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Information on BMW idle speed problems.

Pin assignments used on BMW idle speed control units. (Used on 1982-1987 5 series and 3 Series. Not used on Motronic 1.1 cars)
Pin Function
1&5 Idle air stabilizer valve (9-10 Ohms between the 2 pins at 73 Deg. F)
2 Battery
3 Engine speed
4 Ground
6 Coolant temp. switch
7 Auto. transmission range switch (12 Volts when in Neutral)
8 Auto. transmission range switch(12 Volts when in Park)
9 Air Cond. on signal
10 Air temp. switch (12 volts if below 18 deg. F, 0 V above 39 deg F)
11 Engine temp. (resistance to gnd. varies with temp.)
12 Throttle switch(Continuity to gnd when throttle closed, open otherwise)
SUMMARY: Tale of fixing a "surge" problem on a '84 528e. IDLE STABILIZER OVERVIEW: Using a solenoid type control (VDO unit), feedback loop, and a control unit to stabilize the rpm to 700 rpm under various conditions. Input to the control box: engine speed engine temperature idle contact switch system voltage Output from the control box: PWM (pulse width modulated) to control the stabilizer valve. When the solenoid is disconnected, or failed, it will remain wide open. The idle rpm should go "open loop". A warm engine will rev until 1200 rpm, causing the the fuel injection system to intervene, cutting off fuel flow. Shutting off the injectors causes the engine to decrease in speed... cutting the engine out. RPM drops back, and the fuel starts flowing again. This will give a surge from 700 RPM to 1200 RPM. Important to realize the idle stabilizer is wide open, so the rpm is actually being controlled by the fuel injection system running, then cutting out. (oscillation period for this engine was about 4 seconds). With the solenoid connected directly to 12 volts and ground, it will fully close the valve, disabling any idle function. Once the car is started, it will stall unless the throttle is depressed to keep it running. Thankfully this is a fairly simple test to check the solenoid. With a direct connection from the battery to the solenoid, current should be around 1.2 Amps with the valve connected between +12 and ground. Should the valve be shorted out, it will have a much higher current flow, possibly damaging the meter. If this is the case, the control unit may have also been damaged. This also explains the dealerships desire to replace both units at the same time. Make an adapter to allow the current flowing through the stabilizer valve to be measured. An analog ammeter is preferable, to see any fluctuations, but a digital one will work since the pulse train is fairly fast, and the inductance of the stabilizer valve will dampened the fluctuations. A neat "emergency" trick is to drill a 1/4" hole into a penny, place the penny next to valve input, replace the feed hose, and leave the stabilizer's electrical connection disconnected. This will result in an idle of approximately 800 rpm, allowing you to temporarily drive without the engine surging. Once the solenoid was checked, both under cold and hot conditions, the ammeter was left attached and placed into the passenger compartment for a test drive. Normal idle should produce a current reading of 470 ma. This was confirmed. On the vehicle tested, the idle current would suddenly drop to a value much lower value than expected, and on occasion completely to zero. Since the valve checked out good previously, [it may have been a heat related open] the most likely cause was the control unit itself. CONTROL UNIT: A small "black box" [about 2" x 3" x 1" thick] is located in the upper portion of the glove box. The much larger unit is the Motronic engine management system, the one we are looking for is usually attached to a support bracket. The control box was marked with a green band of tape, showing it to be of the later recall that affected the '84 model year. The control valve is silver, as the earlier [recalled] units were black. Once I had the box out, I was able to open it with care using a half dozen paper clips, a small screw drive, and lots of patience. First glance didn't show anything unusual, only four Integrated circuits (IC's) all of which were LM2902 quad op-amps, a few active components (transistors), and maybe five dozen passive components (resistors, capacitor, and an inductor). The output transistor, an BD438 [PNP 4Amp] , showed a fair amount of heat being dissipated, blackening the PCB (parts component board), and was a fair suspect for a "heat related failure". The heat, and vibration, of the unmounted device had caused the solder joints to degrade in appearance. Desoldering to component it was obvious the collector lead was barely making contact. Once I had the power transistor out, I tested it over night on a curve tracer, looking for an intermittent failure. The transistor performed satisfactory, but left me perplexed as to why VDO didn't use a heatsink on it, but left it with only its leads to hold it in place and provide a heat dissipation path. Further examination of the PCB showed a hairline crack in the trace leading from the transistor to the connector. This would close at cold temperature, then OPEN at higher temperature. Since the contact was marginal it will allow some current to pass, but not as much as expected. A repair was carried out by soldering a wire direct from the transistor to the connector. So far everything has held together, and the car is back into operation as hoped! While a new control stabilizer valve and control unit would have set me back almost $500, I was able to track down the failure which give a higher sense of satisfaction, al a bit at over 40 hours involved! [It would not have been unusual for a BMW mechanic to have billed over 20 hours labor, and the new parts, to a tune of $1300 to repair this!] Since a new control box, from the dealer, cost $250; wrecking yard would be approximately half that cost. But given the failure I described, the life expectancy of a used unit would be questionable. It seems a simple microcontroller with built in diagnostics could be built to replace the original. Advantage of the aftermarket unit would be a proper heatsink, current limiting (so a bad valve can not destroy the control box), and could incorporate an LED to flash a diagnostic code allowing a quick check of the system functions. In order to develop such a box, its full cost would have to be around $20, to insure a resale price less than $120. How many BMW's use the "green strip" or "green box" control system? Typical life expectancy of the VDO unit? Cheers, Tom Walter Beaverton, Oregon. P.S. for BMW owners of the similar model: BMW recommends a full tune up, vavle adjustment, fuel pressure check, before proceeding with any electronic diagnoses. Also the 528's were prone to developing carbon deposits on the intake valves, causing driveability problems [they developed a system to "walnut blast" the intake valves, with the intake manifold removed]. Hats off to someone who could FIX one of those boxes Heres my knowledge of the coding of said ICUs/ICVs Early Unit Recall Current Engine (Recalled) Replacement Unit 1.8l blk/blk blk(grn stripe)/blk grn/yel 2.8l grn/red (looks purple to me!) 3.5l grn/blue The color is Body/Connector Early ICV's were black plastic with INTERNAL current adjust Newer ICV's are silver with external current adjust BEWARE .. some parts places and dealers are selling BLACK boxes with yel/red/blue ends these are NOT the current and best option ... the green (olive green) bodied boxes are purported (by CCA tech tip reps) to be the BEST as they are designed to NOT burn out (as much)