What are the signs of a dying tachometer?

[Infinitiguy19]

I have what I think is a bad tachometer, How do I know: Well once a while when entering the highway stomp on it while doing 30 MPH or less and I see the tach jump to 7100 RPM, not this is without a comuter consult hooked up to the ECU.

I have 3 ECU's but they all do the same, I have a 1991 Q45 with TCS ECU, 1993 Q45 with TCS ECU (Stock), and I also have a 1994 Q45 ECU with TCS Cali model.

I also have the stock TCU if it matters.

AT-CHECK stays at AT-CHECK when perfoming the test.

I will record with a consult what the ECU reads and then see if there is a differnce.

Also when reving at 7100 RPM's it feels like it just stays there, it does;nt bounce off it or anything.

And yes I know the Tach is incorrect after a certain RPM, I think 4000 RPM's or so.

And the Tachometer always works, Meaning it never goes to zero or bouce arond when driving or at idle or any other circumstance.

But is the Tachometer bad or not?

[maxnix]

What you describe sounds like a toasted transmission.

Any maintenance history you would like to share?

[Infinitiguy19]

Talked to Wes on this one and he said the tach is off by +/- 200 RPM's and thats normal.

Only real problem I am at a loss for is that the tach stays at 7100 RPM's and does'nt bounce off it? But othere than that that and the fact that 1-2 injectors are SOMEWHAT bad the car is fine.

By somewhat i mean they are kinda clogged, but enough so that BG44k and Redline is useless, hence the reason I was wondering about the power flush. But I will just OHM the other injectors and keep the good ones.

But now that you ask the transmission is fine, but the gas sucks up here and thats what I though was a transmission problem on some of my other topics.

The fluid is red but not bright red, So time for a flush. :

[qship96]

What makes you think some of your injectors are clogged?

[Infinitiguy19]

Thanks for asking, Well I used that laptop consult to turn off each cylinder one by one, I felt what it was like to have a bad injector/spark plug. And It only feels only half as bad as a dead cylinder. And its only felt at idle hence causing shaking and a high idle.

[Q45tech]

A tachometer is just an analog dc volt meter reading the voltage generated by ecu that emulates some approximation of rpm.Both speedo and tach have suffered from lifetime issues where they stick read zero, full or inbetween in error................common to replace them with new units on almost every 90-91-92.

[Infinitiguy19]

Why only those 3 year?

Were they made better later on?

[Q45tech]

1994 had hundreds of changes in designs and almost a thousand different parts.

[maxnix]

1994 had hundreds of changes in designs and almost a thousand different parts.

The 1993 was the (improved) bastard of the first series of G50, the 1996 the (not so improved) bastard of the second series of G50.

[Infinitiguy19]

Hence a hybrid.