Megasquirt 3 - Sync loss and remedies


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Intro - Symptoms - Reason codes - Sync error logger - Solutions


Many Megasquirt installs use some kind of trigger wheel with a tooth pattern on it. The code in the Megasquirt decodes these patterns and determines engine position for fuel and spark. Under certain conditions (typically noise) the incoming signal is disturbed and the Megasquirt loses syncronisation with the wheel. This is called a sync loss event or a sync error.


Feels like a misfire or a false rev limiter
RPM in datalog suddenly drops to zero
RPM in datalog suddenly spikes up
"Lost sync count" increments while "SecL" counts up normally.
(NB. If SecL suddenly drops to zero this is a "reset" which would likely be caused by a major voltage spike on the power feed.)

Taking and reviewing a datalog is a required step.
This datalog extract (viewed in Megalogviewer) shows the rpm (yellow trace) dropping to zero and at the same time the "Lost sync count" increases. The "Lost sync reason" shows reason code 2.
sync loss datalog

Reason codes

0no problem
1init error
2missing tooth at wrong time
3too many teeth before missing tooth (last)
4too few teeth before missing tooth (last)
51st tooth failed test
6nonsense input (last)
7nonsense input (mid)
8too many teeth before missing tooth (mid)
9too few teeth before missing tooth (mid)
10too many teeth before end of sequence
11too few teeth before second trigger
12too many sync errors
13dizzy wrong edge
14trigger return vane size
17second trigger not found when expected
20subaru 6/7 tooth 6 error
21subaru 6/7 tooth 3 error
22Rover #2 missing tooth error
23420A long tooth not found
24420A cam phase wrong
2936-2-2-2 semi sync failed
3036-2-2-2 tooth 14 error
31Miata 99-00 - 2 cams not seen
32Miata 99-00 - 0 cams seen
336G72 - tooth 2 error
346G72 - tooth 4 error
36CAS 4/1
40Twin trigger
41Twin trigger
42Chrysler 2.2/2.5
44Suzuki Swift
47Daihatsu 3
48Daihatsu 4
50Rover #3
51GM 7X
5236-2-2-2 tooth 30 error
53rc51 semi error
54rc51 re-sync error tooth 6
55rc51 re-sync error tooth 16
56rc51 re-sync error tooth 18
57fiat 1.8 tooth 0
58fiat 1.8 tooth 3
59fiat 1.8 tooth 6
60fiat 1.8 tooth 9
6136-1+1 first
6236-1+1 second
63QR25DE semi failed
64QR25DE lost running sync
65CAS360 running
66NGC8 semi failed
67LS1 semi failed
68LS1 resync failed
69YZF1000 resync failed

Note! Most of these simply show that there's a problem and don't particularly pinpoint where it is.

Sync Error Logger

The "Sync Error Logger" can be very useful to record the defective tooth pattern and display it in the tuning software. See the Tooth logger manual


Having identified that you have a sync loss problem, you need to attempt to eliminate it.
Check for any basic problems

  • Loose sensor
  • Loose cabling
  • Sensor wires running close to spark plug wires or coils
  • Slop in timing belt
  • VR sensor wired backwards
  • Fouled plugs causing misfire
  • Non-resistor plugs
  • Excessively rich mixture causing misfire
  • Excessively lean mixture causing misfire

    Having ruled out all those possible causes, you should enable some noise filtering. See the noise filtering settings

    If you have a question, comment, or suggestion for this FAQ please post it on the forum.

    No part of this manual may be reproduced or changed without written permission from James Murray and Ken Culver.