i'm at my wits end, and i hope gurus here can help.
driving a 75 2.0TS and have been having this weird problem with the speedo. i'll describe the problem.
when the car is moving on a highway at a relatively constant speed, under very hot ambient conditions, after some time the speedo will start to go haywire, where under slight pressing of the accelerator the speedo will drop speed, and if u release the accelerator it will seem to jump back to the proper speed again. if u keep accelerating the speedo will go down again and will even drop straight down to 0. even when u try to just drive at a constant speed when the trouble happen, pretty soon the speedo needle would just drop back to 0 and will stay there. however, if u stop the car (engine still running), and drive back up again, speedo will work again but not for long (1 - 2 minutes).
the funny thing is, this only happens when ambient temperature is hot and car has been driven in such condition for 20 minutes or so.
instrument panel has been checked, and doesn't seem to be a prob. i've taken out the speedo signal amplifier (under the rear seat) and even solder the joints (they were fine, but i just want to be sure). also cleaned the connectors. secured all the cables and amplifier so that they don't move under the seat.
the only thing left is the actual sensor at the gearbox. is it possible to dismantle that and see what's wrong? it seems to be heat related somehow, and i worry it might be connections at the sensor expanding too much thus loosing connectivity when the temperature is too high. but that's just a guess.
any hints and help is apprecieated. thanks!
mcandiago,
doesn't seem to have that, though when i first got the car, it used to work intermittently even when the weather is cool (like at night, for instance). but after cleaning those connections, it doesn't occur when it's cool...only when it's hot and the car is driven fast (not necessarily driven hard).
doesn't seem to have that, though when i first got the car, it used to work intermittently even when the weather is cool (like at night, for instance). but after cleaning those connections, it doesn't occur when it's cool...only when it's hot and the car is driven fast (not necessarily driven hard).
coach123
There are obviously 5 things that can be wrong with your speedometer system
1. The speedometer itself
2. the wiring from the amplifier to the speedometer
3. The wiring from the speedometer sensor to the amplifier
4. The amplifier
5. The speedometer sensor
1. You said you had the speedometer checked, so we'll rule that out.
2. Although you have cleaned some terminals use an Ohm meter to check the continuity of the cabling between the speedometer and the amplifier. If I am correct, your speedometer is not an open collector configuration so the resistance of the wires to the amplifier should be low (~0-2 Ohms).
3. Similarly check the resistance of the wires to the speedometer sensor, they should also be in the 0-2 Ohm range.
4. The amplifier will be difficult to test without some specialized equipment. Essentially you need a signal generator to provide a signal and an oscillocope to measure it. If you have access to this equipment and know how to use it great. If not you will have to take it to an electronic repair shop and they should be able to test it for you.
5. For the speedometer sensor, you will also need an oscilloscope to check the output. Since I believe that this sensor is a hall effect type, it is best tested installed in the gearbox, using an oscilloccope to measure the output. There really is no easy ancedotal way, to verify this part is working.
So what can you do?
If you have determined the cabling is O.K. Then you can simply replace the amplifier and check the system and if this does not work, change the speedometer sensor. This may be cheaper than takiing it to a specialty repair shop. If you can get a hold of these parts used, then this method will be very economical. If you know you have a good amplifier and speedometer sensor and you have verified that the cabling is good, then it can only be the speedometer itself!
Good Luck
There are obviously 5 things that can be wrong with your speedometer system
1. The speedometer itself
2. the wiring from the amplifier to the speedometer
3. The wiring from the speedometer sensor to the amplifier
4. The amplifier
5. The speedometer sensor
1. You said you had the speedometer checked, so we'll rule that out.
2. Although you have cleaned some terminals use an Ohm meter to check the continuity of the cabling between the speedometer and the amplifier. If I am correct, your speedometer is not an open collector configuration so the resistance of the wires to the amplifier should be low (~0-2 Ohms).
3. Similarly check the resistance of the wires to the speedometer sensor, they should also be in the 0-2 Ohm range.
4. The amplifier will be difficult to test without some specialized equipment. Essentially you need a signal generator to provide a signal and an oscillocope to measure it. If you have access to this equipment and know how to use it great. If not you will have to take it to an electronic repair shop and they should be able to test it for you.
5. For the speedometer sensor, you will also need an oscilloscope to check the output. Since I believe that this sensor is a hall effect type, it is best tested installed in the gearbox, using an oscilloccope to measure the output. There really is no easy ancedotal way, to verify this part is working.
So what can you do?
If you have determined the cabling is O.K. Then you can simply replace the amplifier and check the system and if this does not work, change the speedometer sensor. This may be cheaper than takiing it to a specialty repair shop. If you can get a hold of these parts used, then this method will be very economical. If you know you have a good amplifier and speedometer sensor and you have verified that the cabling is good, then it can only be the speedometer itself!
Good Luck
Marco
1985 Alfa 90
1985 Alfa 90