Need an electrical expert

[donm]

Like most cars these days my daily drivers have a dash display that shows the outside air temperature and I thought it would be great to have something like that in my Fiat. So I went on-line and bought a little digital display which I mounted under the dash and a remote probe that I mounted under the front bumper. I wired it up to come on when the key is placed in the ignition, like my radio.

It works great when the engine isn't running but when it is running, the temperature display jumps around a lot instead of showing a steady reading as it does when the engine is off.

I'm thinking it might have something to do with being powered by the alternator as it is when the engine is running, as opposed to running on just the battery as it is when the engine is off. If that's the case, I was wondering if there was something I could do to smooth out the current, like some kind of capacitor other device. Anyone have any thoughts?

[RRoller123]

You are probably on the right track, and it could be a dirty signal on the voltage, like radio crackle from points, coil, spark pugs, etc. (?). Or could be the vibration and rapid air flow in the outside front sensor location?

One other thought:

I grounded my driving lights first on the bumper mounts, because, duh, they are bolted right into the frame and should provide a good ground, right? Lights worked for a short time and stopped. Re-grounded by running a wire from the ground pod back in the engine compartment out to the lights and they immediately worked perfectly. So maybe if you grounded the sensor outside on the bumper that could be the problem?

[spider2081]

The jumping display can be caused be a noisy electrical system or by radiated noise like ignition. I would see if the temperature display will operate of a 9 volt battery. If it will retest your display with the 9 volt battery. if the display still jumps around it could be caused by the routing of the probes wiring close to other noise sources like the spark plug wires or ignition coil.

[donm]

It works great with a 9v battery, in fact instructions say it will operate on anything from 4-24 volts. The probe itself isn't grounded; it has a cable that connects directly to the display and it's isolated in a rubber mount. I doubt vibration or any kind of interference is the culprit
because I tried using the 9v with the engine running and it works fine, which leads me back to the alternator output not being smooth enough and that's why I was asking about some kind of capacitor but I'm just speculating.

[baltobernie]

What circuit did you use for the temperature display (fuse number)?

[donm]

I used the same circuit as the seat belt buzzer, same as the radio, nuso it comes on when i insert the key.

[Nut124]

Try some cheap capacitors like you suggested and see what happens.

[Nut124]

Try some cheap capacitors like you suggested and see what happens.

[donm]

But what kind and how would I hook it up?

[spider2081]

here is a link to an alternator filter. https://www.amazon.com/Heavy-duty-suppr ... B00MU2HM2U This type of filter is used with aftermarket audio systems that often are affected by alternator noise.
Is your alternator in good working condition? Usually the cars battery is all that is needed to filter the noise an properly operating alternator is making. If you believe the alternator and battery are in good shape making the power ground connection as close to the battery as possible sometimes helps.

[TX82FIAT]

You could wire it to your lighter or clock circuit and put a switch someplace so it is on when you want it on. It's a non standard piece of equipment so a hidden switch should not present issues. I did something similar with the cooling circuit in the event I ever have a cooling system issue that prevents the fan from working I installed a manual override under the dash. Occasionally when in stop and go traffic I will trip the fan on before the car gets to the +/- 190 range driving on this Texas 100+ degree heat.