Confused, tired..... Need help with project.

[Dvt Dopesick]

I have a bit of a snag that I must solve.

On my car, I am installing a Air Fuel Ratio Gauge. Now this gauge works on a 0-1 volt DC Signal. The A/F Guage I am install works on a 0-1 volt DC level. Unfortunatly the cars PCM Sends a 5 Volt Search Signal to the O2 Sensor at start up. If I hook the guage up to that it will definatly blow out the gauge at startup. However, that pulse is exactly that. A Pulse. I need to think up a way to stall the gauge from activating.

I was thinking a timer based 12v Circuit with a relay section to "null" the input going to the gauge. The unfortunate problem is, the signal wire going to the gauge CAN NOT be modifed or buffered in anyway. I thought of a timed "soft" turn on period. I will only need to create an "open" circuit of the input wire going to the device.

Also I was wondering, since it is a signal wire, should I place a diode on it to not send any transient or problem voltage from a circuit failure, back to the O2 sensor & PCM. I want to make a protection into the circuit if it goes over 3 volts it shuts down.

Keep in mind I am only going to "tap" into the circuit going from the O2 Sensor to the PCM, not interupt it.

I need to contact my source at dodge, about the 5 volt search signal, to find it's duration, and if any recurance of that signal while running.


I thought of the cap in parralell but I wish to build up more of a circuit so I have some sort of protection. The A/F Guage I bought was $65. I would wish to NEVER replace it for a over-volt problem caused by myself.

Also Vise-Versa with the PCM and O2 Sensor. I wish to make sure the circuit that meter will monitor, I want to make absolute sure the gauge if it fails, or circuit if it fails, has no way to back voltage the O2 to PCM circuit.

I thought of a zener, with a 3 volt cascade. perhaps someone could help me with that. I've never dealt with a circuit in this manner. The Timer and relay was the way I was leaning toward, but I still had to think of a manner of protection.

 

[Roff]

Can you post a url to the datasheet for your A/F gauge?

 

[Dvt Dopesick]

http://hp.autometer.com/instruction_download/instructions/837j.pdf

Is all that is available.

I do have the printout from DODGE on the O2 Sensor I can copy that and post it up if you need it.

As you will see in the pdf, I can get 12v power from the circuit running tha gauge, but I need to protect the violet wire from the 5v input from the PCM when it sends it's pulse.

Like I said the pulse is also just that a pulse, supposedly 1 time only, at startup. IF I can counter-act that pulse I'm set.

 

[Oznog]

Well, several things:
1. Are you sure 5V will blow the gauge? This is not high voltage, and I would expect the gauge to just read full rich/lean during the pulse. I find it hard to believe that a gauge would be made in a way where it would be so easy to blow in the intended application.
2. You should definitely not try to solve this by switching off the 12V supply. Amplifiers do not like to accept inputs while there is no supply voltage, it can result in engaging protection diodes. This actually could blow the gauge, or prevent the 5V pulse from being successfully read by the engine. It is more realistic to switch off the connection between the sensor wire and gauge.
3. You wouldn't do a solution which relies on the computer's timing. If you flick the switch back and forth so one resets and the other doesn't, all bets are off.
4. Sounds like the computer always applies 5V open circuit, with a calibrated output resistance, forming an resistor divider with the sensor's resistance. As such, the voltage will not be 5V at startup unless the sensor's disconnected, in which case it will always be 5V.

Again, I don't think there's any way the gauge could be designed to blow on a 5V signal.

 

[Roff]

Have you seen http://hp.autometer.com/techtips/faq_tech_writeups/air_fuel/air_fuel.html and http://hp.autometer.com/techtips/fa..._ignitions/af_and_0to5v_ignitions.html#anchor ?
These pages imply that 5 volts will not damage your meter. They also mention several models of Dodge that use 5 volt sensors. Are you sure yours has a 1 volt sensor? They have a couple of solutions, if you do have one of these models.

 

[Dvt Dopesick]

It looks as if more investigation is needed then. I was told by dodge the sensor is 0-1 volt. And others are listing it as a wideband 0-5 volt.

back to the think tank.