Tacho Pulses

[eboggs_jkvl]

Tables G1202 & 03. Tacho Pulses High & Tacho Pulses Low. Right now they are both set to 15 pulses. What happens to the tach if these numbers are adjusted?


Elmer

[Taz]

Hello Elmer,

G1201, G1202, and G1203 on E38 ECMs control the type of Tach signal output, and the number of pulses sent to the Tach. Both G1202 and G1203 must have the same value, and must be a whole number.

Stock settings for your vehicle are probably:

G1201 - Crank
G1202 - 15
G1203 - 15

Changing G1201 alters how the signal is sent to the Tach. Decreasing the value of G1202 and G1203 will increase the number of pulses sent to the Tach, etcetera.


Regards,
Taz

[eboggs_jkvl]

OK, that's logical but will that affect the response of the tach? Suppose I set it to 50 in both tables. Will that slow the response of the tach? Make the tach more responsive?

[Taz]

Think of it like the selector switch on the back the Sun Super Tachs we all used to love. There was a switch with a setting for 4, 6, or 8 cylinders.

Changing G1202 and G1203 does the same thing. With your 8 cylinder engine at 2000 RPM:

G1203 & G1203 set to 15 - Tach displays 2000 RPM
G1203 & G1203 set to 7 or 8 - Tach displays 4000 RPM

Most (if not all) GM Tachs are calibrated to receive what would have historically been a “4 cylinder” signal. Rather than adjusting the Tach, or producing different Tachs for engines with varying numbers of cylinders, GM uses the ECM (or PCM) to alter the Tach output.


Regards,
Taz

[eboggs_jkvl]

Great answer. I have it now. Darn, I was trying to slow down the tach response a little to test a theory on my dieseling condition. Oh well, on to more things to try and solve the problem.

Thanks a lot.

Elmer

[Lennart]

I recently had to adjust G1202/3 on our 99 Silverado conversion (to L92 - E38).
Assumed 6 pulses for a 24x crank would result in 14.5 pulses on a 58x crank.
Right now I set both to 14 with the tach reading a little high. Should I try to set one to 14 and the other to 15 to get the needed 14.5?

[Taz]

Hello Lennart,

Sounds like an interesting conversion you have on the go. In the E38 tune you are working with confirm G1201 is set to “Crank”. G1202 and 1203 must be set to the same value - this can be any whole number. Setting both to 15 is usually a fairly close output for a “4 cylinder” type Tach (the OEM Tach in your Silverado). You can adjust these values as needed (as long as they remain equal) until your Tach reads accurately.


Regards,
Taz

[Lennart]

Thanks for the reply will try 15 on the next tune.

[Sleeping]

So would changing the pulse help me in anyway instead of wiring a circuit up like the attached picture for my 04 GTO swap?
10710

[Taz]

That diagram is a "Tach pull up" circuit - used when the output from the PCM / ECM is insufficient to cause the needle on the Tach to move at all / move consistently. This will not assist with the accuracy of the Tach - just its function.


Regards,
Taz

[Lennart]

I got the tach working in the cluster, but the aftermarket tach is not moving. Should I try the pullup as well?

[Taz]

If the Tach in the gauge cluster (IPC) is functional and accurate, then the parameters in your tune are appropriate. Some aftermarket Tachs will require a pull up circuit - give it a try - can't do any harm if wired correctly.


Regards,
Taz