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Mitsubishi Montero & Montero SportThis sport utility vehicle offers more size than the other Mitsubishi SUVs, but manages to keep a sporty look and comfortable feel, unlike many larger SUVs.
New error code: "Vehicle Speed Sensor (Reed switch)" per page 14-77 of the manual. See the waveform below. But how can this be relevant if I haven't moved the car since I replaced the power transistor and coil? Not sure how the computer figured this out, but the test procedure on page 8-173 determined that the reed switch's transitions from conductive to non-conductive 4 times per revolution of the speedometer cable were not smooth. This is on the new (used) instrument panel I had installed to fix the non-working fuel and temp gauges. This same test confirmed that the old panel had a perfectly working reed switch, so took out the fuel/temp gauges unit from the new panel and put it in the old, and re-installed it. After clearing up the error codes by unplugging the ECU for 10+ seconds, no more error codes.
Also opened and cleaned up (with a Dremel tool) corrosion in the contacts of the hazard lights switch, through which the blinker signals also pass. It wasn't working. The cleaning fixed it.
The low idle and stalling when warm persists, but the misfires seem less pronounced. They still mostly occur at 3000-3500 RPMs, but sometimes they don't. The worst kind of problem: intermittent. I'm now wondering if some sensor is not working properly at that RPM range, but in any case the computer is not detecting any failure. Maybe bad contacts? But this doesn't explain the extremely poor mileage. Maybe one or more injectors? Getting to the injectors requires more disassembly. More research needed. It is possible that the misfires and the low idle/stalling problems are caused by different parts.
Any further hint welcome.
Last edited by 1990-montero; Apr 15, 2025 at 10:07 PM.
Yep these early 6G72s run an optical sensor in the distributor body only. It is driven off the right hand camshaft. The bloke at the wreckers said that these early 6G72s were well known for ignition problems, and distributor problems in particular. I think that later engines do use a different type of crank angle sensor - maybe Hall effect and driven off the crank?
The ECU used in these early Pajeros is pretty basic - well, they are nearly 20 years old after all. For example, there is no fault code if the crank angle sensor fails. If there was, it would have saved me a lot of time!
My particular engine is now 35 years old, not 20... and the manual does list an error code for crank angle sensor.
Tested the crank angle sensor. It is at the base of the distributor. It has 4 wires going to it: from right to left: power, ground and 2 signal outputs. Connected the scope to those 2 lines and the resulting trace is below. Looks like it's working correctly. If anyone knows that the voltages are not what they're supposed to be please let me know. I suspect that if this weren't working the engine wouldn't start at all. The manual doesn't give a test procedure, just that if it doesn't work, replace the distributor. The low idle persists and when the engine warms up it stalls. Just doing these tests, the gas gauge shows 1/8 of the tank was used. The only thing I can think of now is that one or more injectors is stuck open, but the computer insists that there is no problem. Other possible items I've read could be the cause are the "idle control valve", the "Iac sensor" and the "fast idle air valve" but it seems that they all describe the idle speed controller, which I've already replaced. I am now out of sensors to test, it seems. Unless I can come up with something else to test, it's off to the dealer.
Checked the timing again, on a cloudy day so I could see the little window by the crankshaft pulley. I see no timing marks with the timing light, at any RPMs. The timing procedure on page 8-148 of the manual says the engine should be at operating temperature, which poses a problem in my situation as when it reaches this stage it stalls. Is it normal that when cold and at RPMs that are lower than they should be I don't see any timing marks? Could this be a side-effect of the timing belt having skipped? I have also checked the dipstick. The oil looks normal. Presumably this rules out gas going into the crank case. So, it looks like the timing is way off, but why? Any hints welcome.
Next item to check: the oxygen sensor. Found it downstream from where the left and right exhaust headers join. The reinforced wire disappears above the transmission and I have no idea where to find connector B-19. Per the manual this 2-wire connector is where the oxygen sensor plugs in. Does anyone know where this connector is? The next connector upstream is B-29. This is another possible test point but I also have no idea where this connector is. This one is a 4-wire connector.
The manual says that proper operation is: at idle: ~400 mV, at 2000 RPMs: 600 mV, at higher RPMs: 600 - 1000 mV. If it is always low: air intake, always high: injector leakage.
The computer doesn't report any error code at all, even though there are error codes for the oxygen sensor, air flow sensor, intake air temperature sensor, barometric pressure sensor and injector.
But first I need to find those connectors. Can anyone tell me where to find them?
Oxygen sensor. Since I couldn't find the connectors above, followed the wires in the diagram (pages 14-60 and 14-61 in the manual) to the connector at the control module (connector C-39) but the pin locations don't match. I had to go by the wire colors. Connector C-39 is the furthest forward, next to the firewall, and the relevant pins for the oxygen sensor are the furthest forward. Ground is the first pin at the bottom row (black wire) and the signal is at column 3, top row (white wire with a red dot).
With ignition on but engine off, I measured 0.5 V, after I turned the engine on it went up gradually. At higher RPMs it was close to 1 V. At rough idle it went down a bit, but not as much as the manual says. After I turned the engine off it went down to 0.83 but then started going up again.
So, it looks like the sensor is reacting and it's high probably because of injector leakage. If my assessment is correct, one or more injectors are putting in way too much gas and I need to replace the injectors.
Oxygen sensor. Tested again, and this time I get 2.2 V, which is way out of range according to everything I read. Still, I found plenty of posts out there of people encountering those voltages. Because the schematic only has 2 wires going to the O2 sensor (see picture below), I assumed it didn't have a built-in heater. I ordered a new one (NTK) and it has 4 wires. Looked above the transmission to see where the wire goes, and it appears to go into the passenger compartment behind the center console. Haven't found the 4-prong plug that supposedly matches the new sensor yet.
Put the scope on the pins described in UPDATE 15 and I got the traces shown below, with engine off and engine on. The width of the pulses decreases with acceleration and the voltage drops slightly in the digital voltmeter (a true RMS meter, so it makes sense).
Latest hypothesis: maybe the heater voltage is somehow leaking into the signal output? Does this output look normal? Can anyone tell if the O2 sensor is bad?
Last edited by 1990-montero; Jun 27, 2025 at 04:26 PM.
It looked like the oxygen sensor wire disappeared above the transmission, so I assumed it went inside. Removed the center console and found where a big bundle of wires comes in from above the transmission, but no plug is to be found there that looks like the new oxygen sensor I got (see picture 1 below). Another dead end.
Assuming that I got the correct oxygen sensor, the plug (which looks like the type found inside the engine compartment) is probably somewhere in the engine compartment, but I haven't found it yet. Furthermore, the oxygen sensor installed at the factory has the wires encased in a flexible metal mesh sleeve so it's not possible to get to the wires, or see how many there are.
Unfortunately I don't know how 1990 model is wired. On my 2001 year model, two O2 sensor plugs are mounted O2 the transmission (rear post cat sensors). Thet are tucker right up between transmission and the transmission tunnel wall - very unconfortable place to get to. The other two pluga are in the engine compartment. Those are for the two front sensors.
After a trip to the local Mitsubishi dealer, the very nice guy at the service desk looked up their online manual, down to my specific VIN. Of course I had been looking at the wrong pin. He printed out the page with the diagram of the oxygen sensor. It indeed has 4 wires, and the output signal goes to pin 4 of the longest connector of the ECU. Ground is pin 17. See the attached diagram.
So I plug in the scope. With ignition off the voltage is around 30 mV. As soon as I turn on the ignition, it starts rising slowly, over about 20 seconds to stabilize at 1.8 V. This seems to imply a bad O2 sensor. The testing procedure says that the voltage should be around 1V when racing the engine, to "Make air-fuel mixture rich by accelerator operation".
The scope trace below shows the dips that occur when lifting the foot from the accelerator. It looks like the "Normal waveform" in the second attachment, minus the DC offset.
Opinions? Do I really have a bad O2 sensor? I still have to find the connector. The dealer says it's a very simple procedure, their system allocates 1/2 hour of labor to replace it. That's next. I'll probably need a better light and some articulated arm for the endoscope.
Last edited by 1990-montero; Aug 1, 2025 at 04:22 PM.
Found the O2 sensor plug. It's under the car on the other side of the transmission. Plugged the new O2 sensor in and tested the output. Since the new sensor is not yet in the exhaust flow, it obviously didn't react to variable oxygen, so the trace is totally flat, but the 1.8V DC offset is gone. This is the oscilloscope trace:
The region with lack of spikes is when I lifted the foot from the accelerator.
I could not remove the old O2 sensor from the exhaust pipe. Not enough room to have enough leverage. I'll have to take it to the local mechanic, but it looks like the 1.8V DC voltage might be the cause of the horrendous 4 MPG mileage. Once the new sensor is in and I test real mileage, I'll report back.
