1977 308 GTB Speedometer help please!

First a little background. When I bought the car the speedo and odo worked fine. Then one day the odo quit working and the speedo was reading low. eventually it stopped working too and would only work after the car had been driven for half hour or so. For some reason the heat from being driven around a while must've made it work. I took the speedo apart and found one of the resistors badly burnt. I couldn't tell exactly what the color bands were on the burnt resistor. It looked like it was orange-orange-black (33 ohms) so I replaced it with a 33 ohm resistor which can be seen in the pic. From that point it hasn't worked at all even when the car is warmed up. I assumed the sender was the reason for the intermittency and replaced it, but still nothing works. Can anyone confirm what the value of this resistoris SUPPOSED to be? It's possible I've just got the wrong one in there I guess. I also want to mention that the SAY115Y looks burnt in the picture, but It's not. it's just got some glue on it from a sticker that was on there. I know it could still be bad, but I really just want to check and make sure I've got the right resistor in there first. Any other ideas would be greatly appreciated too. I've read all the other posts on speedo problems and think the problem MUST be with the speedo now since I've replaced the sender and obviously something caused that resistor to burn up in the first place.
Sean
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Looks like 33 ohms to me with black as the multiplier, especially comparing the color with a resistor at the bottom of the photo which is definately a brown multiplier.

Have you tried to draw out the circuit to understand the function of that resistor?

Normally, one would send the speedometer to Palo Alto Speedometer (if you have 4 months to wait). They are good but slow. I've never seen the schematic posted here or on another site.

 

Thanks Paul. The one that's in there NOW is definitely 33 ohms because that's what I put in. I mistakenly said 330 ohms in my original post and just now went in and corrected that. Sorry, My mistake. It was so badly burnt I couldn't tell for sure if that's what it was supposed to be so I guessed at the 33 ohms. Any idea what it's SUPPOSED to be?

 

Oops. To answer your question, no I have no idea how to figure out the circuit or what that resistor does. I know just enough to be dangerous with electronics. At least if I know what that resistor is supposed to be, I can get the right one in there and see if it works. If not, I'll probably just send it off for repair.

 

I had spedo problems for years, and almost everyone said 'change the sending unit', but it always acted like a bad ground. I replaced my fuse bus with Birdman's several weeks and several hundred miles ago and since then I have not had a single problem with the spedo.

I cannot say for sure that it will help, but as many on this board will tell you it won't hurt. My windows operate quicker, my instrument lights are brighter, and best of all - my speedometer works ALL THE TIME.

Good luck.

 

Where do I get this "Birdman" bus ??

 

http://www.birdmanferrari.com/
or you can pm him via any of his posts on this board by clicking his name.

Birdman is one of our better posters. His tutorials are very helpful in keeping your car on the road.

Good luck!

 

Hi Gecko4456
Search is your friend here: http://www.ferrarichat.com/forum/showthread.php?p=138315268&highlight=schematic#post138315268

RJay's board is not quite the same as yours but that resistor is either R1 or R3, so 33ohm or 100 ohms. The actual circuit is very close, the board layout is different. If I had to guess it is R1=33ohms.

And if you search for my name, there should be a post from the last month or so describing how to use your PC sound board to test the speedo/odometer.

The resistor that burned up is part of the power conditioning to the board, from the car's 14.x volts. Did you perchance reverse polarize on battery boosting?
Chances are that the diode nearby is also toast. A 1N4001 will do if you have one handy.
Gerrit
http://dino308gt4.com

 

I'm sure it's 33 ohms then. I could definitely see two orange bands on the burned resistor, but just couldn't tell what the multiplier band was. Thanks. I'll check out the link you posted and test it. Hopefully I can get it working soon. Thanks again! Sean

 

This place is AWESOME !
Thanks !

 

I'm wanting to test the speedo with my new sender while not installed in the car to eliminate a wiring problem being the possible cause of all my frustration with this thing. Can anyone tell me how to connect the three wires on the speedo and the two on the sender along with a 12v power supply so I can perhaps just spin the sender with a drill or something and see if this thing works while out of the car? I found a post that told how to bench test the sender, but havent tried it since i just bought it brand new and can't imagine it being bad right out of the box. I just want to hook it all up on my kitchen table and see what happens. At least then If it doesn't work, I've eliminated the car's wiring and I can just send in my speedo for repair and be done with it. Thanks for all the help so far!

 

Dear Sean,
I have a Dino GT4 that I bought layed up 8 years. Stripped down engine. New bearings - all underside rustfreed, waxall pumped into all chassis members, etc.
When completed in garage, turned ignition key and speedo read 50mph. Disconnected gearbox impulse generator (sender) wires. Turned on ignition. Still reads 50mph. Took speedo apart. Printed circuit board has no sign of overheating. Upper left hand 33 ohm resistor seems to have been hot. I have no wiring diagram of speedo with resistor values. This is unavailable from Veglia. I am still trying for info. One source claims driver (SAY 115Y) has several oscillators within and often fails. Mine is USA ITT same as yours. But not possible to source.
Reply to James. Good luck.