Instrument fuse blowing and alternator stoping

Awesome work, and thank you for posting the steps you took in tracking it down. I guarantee this thread is going to save someone's ass in the future.

 

What a Rube Goldberg design for a sensor! Seems that I've seen pendulums in these things before, but never a sliding brick, and especially an apparatus that can allow the brick to slide off its track..... I'm wondering if that set screw on top of the cage for the brick is intended to keep it from sliding past that point?

I've got the sensor dismounted from my car so will have a looksee as you did, Pete.

 

Nice sleuthing Pete! Thanks for posting! Now that you have provided leads, following are some references to that same part. Item #14 in the Ricambi Electrical Boards section..part # 154310. Apparently also used in 355 and 456 models. Discussed starting on page F19 of the WSM. and particularly defined on F24 of WSM.

ELECTRICAL BOARDS - Ricambi America, Inc.

154310 FILTERED ACCLEROMETER - Ricambi America, Inc.

Ferrari 550 456 GT Vertical Acceleration Sensor 154310 Filtered Accelerometer | eBay

 

Great job much respect.

What I do not understand is how unplugging alternator fixes the short. In your drawing the alternator does not play a role.

 

The alternator was a wild goose chase, as was the water pump. That can happen with electrical problems like this.
That sensor is either using piezo effect to measure acceleration, or bounce of the brick against that setscrew pin. I suspect that the brick is not supposed to be held in by friction but should be glued to the white ceramic material. It is definitely not designed to slide and I think it will fail again if not glued. The deflection of the ceramic provides a raw signal (by piezo or bounce) that can be measured and converted into a useful signal. The setscrew under the black goo is a calibration, do not tamper with it. I suspect that the white silicone is for damping and to cover the fixture of the ceramic.
Be careful as the ceramic can break easily.
If it is using contact bounce, then I think the brick should be glued in with a conductive glue.

 

I would have thought the brick had to be glued as well, except there are no signs of glue there. Instead, it appears to be pressed against the pressure sensor (as you said it is most likely piezo) by a calibration screw which applies a preset force. Any measured deviation from this preset indicates a vertical acceleration.
This force also stops the brick from sliding out when you turn hard!
In Pete's case it looks like the screw came loose and calibration was lost, so I would just replace the sensor...

Stefano

 

Hi Stefano, it could be piezo or it could be using a bounce effect, I edited my post since your reply. Anyway I am convinced it should be affixed to the ceramic somehow. If not then one jolt has the potential for ruining the sensor, that surely cannot have been the engineer's choice...?

 

It is an unusual design for an acceleration sensor. The arm onto which the brick sits does not appear to be free to swing, there is a metal protrusion from the base beneath it. That's why I thought the brick was sandwiched between the pressure sensor and the screw.
Magneti Marelli must have patented this, so I'll try to dig out the patent later, I'm curious now....

Stefano

 

Hey at least I had the gold part right!

 

Outstanding diagnostic work, Pete! (I couldn't agree with you more; the 550 WSM wiring diagrams are some of the worst ever to come out of Ferrari...)

 

No, this brick seems to be intended to freely move. The surface the brick sits on appears to possibly be an electrical contact point, and the brick does NOT touch the contact point above when it is sitting flat on the surface. There is less than a mm clearance between the brick and that contact point. There were no loose screws anywhere else. I am guessing that when the car "jumps" vertically the brick flops up and touches the upper contact while still making contact with the lower contact "mat".
Would love to hear someone who is actually an engineer comment on this (not me).

 

Yes, as others said it was a complete red herring. I was rooting around under the car bumping stuff as I was doing it and I may have jostled the brick inside that sensor a bit right prior to unplugging the regulator, which may have temporarily resolved the short.

A few days back I briefly had the car working again without a short, but the fuse blew immediately when I slammed the hood to take it on a test drive. That was another clue that it was likely something in the engine compartment causing the problem.

 

Very well done Pete! Great post as usual.
Many thanks!
Bill

 

I do have an EE degree so my comment should have some validity . From a mechanics standpoint, if the "jumping brick" were true, it should be better contained. Electrically that would produce an unpredictable signal relative to acceleration. A piezo element with an attached weight is quite a common way of making such a sensor. I would look for signs how it would be affixed to that ceramic, I am sure you will find signs that it was.
Oh and be careful, that ceramic really is quite fragile. Amhik

 

Congrats that is some nice sleuthing
Ron

 

Marelli patented this capacitive acceleration sensor in the 90s:
https://worldwide.espacenet.com/publicationDetails/originalDocument?FT=D&date=19930127&DB=&locale=en_EP&CC=GB&NR=2225433B&KC=B&ND=4

It is the closest I could find, with a mass (8 in the figures) suspended by a cantilevered beam. The patent only explains the principle, not how the final product actually looks.
In this design the mass must be firmly fixed to the beam, as suggested by 166&456.

Stefano

 

Given this info i would assume this unit part of traction control. So pete does the asr work like before? Can you do a high G turn and then get an asr fault implying the brick slid off?

 

From what I see in the WSM, the sensor is part of the adaptive suspension system, but I suspect since the instrument panel died when the fuse blew, you wouldn't see the suspension fault warning light in the display. That warning light would have made it easier to track down the faulty part! I think the traction control uses input from the suspension ECU, so it is probably used for traction control indirectly.

 

I think the metal brick needs to be glued.

Why would you use an overweight brick in such a contraption. So that it touches the black pin at the top when the car rolls over (or gets a violent schock).
I have the impression that the metal brick is at the end of a plastick board and that plastic board is suspended (as in not supported) for 2/3 of its lenght, basically acting like a jumping board.

Who is going to solve this mysterie?

 

It has to be glued or else how would they be able to be shipped as a spare part?

 

To me it looks as if the brick is not glued but held in place by some kind of retainer covered in black at the top of the housing Look at the apparent dimple impressed in the brick. I suspect that the retainer applies enough "pressure" in retaining the brick to allow it to move about the pivot point and provide input to the sensors in the main flat contact surface of the brick. The brick likely need to be able to move in order to work effectively. If this theory is accurate, a safety issue (non-performance of traction control) could result if the ECU erroneously believes that the car is stable.
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If as the patent implies "capacitive", then the brick must move to change capacitance the sensor can read. So the black looking thing probably is a set screw to hold the brick in some tight usable range which has failed. That is probably why there's no protection on the other screws. The brick coming off would be a failed sensor. How much pressure on the set screw is anyone's guess. Oh how ferrari!

 

I actually DID see the suspension warning, as it was only the instruments that went out. The multifunction center display is apparently on another fuse.

 

Moorfan,
Thank you for all the investigative work, the explanation and the pictures....
Was really getting to that point where one would start to pull some hairs... Have had 100% same issue... You contribution was a life saver!!! Thanks!