Hello all together, I have the issue, that the RPM reading is wrong. At around 3000 RPM you can see, that the Tachometer ist standing still for a couple of 100 RPMs. Is this a common issue? What should I check/prove? Thank you very much
Never heard of this, but check your connection under the coolant fill tank. Disconnect it, clean the sides and reconnect. Maybe there's an intermittent connection around that harmonic.
It is not uncommon for the tacho in the Testarossa to read wrong. I've seen it at least a couple of times where it studders into the rpm limiter and only showing about 5000 on the tacho. Without knowing for certain,but comparing with other italian cars I have repaired,there could be a potensiometer inside the instrument. This can be adjusted/calibrated so the tacho is reading correctly. Worth checking !
Are you saying that the RPM of the engine is smoothly increasing, but the tach needle "sticks" for a bit at 3000 RPM and then jumps up suddenly to catch up and continues to increase smoothly with the engine RPM? If so, I'd put most suspicion on the mechanics of the needle motion inside the tach itself. Is your ignition system still stock?
Well, the needle does not catch up. It stays behind. You can see, it goes up as it should to around 3000 RPM, the engine pulls on, the needle slows down and after some RPM, it moves in normal speed again, but more than 1000 RPM behind. Ignition still is original. No problems so far and works as it should (besides the RPM reading).
That (bad) tach needle behavior could be caused by either a mechanical or electrical fault inside the tachometer.. Your OM schematic shows that the tach signal is generated inside the Dinoplex ECU so it is possible that only that section is bad, and the section running the coil is still OK (since you report that the engine seems to rev properly and smoothly), but I'd give that a lower probability than something being wrong in the tach itself -- but just a guess. This is where you wish you had a spare BB512 laying around so you could just try another Dinoplex ECU for a quicky test
as steve says easiest way to check changing the dinoplex. but this "needle-problem" I also have sometimes on other cars and always it was the rpm-meter itself. so you may try if possible an other rpm-meter? if you would be in my area I would give you mine for testing but unfortuantely you have not published where you are located. may be someone in your area could help with an other rpm-meter?
I was having a conversation with Adrian at [email protected] regarding the tach in my testarossa and this was his email to me regarding the tach. "a good test would be to have a passenger check the tacho input signal with a portable scope while driving, this should help you identifying if there is a bad ground problem or if the signal itself drops out or is erraneous. Bad ground issues are more common between the sensors and the Microplex, you should be fine if the ground connection between the six terminal AMP connector and the tacho is good (e.g. less than 2 ohms between the tacho housing and a good earth). Also check the battery minus terminal for corrosion, as well as the engine ground to battery minus. On your Testarossa, the tacho signal is generated by the Microplex MED120B based on the RPM sensor input. You might want to check the RPM sensor signal going to the MED120B with a portable scope as above to rule out any broken sensor or ground probems. On the MED120B circuit board is a BC489B PNP transistor (T8)which drives the output signal going to the tacho. This transistor might be broken (or have bad solder joints), so might be a good idea to replace it if you discover that the tacho output signal coming from the MED120 and going to the tach drops out or is erraneous."
