specific application. The reason I asked about the part number is because the European market version uses part number 000-141-14-25 (USDM is part number 000-141-12-25) - if you were to use the European part, you will get high idle. Unfortunately, they are not interchangeable as we would like to believe - it will fit right, but that's about it.

 

[ABursell]

I'll just add that if you do decide to try the Motorcraft route and end up with the older style that was used on the pushrod 5.0 engines, Ford makes (made?) an add-on plate that fits between the throttle body and the IAC with a couple small screws in it for fine-tuning type adjustments. You might be able to make one or add one to the Ford IAC or whatever IAC you end up with if you can't get an exact match and get it tuned?