I should never have counted them!

Sigh... Last night I got the new OVP installed and whammo the problem was back. Weird idle, stalling etc. Waste of £40. Never mind it forced me into looking elsewhere.

I checked the coolant sensor, the two pronged one for CIS-E & EZL. Thing is open, no resistance - infinity. Aha. The 4 pin coolant sensor - yes I have two temp sensors on my car - I think this one goes to the ECU - was working fine. Not sure if the two pin one is needed but I will replace.

Now, there is a sensor attached to the Air Flow Meter, it's called either a "air flow position sensor" or a "throttle valve switch". I'm really not sure which it is, because there is a short cable that connects the sensor to the switch.. Anyway, the sensor I think is supposed to show 0 ohms on one of the pins. At idle or engine off all of the pins show infinity. I gather this means it's nuked. Anyone with experience on these things?

Cheers,
Neil
190 2.5 16V Evo 1

Before you replace the coolant 'sensor' make certain it is not a switch. Temperature sensitive switches are used for the fan clutch and the front electric fan. Switching temps are 70C and 105C, I believe. They will of course read 'open' below the switching temperature.

Steve

The airflow meter is mounted above the throttle unit, which are connected via a rubber air duct. The throttle unit is bolted to the intake manifold plenum, from which the intake runners rise up to the cylinder ports.

Using this rubber air duct as a guide, if the sensor you are testing is above that, it is the airflow meter sensor, if below, the throttle valve switch. The former is a potentiometer that should vary smoothly as you push down the sensor plate below the air cleaner. The latter should close contacts (two pins shorted together) with the throttle at idle, and close other contacts with the throttle full open. In between all pins should be open to the others.

Do not confuse the airflow sensor plate with the throttle - it is in no way connected to the throttle linkage.

Steve

I tested the air flow sensor pot and I see infinity on each of the pins. When I have everything hooked up and idling I see 4.88v on 1-3 and a range from 0V to 1.3V on 1-2. It swings wildly stays at zero for a second or two then ramps up fast to over a volts then falls dramatically, mirroring the result in the idle - up and down. The higher the voltage the lower the idle.

Can someone please explain? I gather the pot really is dead, no resistance for a start..

Oh great I feel a severe lightening of the wallet... warrgh...

Well, note that you would expect it to go up and down with rpm change. Going to '0' is likely not good, though. The wiper is probably hitting a bad spot on its track.

Steve

I can understand it changing with rpm, but why is the rpm changing by itself? With the sensor/eha removed the idle is stable and the car is tuned correctly - it drives great with no electronic control. It's been tested on a Lamba/CO2 sensor with everything disconnected and the duty cycle is around 40% slightly rich. Once I connect the sensor back up again the idle goes mad. In the factory CD it gives resistance valves for the pot, mine is showing infinity.

If you get a varying voltage reading as the rpm varies, this means the resistance is not always infinity. Best is to clip the leads on the unplugged sensor while slowly moving the speed sensor vane (engine off). Across some 2 pins, you should see the resistance vary smoothly across some range. If the reading at any point jumps to infinity (or a much larger value), the sensor is bad. Be careful, though. All meters have a maximum resistance capability, and will read infinity above that. If your meter is not autoranging, this point will occur in each range at a different value. Too, digital meters can be frustrating for this sort of measurement - an analog meter is better for trying to judge smooth pot operation.

You will likely find the pot operation is acceptable for readings off-idle, due to the larger amount of time the wipers spend moving about at idle than at higher load conditions. If you replace the sensor, take as many resistance readings as possible corresponding to different positions of the sensor vane. Also measure the total resistance from 1-3 (where you measured the 4.88V). This information will allow you to mechanically calibrate the new sensor (assuming that is possible, I haven't yet had to do this on ours).

Steve


Steve

Thanks Steve will give it another test. Is it possible to replace the Pot or does it mean an entirely new Air flow meter?

Some knowlegable soul (I believe M.B.Doc) reported they are available aftermarket from Bosch - not MB. Do a search on it.

You could also have a look at its repairability I believe there are two wipers inside connected together. One would move on a metal track, the other on the resistance element. It seems unlikely the wipers themselves are worn out, or problems would occur at all speed ranges. If there is some way to shift them so they 'wipe' a new section of the two tracks, operation might be restored. This is speculation, but something I would look at before replacing the sensor. I also wouldn't risk it unless I KNEW I could replace the sensor with a new one quickly, if need be.

Steve