CEL on 550 Maranello

Fuse 21 powers the OBD connector. I also believe it powers the following:

Luggage compartment light
Dome Lights
Cigar lighter
Trunk release (solenoid?)

I would try disconnecting them one by one if that's possible (to see if the voltage changes).

Check to see if you have 9 volts at the fuse also. Maybe the fuseholder has dirty/corroded contacts? (EDIT: might be a good idea to try this first). Leave the fuse in place and check both sides of the fuse (There are normally holes in the fuse to allow a multimeter probe)

 

Here's the diagram showing the fusebox link to the OBD connector:

550 Alarm System Diagram

I have other diagrams, but none show dome light and trunk release wiring. I believe the wiring for the cigar lighter will be similar to the 456's. I've added the (hypothetical) wiring to this diagram:

550 Lighting Diagram

 

have your scanners worked on this car in the past?

 

Now that I think about it, that won't tell you if the downstream side of the fuseholder is corroded. Also pull the fuse and see if both contacts on the fuseholder are ok.

Are you using the cigar lighter socket for anything at the moment (like running the garage lighting on it?)

 

I forgot you already have them.

Bouquets one day, brickbats the next? Perhaps you're not looking in the right spot?

 

Here's the information I extracted from the diagrams I gave you on the 550 forum:

Fuse 21 systems

Power for the fuse comes from the battery terminal board above the passenger footwell fuse/relay panel.
The fuse numbers I've shown in these diagrams are taken from the owner's manual which most owners have access to (Fuse 1 is on the far left, fuse 24 is on the far right of the footwell fuse/relay panel). This is contrary to how the fuse numbers are shown on the original black and white Ferrari wiring diagrams.
Fuse/Relay Panel connector numbers are probably imprinted on the plastic of the panel.
I don't know if connector "H" on the panel feeds things like the luggage compartment light, dome lamps and trunk release, but you may be able to pull this plug to see if your voltage changes.
Connector C feeds the OBD plug and the cigar lighter (as far as I know) via splice [216]. Ferrari doesn't tell us where splices are, unfortunately.

Let me know if you need further information.

 

Looking at an online photo of a 550 fuse panel, I'm not sure if connector "H" is used at all. This might make faultfinding a little more difficult. Look for additional wires on connector "C" pin 7. These may feed the lights and trunk release.

Here's a graphic which I think represents the fuses, relays and connectors on the fuse/relay panel:

Relay panel graphic (theoretical)

 

Not too many SD2s in Maine, eh?

 

Has this car had the recall performed to add the ground wire to the OBDII port? All Ferraris from 1996 to 2005ish are missing the ground. The feds required a recall when states could not conduct emission tests through the OBDII port due to the missing ground. It also prevented many independent Ferrari specialists from diagnosing the cars

 

Good to know, but I don't think a missing ground would explain a low voltage on pin 16.

Dennis, what were you using as a ground for your voltage checks? A chassis earth or pin 5 on the OBD port? How healthy is your battery? With the car ignition on, it can put a drain on the battery.

Not unless the disconnected plug pins are shorting to something.

 

Could you pull plug C on the relay panel and see if you have 13.8 volts coming out of the relay panel? (pin 7).

If the volts are ok, the problem will be downstream of that plug. Do your dome lights, trunk release and luggage compartment light work?

 

Note that the ECU fuses and OBD2 fuse use a different relay panel battery contact. The battery feed connector (plug "K") on the relay panel has two (positive) pins. It would be better to check the power on fuses 13~24 on the right hand side of the relay panel.

 

Refer to the link in post 8 for the relay panel plug numbers. I've incorporated the graphic in some of my diagrams which I'll upload later.

Thanks for the feedback on the fuses, dome lights and trunk release. It could still be a fuse panel problem. These other devices will probably run on 9 volts without a problem. Unfortunately, if the problem is outside the fuse box, it will make finding the problem a little harder because there are no disconnect plugs between the fuse panel plug and the OBD port.

Since your cigar lighter is disconnected, could check the voltage on the lighter plug?

 

Just to confirm, all your measurements, including the one on the OBD2 port were made with with the ignition off and battery conuditioner on?

13.8 is a little high for battery only. I'm not sure why the voltage is lower at the cigarette lighter. At least not until measurements are made at plug C with the plug off. If that is lower than 13.8, you could also check the voltage at plug "H" as a comparison (with plug C off). i.e. the unused plug.
This should confirm if you have an problem in the fuse box or outside the fuse box.

 

According to the diagrams, the headlights do need the ignition key (this is typical Ferrari), but the diagrams show power for the OBD port coming directly from the battery. Seems that the diagrams are wrong (or your car is wired up differently from other cars). I'm not sure I can give you much help from here if this is the case.

You do normally need the ignition on and the immobiliser off to interrogate the Motronics ECUs (using the OBD2 port), but that's only because the Motronic ECUs themselves need ignition power.

Maybe someone here or on the 550 forum can see if they have voltage on pin 16 (without ignition)? I don't have access to a 550. By the way, does your cigar lighter require the ignition to be on? My late model F355 does (but earlier cars don't). It also uses a different fuse from the earlier cars, despite what is printed on the fusebox cover. This may be the case on your car. Perhaps you could try pulling fuse 21 to see if you still get that voltage on pin 16? (ignition on)