| Author | Message | ||
| David Feinberg (Fastradio2)
New member Username: Fastradio2 Post Number: 47 Registered: 4-2002 |
Mitchell, Please be forewarned...disassembly is not for the faint at heart, as they are pretty involved. Doable, just a pain... As to the possibly of a DC offset, right idea...but not a voltage issue, rather calibration. If we presume that a) the "correct" tire size are on the car, and that the gear in the speedo drive assembly (in the transaxle) are correct...or even if both are incorrect!, it is possible to get the speedo back into calibration. From the pictures below, you'll see a brass strap around the speedo "motor" assembly, held in place by 2 screws. By adjust the relative position of the strap, you're setting the "offset", or pre-tension on the speedo. Unfortuately,I don't know the output frequency of the speedo driver, hence from here on...it's trial and error to get the proper speedo reading. It can be done...again, just a bit tedious. A handheld GPS unit can be used to obtain accurate constant speeds for calibration purposes. Likely, this is why the speedo specialists get so much money to fix these things... Hmm, another thought...Check the position of the speedo needle versus the stop. Many years ago I had a BB whose speedo was off (linearly), and it was due to the speedo needle being loose (and hence the incorrect initial position on the shaft). The speedo needle will come off the motor shaft with some very gentle persuassion...or can even be moved (turned) in need be. It is extremely easy to break the motor shaft doing this so....gentle!   | ||
| Mitchell Minh Le (Yelcab1)
Junior Member Username: Yelcab1 Post Number: 183 Registered: 11-2001 |
David I actually have 2 problems. 1) the indicated speed is always 10 MPH too high at all speed. 2) the tripo sticks and affects the odometer counting. From your description, it sounds like you know a thing or two about these. The number 2 problem can be cured by careful disassembly and cleaning. How about number 1 problem. That sounds like a permanent DC offset problem to me. Could be the F-V converter, would you happen to know the actual part number so I can order it? | ||
| David Feinberg (Fastradio2)
New member Username: Fastradio2 Post Number: 46 Registered: 4-2002 |
To clarify the speedo/trip meter function: The transducer simply generates a waveform. The chip in the speedo is a very basic frequency to voltage converter...hence driving the speedometer (which is acting as a voltmeter (but calibrated to read vehicle speed). The trip meter is purely mechanical, and driven, via gears off the speedometer. When the trip recorder sticks (mechanical problems, not electrical)..it will stop the odometer from reading correctly. I have seem this problem in 308s and BB. Careful disassembly, cleaning and lube of the gears usually fixes the problem. David | ||
| Mitchell Minh Le (Yelcab1)
Junior Member Username: Yelcab1 Post Number: 182 Registered: 11-2001 |
There is a signal that comes off the transaxle. This signal is just a waveform that supposes to tell the meter how many to count. There are only 3 wires going into the speedometer that are signals, the rest are for lights only. I am guessing the 3 signals are 12V, GRD, and count. | ||
| Harry (Harry)
New member Username: Harry Post Number: 14 Registered: 3-2002 |
I am pretty surprised that there is a chip in a '75 Ferrari's odometer! But if it is electronic, where is the transducer which converts the wheel rotation into a digital signal? And how is the meter driven? Did not find anything in the manual. | ||
| Mitchell Minh Le (Yelcab1)
Junior Member Username: Yelcab1 Post Number: 180 Registered: 11-2001 |
Harry I had the same problem. The trip meter sticks at round 100 miles and then the odometer stops counting too. Reseting the Trip meter takes care of the problem for 100 miles. Took it off and took it to Palo Alto Speedometer. They said that the main chip needs to be replaced at a cost of $230, plus a new case for $75. $300 total. I am going to live with it for now. I disassembled the whole unit to change the case (I got it from GTCarParts for $50) and found that the "main chip" is a small IC that is probably a counter. It was covered by a label so I could not see what the chip really was. If someone knows what this IC is (if you had a schematic) I would like to know and buy one to replace it. Supposedly it is made by Motorola. | ||
| Harry (Harry)
New member Username: Harry Post Number: 13 Registered: 3-2002 |
In my 308gt4 the odometer does not work properly. The trip recorder sticks at random x9,9 km readings (49,9 59,9 ...) and also the odometer stops then. When I turn the reset knob a little forth and back the whole thing starts to count again. Probably the mechanism in the odometer has a little play. Anybody had the problem and know how to fix it?? |