The Lotus Cars Community banner

1 - 4 of 4 Posts

·
Absolute power does what?
Joined
·
3,201 Posts
Discussion Starter #1
This weekend while running at the track I noticed my tach was reading incorrectly. Basically it progressively lost revs - meaning at the beginning of my session it was correct, after say 5 minutes it was 1k revs low, another 5 minutes another 1k revs. I noticed it when I popped onto the second cam at an indicated 4200 - as I rarely look at my tach when on track (I listen for the shift point and light) it surprised me. I noticed that the shift light was still coming on at the correct revs so it feels like something with the needle.

The odd thing - as I come in the pits at say 2k revs it's showing 0 - not below. I pulled to my spot, shut it off, then restarted - perfect. Verified the revs with my Traqmate - dead on. Oh well I thought - it's a Lotus. But next session it did the exact same thing (and the following final session).

Driving home all was fine - never noticed any issue but as I was on the street I was driving easy/slowly. I've seen a few threads where the needle needed to be reset to 0 but this doesn't seem to be my issue. Could there be some sort of resistance issue with the physical needle that after a certain amount of high reading stress it slips?

As mentioned above I have a Traqmate and have tapped the coil pack at cylinder 1 to read revs. The logging of that looks correct, no odd curves/spikes. Not saying they aren't related but they don't really appear to be.

Ideas anyone? In one way I'm not really worried about it as my season is over for a while and even on track it was more a "uh, WTF is that?".

-Ross
 

·
Premium Member
Joined
·
2,637 Posts
Most likely an intermittent connection between ECU and the Tach display. I believe the tach needle is driven by a stepper motor, when it loses a couple cycles of the quadrature signal it gets out of sync with the phase progression of the quadrature signal. One full cycle is (I believe) 1k rpms. When the rpms pass the next 1k cycle, it resyncs, but 1k rpms off. A buddy of mine discovered the connector on the back of his display had some fractured cold solder joints between the connector and the circuit board. He re-soldered the printed circuit board and his problems went away. The loss of connection could be anywhere else, not just the circuit board.
 

·
Absolute power does what?
Joined
·
3,201 Posts
Discussion Starter #4
Most likely an intermittent connection between ECU and the Tach display. I believe the tach needle is driven by a stepper motor, when it loses a couple cycles of the quadrature signal it gets out of sync with the phase progression of the quadrature signal. One full cycle is (I believe) 1k rpms. When the rpms pass the next 1k cycle, it resyncs, but 1k rpms off. A buddy of mine discovered the connector on the back of his display had some fractured cold solder joints between the connector and the circuit board. He re-soldered the printed circuit board and his problems went away. The loss of connection could be anywhere else, not just the circuit board.
Thanks Adder - that makes a lot of sense. I've not only had my cluster out in the last year but I actually cracked it open to put a piece of tape over the airbag light. That plus the 21 track days I did this season add up to a lot of stress and I could see how something would get loose in there. On my list to fix over winter when I take the interior out.

I'll report back after I give this a try.

-Ross
 
1 - 4 of 4 Posts
Top