RPM's tachometer issue

[baihui]

Hello People,

the issue is that I am currently using standard tachometer from 5 cylinder engine, however the new engine is 4 cylinder engine, hence the RPMs are not correct because the tachometer is counting it differently for the 5 cyl engine.

So is there simple solution for having this issue resolved, say by spoiling some resistor or something ?

Thanks !

[UrS4boy]

Hello People,the issue is that I am currently using standard tachometer from 5 cylinder engine, however the new engine is 4 cylinder engine, hence the RPMs are not correct because the tachometer is counting it differently for the 5 cyl engine.

Actually, the RPMs are determined by the G28 engine speed sensor that counts the number of starter ring gear teeth (on the edge of the flywheel) that go wizzing past in a given time. I presume that the 4 cylinder (1.8t?) flywheel has a different number of teeth than the 5 cylinder you removed. I don't think there is going to be an easy fix other that somehow swapping in the 4 cyl tachometer (and that might be difficult)

http://forums.quattroworld.com/s4s6/msgs/20986.phtml

[Christophski]

Some tachos come with a switch on them that you can set to however many cylinders the car runs on. usually located on the back or under the guage somewhere.

[UrS4boy]

Some tachos come with a switch on them that you can set to however many cylinders the car runs on. usually located on the back or under the guage somewhere.

Hopefully it is that easy (especially since the 80's did come with 4s and 5s).

[Kieron20V]

But the signal from the g28 is sent to the ecu, then the ecu sends a pulse relative to the number of ignition events to the clocks.

So if you have 5 cyl clocks and a 4 cylinder pulse, the clocks will read 20% higher than the actual engine.

I remember reading somewhere about possible jumpers on the back of the clocks pcb, might be worth a search.

Are you using the standard 1.8t ecu?

[Kieron20V]

Sorry, lower.

[baihui]

But the signal from the g28 is sent to the ecu, then the ecu sends a pulse relative to the number of ignition events to the clocks.

So if you have 5 cyl clocks and a 4 cylinder pulse, the clocks will read 20% higher than the actual engine.

I remember reading somewhere about possible jumpers on the back of the clocks pcb, might be worth a search.

Are you using the standard 1.8t ecu?

I am using megasquirt, not the default 1.8t ECU.

I know for sure that there is resistor fix for this, but I don't know what should it be.
Another fix is to replace the RPMs tacho with one from 2.0 B4 engine that should count the same, but I would do that only as final option.

[UrS4boy]

I know for sure that there is resistor fix for this, but I don't know what should it be.

I don't think so. I understand that the G28 sender is sending pulsed signals (square waves or whatever), not a voltage signal so there is no resistor fix.

But I could be wrong.

[baihui]

I don't think so. I understand that the G28 sender is sending pulsed signals (square waves or whatever), not a voltage signal so there is no resistor fix.

But I could be wrong.

so does this mean that if I change the tacho with 2.0 engine, that wont fix it as well?

[Kieron20V]

4 cylinder clocks will work, but if there is somewhere in the megasquirt setup to make the tach output emulate a 5 cylinder engine then do that. Im not sure on megaquirt but I can do that on m7 034 ecu. Im running 5 cylinder clocks off my v8.

[ManiacuV6]

Stock 1.8T ECU doesn't use the signal from starter ring gear . It uses a trigger wheel that is fixed on the crankshaft and the position and speed sensor is fixed on engines's block. So use a tach from a 4 cyl engine or try to simulate with Megasquirt a "5 cylinder tacho signal" .

[baihui]

can you tell me what to look for in the mega in order to simulate the signal, or there is hardware stuff that should be mounted ?

Thanks much !