The speedometer just stopped working on my 1980 308, it was working fine. I replaced the mechanical sending unit, as I read they frequently go bad, but that did not fix it. Are there any other diagnostic tests I can perform? I have the schematics and the wires from the sending unit othe speedometer are not broken.,
Have you confirmed that the green (V) wire at the sending unit is +12V with the key "on"? If not -- most likely fuse/fusebox trouble (fuse #2 position -- one of the wires leaving the bottom of fuse #2 is the V wire); If so (and you've got a presumably new, working sender) -- this does not bode well for the speedometer itself. Good luck (and welcome to FChat!)
Palo Alto Speedometer will do an excellent job repairing your speedo head. Just send it to them with a note stating your request. They'll give you a call when it's done and they'll be prompt about it. Check their website: http://www.paspeedo.com They've done a lot of work for me and I've always been happy with their service. They are authorized to repair VDO, Veglia, Jaeger, MotoMeter, etc..etc... Cheers, Aaron '82 GTSi
The next step (if you want more certainty before sending the speedometer unit out for repair) would be to measure what's coming out of the RN terminal with the key "on" (i.e., V wire at +12V) when rolling the rear axles together in the forward direction. I don't know what instrumentation you have available, but unless the pulse is really, really narrow, I would guess that an AC voltmeter would give some indication if you were actually getting a signal -- i.e., detecting something here under the proper conditions (and then at the speedometer end of the RN wire) would be a clearer indication that the speedometer head is at fault assuming that other input signals (power, ground) to the speedometer head are OK. And likewise, getting no AC voltage indication under those conditions would warrant further digging elsewhere IMO. Of course, if some one can post/describe the expected voltage waveform, that would be helpful (I've got the oscilloscope, but no 308 to measure ). My impression is that 328 and 308QV have had the most reported speedometer head troubles here, and no so many instances on the carb 308 -- do you know if your 308i speedometer PN is the same or different PN than the other siblings? Good hunting!
Just hooked up the Traqmate and here is the info: The tach wire color AT THE CAR'S WHITE MOLEX connector (not the gauge's) is brown striped black (the second wire over on the Molex). NOTE: The wire color from the tach gauge itself is DIFFERENT than the wire color on the car's harness after the gauges molex. As i recall, it maxes at about 5 or 6 Volts and is easy to read with a Volt meter because if you rev the engine, naturally the voltage goes up and down accordingly.
Im not sure what the tach wire has to do with the speedo not working? Anyway I tested the pulse generator for a DC pulse, not AC. So it should be as simple as hooking up a AC meter to it to check the pulse to make sure the new one is working?
Cue55, I hope this helps you in your troubleshooting. http://www.ferrarichat.com/forum/showthread.php?t=101159&highlight=rjay -Rjay
OK, I still have not been able to fix this problem and I am stumped and don't know what to do next. I bought a new sending unit which did not fix it, after some testing from the advice in that other thread it seemed that the new unit may have been bad as it didn't show any movement. Well I ordered another one and it also did not fix the problem, I did the test like on the other thread and it does not work according to it. I find it hard to believe that 2 new sending units are bad. I have tested the wire to the speedometer itself and the wires are good. I also quickly shorted the 2 wires at the back of the car that go to the speedo which puts 12 volts to it like a pulse and the speedo moves so I do not think the speedo is bad. Any suggestions? Im $400 into sending units and no closer to fixing it.
Can you put a meter (or scope) on the sender and spin it by hand (or with a drill motor)? Any output?
If you have proved the wiring harness from the rear to the front, it is probably the speedo unit, it's a pretty common problem....a rare case has found broken wiring harness under the hold down clamps on the frame, with both sending and speedo still good. Report one more early 1977 308 unit failure! *hand waving* There is even a page in my Owner's Manual, to record the mileage, when replacing it..... Good luck, although it's a hassle, Palo Alto Speedo can verify or repair it, eliminating another possibility.
?? There would be no output on just the sending unit doing that, no there is nothing using a meter. It would need 12 volts to generate a pulse. Maybe something on a scope? As I said I put 12 volts to the speedo and it seems to work fine, is that not an accurate test? Thats all the sending unit does to it.
Cue 555, Like most electrical problems, this will require very methodical approach if you are to fix it. Did you try the following? "Easy sender checker - Connect a +9V battery plus side to the green wire of the sender. Connect the minus side to the housing of the sender. Connect a 560 ohm resistor from the red wire on the sender to the housing (minus). The resistor is in a pulldown configuration. Connect a voltmeter negative lead oto the sender housing (minus) and the positive lead to the red wire of the sender. Rotate the sender and you should see the voltage change from 0.5-1V (Low) to 5-6V (High). I dont know if this works on all senders but it works fine on this one." Remember, this is basically a three wire assembly. The Green Power wire, the Red signal output wire and the housing (good ol' ground). If you can't get a signal on your meter, then you have a bad sender or something loose or misconnected. This is fundamentally how this thing works. Without this, there will be no signal to the speedo and your needle and odo won't register a darn thing. I would focus on this part. When you get signal, check it all the way up to the speedo. If you get signal and your speedo is still not working, you could have a bad speedo. When I ran into my problems, I started with the sender. When I got a signal, I started working forward to the speedo and then internally. If you are not comfortable with electronics, I wouldn't mess with it. My only disclaimer is that I don't know if your 1980 is the same as my 1978. JMO -Rjay
Yes I tried that procedure with the original sending unit and the 2 new ones on the bench with no movement on the meter, I find it hard to believe the 2 brand new ones are also bad so maybe that test doesn't work for the 80 as it does on the 78?
Hmm.... Cue555, do you mind setting up the test and then taking a quick photo? Please make sure you capture all of the wires and connections in the photo. This should help us both step through it. Here are a few more questions. What type of meter are you using and what is the setting? What are you doing to rotate the internal shaft on the sender? -Rjay
My father who was an electrical engineer is setting up the test. Using a voltmeter and tried DC and AC settings. I am turning the generator with a drill. BTW anyone know which way it is supposed to turn when looking straight at the shaft?
Cue555, Glad to see your Dad on the job too. I am also an EE. You need to put the meter on the DC setting and rotate it slowly by hand. If you use a drill, you will get a higher frequency waveform that will fool the DC meter and give you an erroneous reading on an AC setting i.e. your meter is not fast enough to show you a good result - you would need an oscilloscope (in that case, you will see a nice waveform). In my write up I mention that you should look for two voltages. Don't be too concerned about the actual readings of these voltages, simply make sure there is a distinct and repeatable "high" voltage and "low"voltage. The input circuit on the speedo is set up to take a digital input like this. When I did this test, the voltage went high and low several times for each revolution and was very repeatable on the meter. If you have +12V available instead of the +9, use that as it will give you even more distinct highs and lows. I hope this is helping. Please post the results of your test. Thanks, -Rjay
On one of the units when it is hooked up I get 2 volts showing but it does not vary at all, the other one shows 12 volts and it also does not vary at all. This is rediculous if these 2 new ones are bad as the places they were bought from do not take returns on electrical items.
Cue555, I don't know how to further help you with this. I wish I could. I would be glad to check your unit out in my car but you will have to wait about a month because I am replacing CV boots and I only have weekends. This way, you would have a check in a working system to dispel any potential bench testing issues. I posted a note to all Fchatters to see if anyone wanted to donate a known bad one so that I can cut it open and reverse engineer it. This way, I would be able to say conclusively, how to test it and then know if one is bad or good for sure. Is there a way that you can ask the manufacturer to prove that your new senders work? -Rjay
I tried asking the second one I bought from if they knew how to or could test them and they didnt know, they just told me that they were new.
