348 speedometer problem

The speedometer sensor is indeed on the transmission next to the left inner CV joint. It is part #116987, you can punch up that number in the search box on the Ricambi America site and see a picture of it. It can be unplugged at the short pigtail right by the sensor, or at the multi-pin connector in front of the left rear shock tower.

What do you mean by "the trans temp stopped working"?

 

Wow....strange!

The piece with two wires by the left inner CV joint should be the "pulse sensor" that drives the speedometer, and it should have a plug connection just a couple of inches from it. Has your car been modified? I have never heard of a transmission temp gauge on a 348.

I'm just going by what I know about my own (stock) 348. Maybe yours is different?

 

Now we're getting somewhere!

The temp gauge for the motor oil is right next to the fuel gauge. The sending unit for the oil temp is on the oil dry sump tank. I can't think why snipping the speedo sensor wires would have any effect on the oil temp reading.

I would guess that the problem with your speedo is this sensor (#29), but of course it is necessary to have the wiring correct, and the engine/transmission have to be grounded properly, and the sensor has to be screwed in tight. You might want to take a look at the pins on the connector by the left rear shock, in case one is loose or dirty. The connector just twists and pulls apart, very easy.

And it isn't difficult to pull out the instrument cluster to check the connections on the back of the speedo, but I doubt if that is the problem.
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Speedo stuck at 200mph???? With key on or off? Make any difference? If it's stuck at 200mph with the key off, I would think that the needle is stuck, and it may come down by tapping on it. If it drops to 0 with the key off, then jumps up to 200mph with the key on, I would GUESS (total guesswork) that there is a direct ground somewhere in the wire between the sensor and the speedo. Maybe where you snipped the wires it is touching on a ground, or the two wires are touching each other? What happens when you twist off that connector by the shock?

 

Okay here we go.

Your little manuver of cutting the wires is what is causing things to get wacky. What you need to do is buy a new sensor. It's part #29 on the picture that Milt posted.

The sensor works is on a Hall effect. It takes a magnetic reading from the Phonic wheel inside the cover on the side of the gear box. An explanation on the Hall effect from the works shop manual:

"This principle is based on the fact that if a current is passed through a semiconductor immersed is a magnetic field perpendicular to the current, then every change in the magnetic field gives rise to a variation in voltage."

Since you cut the wires, it cannot get the reading from the wheel. I'm guessing that the voltage on the cut wires is such that your speedo thinks it is going 200mph. So you will need to get a new sensor.

 

First get a new sensor then worry about the connector. If after you get the new sensor you are still having problems, then I would go through all the wires and connections with a fine tool comb. Ferrari's are know to have bad electrical connections. You will want to look for dirty, frayed, or broken wires. Make sure all the connections are clean and connected properly. What ever the case you will still need to get a new sensor because you clipped the wires on the old one. Until you get the new sensor you will not be able to rule it out as the cause of the problem.

 

Per your request, I just disconnected the multi-pin connector by the shock tower, then turned on the ignition. The speedo needle didn't move at all. But the oil pressure reading jumped to the peg, even without starting the engine.

Since I have never seen this problem before (speedo jumping to the peg) I can only guess at what is wrong. If it jumps to the peg with the multi-pin connector unplugged, then you may have fried something in the speedo when you cut the wires. Was the ignition turned on when you cut them? You started out with a problem in the sensor (most likely), and now it appears that something else is wrong. Sorry I don't have a specific answer.

 

I'd guess the other way. If his speedo needle goes all of the way to 200mph and back to 0 based on ignition power or not, then I'd say that the needle mechanism physically works so that he must have a signal problem (e.g. wiring, connectors, or sensor).

 

It may be repairable but I wouldnt bother, just replace it with a new one and keep the old one as proof of mileage before (you could even go as far as getting a main dealer to replace and have them issue you with a statement to back up what happened)

As for tampering with the mileage....its just not worth the trouble, unpluging the speedo's is easy (I don't do this but do know people who do) but once the miles are on leave them on

 

Can you still buy a new speedo for 348?? Arent they obsolete?? Not that ive tried to buy one,but yeah. Id prefer to fix the old one anyways if possible? With the old speedo in hand and a new one fitted,it would seem rather suspect about the new mileage. Your car could have over 60K miles,which means you could go to the wreckers,pick up a speedo from a wreck with only 20k miles and put a new speedo in your car and bull**** the rest,you know? Ive seen it before when guys sell cars ect......(havent seen ferrari's sold). Maybe get the dealer to fit?? But id get that one fixed if you could. Good luck mate!

 

Glad to hear to found the problem. I don't see why a speedometer shop couldn't repair it. Try having it repaired before you buy a new one. It could save you several hundred dollars. Don't forget to get the sensor for the trans before you hook every thing back up.

 

Credit to Miltonian. My theory on the problem being a signal issue instead of a bad speedo was all wet.

 

Sweet! Go for it man,if you can get away with it for $3. LOL!!

 

Were you able to identify the resistor/transistor numbers from the old melted ones, so you know you are getting matching ones (per your earlier post)?

I wish I had the talent to do electronics/soldering work like that, but my old eyes and shaky hands just aren't up to it.

Did you ever find out if there was a connector next to the speedo sensor on the transmission on your car? There certainly should be.

Good luck with the project, hope it works for you!!

 

I agree, no harm in attempting to fix it yourself.

Just for a 348 Brotherhood reference, Parts R Us carries that speedometer gauge IC (HCF4024BE) for sixty cents: http://www.partsrruss.com/digital.htm

 

Please post the transistor part number that you found (and purchase source) for our future reference.

Also, send a private email to F-chatter FMOMEN. He's got (or had) a spare speedo gauge.