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Mazda MPV: throttle sensor or short or?

1990 Mazda MPV 2.6L Manual

Driving down the freeway my van suddenly lost power and the check engine light came on. Pressing the accelerator made the engine want to stall. After pulling over and waiting a while the car ran again for a little while before losing power again so I was able to limp home where the car currently sits.

I have a Haynes Repair Manual which tells me how to get the check engine codes and there are four errors: (09) water thermostat, (11) Intake air thermosensor, (12) Throttle sensor, and (14) atmospheric presssure sensor. Using a meter 09, 11 and 14 seem OK. The throttle sensor is suspect though - one of the wires is supposed to supply around a 5V charge and I only see some small amount < 1V. So unless I’m measuring wrong this seems to be the problem.

I have checked the fuses and all are good. Is this a wiring problem? Or is the throttle sensor bad? Shouldn’t the supply still provide 5V even if the sensor is bad? Is there some better way to diagnose this? This has been a great van - but it has 230,000 miles on it and I don’t want to throw money at the problem. Any ideas would be greatly appreciated.

What voltage do you read on the supply wire if you disconnect the throttle sensor? If it’s still low, then check the wiring. If it’s not the wiring, it would be the computer (rare, but possible).
If it goes to 5V on disconnect, then maybe the sensor is shorting out the supply as you posit.

Thanks for the reply. The reading is the same connected or disconnected. But I’m not sure how to start checking out the wiring - everything I can see looks good - but most of the wires are bundled and wrapped up - and going who knows where. Any general hints on how to tackle this? I suppose if I knew where the 5V originated that would be a start.

You’re going to need the factory service manual and/or wiring diagrams. The supply wire comes from one of the pins on the connector to the computer. Normally, you would disconnect this connector, use an ohm meter to verify continuity from the throttle sensor connector to the computer connector, and then use an ohm meter to verify that there is no leakage from that signal to ground or any other signal. You would first need to verify that the TPS supply signal did or did not run to any other sensors too.

Thanks again. I’ll look in to it. There is a simple wiring diagram in the Haynes manual that I’ll start with.

Here is the wiring diagram. Click on Fig. 6.
To check the TPS (throttle position sensor), disconnect the wire connector from the TPS. With the ignition switch ON, measure the brown/black (BR/B) wire for 5 volts. That comes from pin 20 on the PCM (engine computer), and goes to IATS, ECTS, O2 sensor, and the TPS.
Do you have backprobes for your digital multimeter? With the wires reconnected to the TPS, backprobe the red/blue (R/BL) wire, slowly rotate the throttle, as you observe the dc voltage on the multimeter. Are there voltage spikes or flat spots? Voltage spikes, and flat spots aren’t good.
The brown/white (BR/W) wire is circuit ground, which may (or, may not) be the same as vehicle ground. It will probably read 0 volts.

Thanks for your help. I have determined that engine computer is at fault - pin 20 is not providing 5V. After removing the PCM and taking a look at the circuit board I can see that one of the smaller capacitors has leaked some black goop and I’m guessing that this failed capacitor is the source of the problem. I’ll either try to replace this capacitor if I can determine what it is, or try to find a replacement PCM at a local junk yard. Thanks again.

You may have found the cause of the problem. Some circuit boards are marked with the components. Do you see something like 0.5 uF? If you look at a salvage yard board, you may be able to identify what the damaged item is.

Thanks. The circuit board has a hash mark with a ‘+’ which I think indicates ‘polarized capacitor’. The leaking capacitor is small, blue and teardrop shaped and is marked with ‘33+’ followed by ‘10P’ and then an ‘m’ in a rectangle. The ‘m’ I think is for the manufacturer - Matsushita. The 33+ and the 10P identify the specs but I don’t know what they mean. If anyone knows - that would be helpful. photo here: