Dash lights out after 15 min of driving ...

my 98 355 dash lights will go out after 15 minutes of driving. This tells me that it’s something electrical with an issue that’s heat related . I’ve read a bad potentiometer can cause this. Anyone have experience with this?

Thanks

 

Is the problem limited to the dash lights? Is the centre console switch backlighting affected? Is the clock, fuel gauge, etc, backlighting affected? I'm just wondering if it's a light power problem in general or if it's limited to certain areas.

Have you tried putting the potentiometer in a different position?

I've had my rear turn indicator lights and brake lights play up after 15 minutes or so, but this was due to a bad earth near the radiator. Even earths can be affected by temperature.

 

I see you've had this problem a while...

https://www.ferrarichat.com/forum/posts/145408033/

 

Regarding earths, there is a common earth for both the clock, fuel gauge, etc and the instrument binnacle gauges, labelled "70090". Unfortunately, I have no idea where this is.

Fig. 8 Lighting (5.2)

Fig 5. Instrument Lighting/Handbrake

 

Can you elaborate further? When the dash lights go out do they come back on the day after or come back on at all? Surprisingly I don’t see a “potentiometer” in my car. Ive never driven at night. Never had to turn on the lights. 1999 355F1.

A potentiometer is more or less an adjustable resistor. It’s purpose is to limit electrical power to a device(s) by limiting the voltage via a dial or sliding mechanism. When these devices fail, it usually happens at the contacts. The likely instigating factor is heat. But we don’t know it’s the problem until you answer question above and check all devices in line. The fuse for example. See drawing.


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Here

 

Dash light's rheostat located on stem, bottom of tach face.

 

In your other thread, you've posted a (still) photo of the centre console with the clocks not illuminated, but the switch backlighting illuminated.. .and the topic labelled "dash lights flicker". Assuming the switch backlighting was ok (and not just in its "on" state during the flickering) this is something we can work with. It proves it's not a fuse or relay but something further down the line (as one fuse powers all these lights). It's more likely to be a rheostat problem or something further downstream, such as an earth.

https://www.ferrarichat.com/forum/attachments/image-jpg.2400516/

In theory, you can remove the tacho electrical plug and put a (temporary) shorting wire across the plug's white and yellow-red wire, bypassing the rheostat. This will give you full brightness on all the instrument lighting (except the tacho which will be dark). If you get no flicking on the instruments on subsequent drives, this will prove the rheostat is faulty. Rheostats are not available new. I have seen used rheostats for sale, but they have silly prices.

If indeed the switch backlighting was in it's "on" state during the flickering in your photo, it could be a fuse or relay contact failure.

 

The "rheostat" is actually electronic, using a potentiometer and probably having just one power transistor controlling the current to the bulbs. I believe it can be repaired by replacing the transistor and/or other failed components inside. Otherwise, it is the same as Alfa Milano unit but these are also very hard to find (one Alfa forum even suggested to look for a Ferrari 348 or 355 rheostat).

 

thanks for the help everyone. I was able to source a brand new rheostat (sat on a shelf for years), I've since installed it and lets see what happens.

 

hi there! I am wondering if you solved the problem with new rheostat.
Thanks in advance

 

Yup looks like the issue was the rheostat, all is well now. Since this rheostat was super hard to find Im going to see if my old one can be repaired for future use or to help out another owner. I suspect this will soon become a common issue for others.

 

Thanks for the feedback, Ryan. I was hoping it was something cheaper, like a relay, but...