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Discussion Starter · #1 ·
Does anyone have experience or

Does anyone have experience or ideas where to start with a tachometer gauge that indicates (apparently) half speed? It is a recent change noticed at startup a couple of weeks ago (may have been the first time I drove it after the winter). I say it is apparently half speed, the charging light now goes out at approx 750 rpm after startup and it indicates about 2000 rpm at about 80 mph. I have removed the sensor from the flywheel cover and there appears to be no damage. I have not checked voltages/resistances. In the past (with motorcycle using magnetic pickup for speedometer sensors) I have used a dremel with a digital readout for revolutions to check (compare) and will try that this weekend.
I believe this to be an isolated problem though I do have another issue I need to address that may be related. The engine is somewhat flat at startup and driving but clears out somewhat (not completely) when fully warm. It feels like lost spark that sometimes clears on neutral throttle when fully warm but flattens back out with heavier throttle.
Recently, I have had to replace an air valve in the air circuit of the emmissions (used a gm valve as recommended by the Ferrari dealer for similarity and cheaper price-the miss existed prior to this change). I also found a spark plug with a broken insulator which I thought would cure the miss, but it did not.
Thank you in advance for your advise or experience with the tachometer problem. Of course, any advise on experiencee curing a similar miss would also be appreciated.
dave
 
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Discussion Starter · #2 ·
David,

What car is this on?


David,

What car is this on?

If it is on a car with duel/same dinoplexes like a injected 308 you might try swapping the boxes to see if it is a sending problem with the dinoplex. Atl least its one thing to eliminate.
 
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Discussion Starter · #3 ·
The car is a 1986 328 gst. It

The car is a 1986 328 gst. It has a Magneti Marelli MED860A (microplex).
I am not very familiar with the system. I see the flywheel sensor going into the microplex unit. I assume it is that signal that goes to the gauge. Is this incorrect, are there signals from the coils (triggers) that (both) must be sending? If that is the case, it may point to a solution for the miss as well. When the miss clears, though, the tacho readout does not change.
Thank you for your ideas.

dave
 
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Discussion Starter · #4 ·
Common problem on the 328 and

Common problem on the 328 and Mondial with the same engine. The fix is to pull the tach and send it to a speedometer shop...they will replace one or two parts on the circuit board on the back of the instrument. I think I paid around $140 plus shipping to have Palo Alto Speedometer fix it.
 
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Discussion Starter · #5 ·
Most people will suggest you p

Most people will suggest you pull the tacho out and send it to Palo Alto Speedometer to fix it for you. Apparently the problem is the calibration potentiometer inside the tacho ... its gets dirty and the contacts aren't being made properly so it reads incorrectly. The solution is to clean or replace it (and then check calibration).
 
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