300E Flooding Out when running

Hi Tom

Oh, I wasn't aware of that story.

When two batteries with opposite polarity are connected, the one with most power will "win" with some amount. I assume that Chucks battery was the weak one, and that some negative voltage has been applied to the car then. The zener diode in the OVP then acts as a normal diode, which effectively would limit the negative voltage to the computer circuits to approxemately 0,7 volts and then blow the OVP fuse. The fuse blew indeed and served its purpose (and maybe the diode broke too).

The computer (and sensors around it which are in fact supplied with power by the computer) apparently survived, and according to Chucks description it functioned well for some months afterwards. It is possible that something was harmed and finally broke down after that long period, but that would be very rare. So, I suggest to see the problem separately from the story.

I will certainly not rule out the OVP relay as the source of the problem. I just can't see how it would give symptoms as described (but sometimes the explanation is first found, when the prove appears), unless poor connections in the OVP relay (which have been seen before) cause a drop in the voltage supplied to the computer, which could then cause the computer to function incorrectly.

So Chuck: You can check the voltage output of the relay (pin 2... circuit 87E), or just try to replace it.

And by the way, the "Check engine" light came on. This i driven by the computer, so the computer must work in general.

SO your saying I should replace the OVP relay? Could it of gone bad? Bear in mind that I replaced the OVP a week after the cables were cross connected, and all was well...-chuck

What I say is, that technically seen I would first replace the the oxygen sensor again, as this is a most probable cause of the problem.

Then I say, that the OVP relay is not impossible to cause the problem, and you could chance to replace that first (it costs only half, but in the end, it may have cost you one and a half).

You never know how long time things last. Both items will normally have a very long lifetime, but if something is wrong with them, the fault will typically appear in the beginning of their lifecycle (if it is a new origininal item, warranty should cover).

You can also try to troubleshoot (if you have the means) by measuring the the oxygen sensor signal at the input to the computer. When engine is running correctly, the voltage should fluctuate between 0,2 and 0,8 volts.

In your case the voltage should go towards 0,8 volts to indicate too rich mixture:

- If so, you should suspect the EHA or the wiring to it.

- If not (the voltage is near 0 volts), you shold suspect the oxygen sensor or the wiring (short circuit or open) to it.

The above assumes that supply voltages to the computer are OK (through the OVP relay).

To measure inputs/outputs of the computer, you can take the computer out of its case, put it on so it is isolated from chassis (on a piece of cardboard) and reconnect the cable. The pins from the plug can be measured at the printed circuit board. The oxygen sensor input is pin

2. Be careful not to harm the computer.

What I say is, that technically seen I would first replace the the oxygen sensor again, as this is a most probable cause of the problem.

Then I say, that the OVP relay is not impossible to cause the problem, and you could chance to replace that first (it costs only half, but in the end, it may have cost you one and a half).

You never know how long time things last. Both items will normally have a very long lifetime, but if something is wrong with them, the fault will typically appear in the beginning of their lifecycle (if it is a new origininal item, warranty should cover).

You can also try to troubleshoot (if you have the means) by measuring the the oxygen sensor signal at the input to the computer. When engine is running correctly, the voltage should fluctuate between 0,2 and 0,8 volts.

In your case the voltage should go towards 0,8 volts to indicate too rich mixture:

- If so, you should suspect the EHA or the wiring to it.

- If not (the voltage is near 0 volts), you shold suspect the oxygen sensor or the wiring (short circuit or open) to it.

The above assumes that supply voltages to the computer are OK (through the OVP relay).

To measure inputs/outputs of the computer, you can take the computer out of its case, put it on so it is isolated from chassis (on a piece of cardboard) and reconnect the cable. The pins from the plug can be measured at the printed circuit board. The oxygen sensor input is pin

2. Be careful not to harm the computer.

The computer can operate in open loop mode (disregarding O2 sensor) or in closed loop mode (using O2 sensor signal).

The computer will enter closed loop mode when O2 sensor is ready. The sensor is considered ready, when the voltage increases towards 0,8 volts. When approaching 0,8 volts, the computer will compensate by reducing fuel pressure, until the signal approaches 0,2 volts, after which it will increase the pressure again. When everything works the voltage will fluctuate between the two values. Any steady voltage will be considered as a fault.

All other signals (temperature, air mass etc.) are used to provide correct mixture in open loop mode and to bias the mixture in closed loop mode.

It is correct, that the increasing temperature basically should reduce the mixture, but once in closed loop mode, the operation of the O2 sensor will compensate for errors in the temperature signal.

If the computer never enters close loop mode (due to defective O2 sensor for instance) and the temperature sensor is wrong, this may lead to the current symptoms. Then we are talking about two simultaneous faults, which of course is absolutely possible.

What code is the MIL showing on read out? ie what code does the check engine light throw? O2 sensor perhaps? O sensor Sounds like it could be the ticket to me.

DON"T JUST THROW PARTS AT IT!

Figure it out for pete's sake.

Sorry for the shouting. Marty