Bouncing Tachometer - F348.

i have seen alternators cause abs/asr/esp warning lights but no charging lights. Has to do with the 3 faze function of the alternator if #'s 1 and 2 are ok charging and basic functions work normally but accesories and non essential systems malfunction due to "low voltage" codes . if the alternator was bad i would exxpect a warning light or two, plus a bouncing tach.
Watch the idle, if the rpms are steady but the tach moves then check the ground points, if the rpms flutuate with the tach movment then check the alternator. A simple draw test will confirm either.

 

Hi, years later did you find the solution?

 

As mentioned, there are multiple solutions. What are your symptoms and what you have done to diagnose the problem.

Does your car start, idle and run normally? Do you have any codes with the engine running? Have you swapped ECUs? Have you checked the connections on the back of the tachometer? Have you inspected the crank sensor and its wiring on the side of the crank pulley?

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Hello Qavion,
i just bought the car and drove it maybe 1 hour, not more.
AS far as i can see there is no problem with motor, it seems driving fully ok. As well as starting and idle.
In Europe we do not have access to diagnose system , i am afraid...
I did not swap ECU, would it change something?
Did not check the connexion yet. Will do that first.
As well as i did not check the sensor and his wiring.

Just to be complete, it doesn't happen all the time but maybe once within 15 min!
i did a video as it happened:

https://www.youtube.com/shorts/WXPbKuh_IQQ

 

The signal from the LH crank sensor is processed by the LH Motronic ECU. The Motronic ECU then sends the signal to the tachometer. It is possible that part of the ECU processing circuits are faulty (the engine may run well, but the output signal to the tachometer is faulty). The ECUs are identical internally, so it is possible to swap them. If the engine was also doing strange things, the crank sensor itself may be faulty. However, with these cars, anything is possible... Sometimes logic doesn't apply .

I have all the wiring diagrams, so if you do need to check wiring, let me know. Do you know if your car has a 2.5 or 2.7 Motronic system?

Sorry to hear you are having issues after such a short period of ownership.

 

oh i am sorry , i am not sure but as it is a 3/94 GTS i think it is 2.7?
Where can i find both ecu exactly to swap them?
behind seats?

 

Checking my video again, with sound this time, it looks like when it was doing some strange things on rev counter, there was also a strange, not regular, sound from engine... Do you hear that?

 

i will order and exchange those crank sensor as they are cheap. (25€ each).
AM I right thiking the two have the same reference, right?

 

Yes, they seem to be behind the seats, mounted on the firewall... (see #1)

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Hard to say. Maybe the experts can chime in.

Yes, they have the same OEM number. See item #40 in this parts diagram

https://www.eurospares.co.uk/Ferrari/348/348_(2.7_Motronic)/PartDiagrams/007/Air_Injection_-_Ignition

When you're changing them (or just the LH bank sensor), also check for wiring damage beyond the sensor connector. The lower sensor (RH bank) wiring seems susceptible to this.