348 speedometer problem

I was driving my 348 for about 2 miles probaby 45mph or so, when the speedometer jumped to about 200+mph, I pulled over and turned off the car, and turned the ignition back on and it went to 180mph, then again it went to 150mph then again it went to 50mph then again and it went to 15mph. I drove it again about 1/4 mile it worked fine then it jumped back to 200+. Where is the speedometer sensor? I found a sensor by the left CV boot on the trans but when I disconnected it the Trans temp stopped working, any ideas of what this might be or where the sensor is located?

Thanks

 

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"?

 

There were two wires going to a sensor about 45 degree's angle just below and infront of the left CV boot going into the trans, I had to cut them since they didnt have a plug connector on it. I was hoping to see if the Speedometer would stop working, since as soon as I turn on the ignition even with the motor not running the speedometer runs up past 200mph. When I cut those wires (wish I had not) the transmission temp gauge stopped working but the odometer is running like crazy.

 

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?

 

No its not modifed but it was a project car, in other words it was in a wreck but most of it was just body damage which I just fixed. I spoke to the original owner and he said it never gave him any problems, he said he just put a new clutch in it, which seems odd for only 11,000miles, and he bought a tubi exhaust, other then that he gave me all the records and it has been taken well care of.

I will look tonight, I'm pretty sure it doesnt have a connector and if it does im going to feel real stupid after cutting the wires. You know the gauge under the radio is not marked as to what its for, I assumed it was for the transmission, but by what you just posted I can see that its more likely for engine oil temp.

So, Now I have 2 problems, #1 get the speedometer to work normaly, and get back to 11,064 miles + the 2 miles I drove.

 

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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Will a loose connection cause the speedometer to stay stuck on 200 mph?

 

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?

 

Ok, did some work on it last night. To answer your question, the speedo jumps past 200mph as soon as you turn on the ignition, with or without the motor running. I took the black twist plug completely off and it doesnt make any difference on the speedo. I pulled the dash assembly off and played with the speedo plug didnt make any difference. How does the speedo work? Is it supplied a positive signal or a grounded signal to it goto 200mph?

 

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.

 

Ok, but if I disconnect the black harness on the shock tower shouldnt the speedo read 0 mph? I don't suppose anyone with a 348 could disconnect there harness and turn on the ignition to see what the speedo does.

 

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).

 

Found the problem last night. I took the speedo out and took the back apart as I did it smelled real bad like something burned. Once out I found 1 resistor and 1 transistor melted so bad that you cant really read the numbers. Do you think a speedo shop can repair it?

by the way thanks for all the help

 

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.

 

I'm going to try to repair it myself tonight. I just picked up some transistors so we will see if I can fix it. If i fix it, I will post a pic of the schematic. All the parts can be found online at semi-conductor places. Infact you could probably make a complete new one for less then 10$ as even the main IC HCF4024BE was less then 2$. The board is fairly old school and looks pretty simple, a 10 year old could probably solder one together, I was a little worried at first being thats its a Ferrari, but my choices are buy a used speedo @ 500$, have mine repaired for 150-200$, or try myself for 3$. If I cant fix it, its still broke so Im not out anything.

 

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

 

Yeah I found the plug by the speedo... I was able to find the transistor part number, but the resistor I could not find, it was just burned up to badly. Then it cracked so I couldnt test it. Tracking the flow path tells me that it should probably be a 1watt resistor but the ohm amount is unknown. I started last night at 1Kohm and it didnt work, I then went down to a 220ohm resistor and I got it to show 25mph at about 5th gear 3000rpm, I ran out of time, so this weekend I will keep working my way down.

I don't suppose anyone has an old 348 speedo that doesnt work that they could look inside for me. Figuered no one really wants to take there good one apart to give me the color codes on a resistor

Thanks

 

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.