Chymos

Post #2 on this forum and I'm already asking for help... I searched for information about an odd "grinding" noise coming from behind my dash instruments after I turn the car completely off (key OUT of the ignition), but only found info on A/C and heater duct issues with the car running. Does anyone know what could be causing this noise when the car is completely off? It is ALWAYS there after turning on, running for any amount of time, and turning her off. It grinds for a decent amount of time (10-15 min) and then just stops. Sound seems to be coming from right behind the speedo.
The blinker fluid has already been changed (smile), and I replaced the gears on the odo that just failed about a month ago, but the grinding was there before and after both of these items.
Thoughts?

pp000830

1. I believe the fresh air servo cycles when the key is turned off and if its internals are not right it may keep trying to cycle.
2. The fan on the back of the CCU has some dust clogging it that can cause a grinding like sound.

Both are, easy to get to and fix items, the servo via rebuilding and the dust by pulling the CCU and cleaning it out.

jrmdir

Very common issue and easy to fix. Normal for the CCU fan to run on after key is turned off. And dirt on the fan makes it noisy. Search here and Google for 993 CCU noise. Many many posts and even some YouTube DIY's. Here's one: https://rennlist.com/forums/993-1995...-993-dash.html

Chymos

Thanks for the suggestions, but I swear it sounds like the noise is coming from directly behind the speedo. I would think the CCU would generate a noise a little lower behind the dash, and I thought the fresh air sevo is located on the other side of the firewall? I would think this would sound more "distant" if you know what I mean. Regardless, I will take a look at both of these to see if I find the little gremlin! Thank you!

Chymos

I'll give it a shot. Thank you!

jrmdir

FYI - I was able to silence my fan simply by blowing/vacuuming out the dust on the fan blade. Did not need to take the motor apart. Maybe next time?

BTW, as you can see from the fact that there's a rubber hose attached to the suction end, the purpose of this fan is to pull cabin ambient air in from the front of the CCU in order to keep the temperature sensor from getting confused by internal heat.

Ron

Chymos

Nice! I'll try some compressed air before I disassemble. Good tip!

skyboy

I had the CCU arpart the other day. What is that thing located in the tube right behind the faceplate? It appears to be able to move in/out a bit and has a couple wires mounted to the pc board.

911F1

Could be the clock

jrmdir

Well, to be clear, I did remove the CCU and get to the fan assy before blowing. I just didn't disassemble the motor housing like in the article. And I may have had to use a toothbrush to get it really clean - don't recall. But I'm not sure I'd blast with too much pressure from the front.

jrmdir

Pretty sure you're talking about the temperature sensor (thermistor.) It sits just behind the small grill opening on the left side of the face plate. Measures temp of cabin air sucked into that tube by the fan we're talking about.

skyboy

I think you’re right, it’s a temp sensor. Any idea how it works? It’s a plastic thing that slides in and out of the plastic tube that is part of the faceplate and goes inside the rubber hose that runs to the back when assembled.

In fact, mine was outside of the guides when I pulled it apart and had to be gently re-inserted into the guides. I just don’t see how this thing translates information as to temp. The sliding isn’t even that smooth. Its operation is confusing to me, a guy with electronics knowledge.

TheBen

The grinding noise in the back of the speedo could be the gears failing. Happened to me. Noise went away when they finally crumbled and odometer/trip stopped working.

Chymos

Thought this was a possibility as well, but I've done the ODO repair (pretty easy too, thanks to Rennlist!), and the sound remains. My next step is the cleaning suggestion above.

ToreB

The CCU temperature sensor is very brittle and easily breaks. Do not pull the two wires apart.
It is mounted on a plastic holder that slides into the duct after the PCB is inserted.

The CCU fan is known to make noises when the bearings wear out, often with rattling and whirring. The fan shall run when the CCU is on. It shuts down about 20 minutes after ignition off to ensure correct cabin temp measurement on short stops. The unit heats up while on, and the fan needs to run to avoid heating up the sensor. This is also why there is foam insulation around the sensor duct.
Cheers,
Tore