ECU rev limiter

Sat Apr 28, 2007 2:38 am

Anyone know how the ECU activates the rev. limiter? 1990 M42 with cat.
Cut the fuel, spark or Both?

I'm getting the same effect at 2500 rpm that prevously only happens at 6500rm... Sensors, wiring all checkout OK. All except the O2 sensor which looks like you need a PhD in Physics to figure out WTF is going on
I reckon the ECU got fried after a battery jump start without any additional electrical load .

Cheers Mark

Sat Apr 28, 2007 12:59 pm

there isint one in the ecu on that motor... look at your cap and rotor first..

Sat Apr 28, 2007 1:05 pm

Thanks for replying Zaust, but the M42 has a 4-coil pack, no dizzy nor rotor... runs with a Motronic 1.7 ECU

Sat Apr 28, 2007 1:14 pm

Have you tried disconnecting the battery to reset the ECU?

Sat Apr 28, 2007 2:03 pm

Rev limiter on an M20 cuts fuel only (No rev limiter when running on LPG) - I wouild expect the iS to be the same.
O2 sensor wiring is simple. Two of the wires are for a heater - one goes to earth, the other comes from a relay. One of the other wires is the signal earth, and the fourth wire is the signal itself.
ECU failure is rare. How is the rev limit operating? A sharp cut off, or just an increasing misfire?

Sun Apr 29, 2007 12:06 am

Not that easy to describe, I would say more like an increasing misfire which disappears when the accelerator is partially released.

I have discinnected the battery for 2 hours. In that time I had the chance to unplug and reseat the large multipin plug that connects the engine wiring loom to the ECU aswell as check electrical continuity between the connectors under the bonnet: <coil pack, injectors, temp sensor, cam and engine speed sensors> and the aforementioned multipin plug (the Russian website is good fort hat!)

As mentioned, the lambda sensor got me bamboozled as it seems to have an oscillating output signal with a warning not the check the resistance as the voltage applied can damge the sensor. True or urban myth?

Sun Apr 29, 2007 12:19 am

Lambda sensor should have an oscillating output - essentially a square wave with a minimum frequency of 2 Hz. Higher the frequency, the better.
Sure you haven't got an air leak somewhere, bypassing the air meter?

Sun Apr 29, 2007 1:13 am

That is possible, but spraying around the manifold area and throttle with carb cleaner doesn't change the idle wjhic is rock steady at 750 rpm.

Opening the oil filler cap or removing the crankcase breather hose gives the expected rough idle

I'm thinking that the AFM connection is the problem... the wiring diagram shows the grey and white cable is common to the TPS, AFM and ECU in a M42. I'll post the result

Sat May 05, 2007 1:51 pm

connections between AFM and TPS show the expected connectivity.

The resistance of the throttle potentiometer shows values between 495 closed to 180 ohms when fully open.

BTW does any know how to get the M42 ECU to display the Motronic fault codes.
The M20 will flash the code at pin 15 after the WOT switch has been activated 5 times in succession.
Since the M42 doesn't have idle and WOT switches, wondered if anyone had figured out how to make it happen.

Sat May 05, 2007 3:15 pm

i put new plugs in my car which made the car rev to 4k put the old plugs in and it limited at normal rpm

Sat May 05, 2007 10:16 pm

Thanks surj, do you mean spark plug or an engine connection plug?

In addition I did open up the C101 plug to clean and reseat the connections plus the same for the main, DME and 02 relays

I still have the feeling the ECU got damaged after a battery jump start, that why I was interested if anyone knew how to access the fault codes.

Sun May 27, 2007 3:06 pm

replacement ECU arrived (thanks Chris W) but unlike Red-devil, no difference...

made sure the battery was disconnected before replacing the ECU connector, so now all I can think of is the AFM.
(or spark plugs)

BTW, does an ECU normally get hot? I've been driving around with the kick panel off and the ECU is very warm to the touch after 10 minutes.

Sun May 27, 2007 6:37 pm

isnt the ECU mounted under a heating duct? so if the heating is on, it will get hot?

i dont think and ECU should be getting hot on its own tbh..

Mon May 28, 2007 12:23 pm

Thanks Craig, ECU gets hot even without any heating on.

OK, hot is a relative term, ECU gets as warm as the power pack on a laptop PC when charging up the battery.

Fri Jun 01, 2007 10:30 pm

CHECK UR SPARK PLUG CABLES! MESURE THE REESISTANSE AND CHECK THE ENDS FOR FAULTY CONTACTS. ALSO CHECK UR CAMSHAFT SPROCKETS MAYBE THERE LOOSE .FINALLY CHECK UR VALVE COVER FOR LEAKS.OIL GOES INTO THE SPARK PLUG HOUSING AND SHORT SIRCUITS THE SPARK PLUGS. ITS THE MOST INSANE REASONS THAT I CAN THINK RIGHT NOW MAYBE IM TALKING NONSENSE BUT HAVE A LOOK .SORRY FOR CAPITAL LETTERS I WAS TO LAZY TO CHANGE THEM!!!

Sat Jun 02, 2007 1:01 pm

Thanks WarlockM42.
Cables have continuity and a plug resting on the block will spark from any of the 4 connectores.
Valve cover is tight, no oil in the sparkplug housing.

Today I checked for leaks using a bottle of propane gas. No response around the inlet hose, throttle or manifolds.
However letting the engine suck in propane through the airbox, the ICV starting clicking like crazy and the engine died. Took 10 seconds of cranking to fire up again.

Checking the AFM, connector pins have the following resistance values in ohms

4&5: 2060 at 23'C
2&5: 58 closed, 860 full open
1&2: 360 closed. increses to 700 then drops to 60 and then increases again to 500 at full open

Anyone know what the correct values should be for an M42 AFM?
since pin 1 on the AFM is common with the TPS, could the weird resistance be the problem?

Sat Jun 02, 2007 1:42 pm

Check the AFM by connecting a 5 - 12 volt power source across it and monitoring the voltage on the wiper pin as you open and close the flap. Should change smoothly with no jumps and drop outs.
The dying and refusing to start after admitting propane into the airbox is normal. Engine was getting gas and petrol, so was running so rich the mixture wouldn't ignite until the engine pumped it out the exhaust.

Sun Jun 03, 2007 10:36 pm

Check the AFM flap for smooth operation some times oil makes deposits making it dificult to open. Chek the inlet manifold for cracks or loose bolds check every hose (air ,vacuum, oil fume hoses) by hand dont depend on propane.Also check the fuel presure regulator.Try to close the fuel return hose whith a pair of plastic pliers and see if the engine goes to full rev.If yes, you have a fuel presure problem. Maybe dirty injectors, faulty pump too.Check theese and ill be back again whith more!!!!