Tach dropuout sanity check

hudsonhawk · Post by **hudsonhawk** » Thu Jan 26, 2006 8:58 am

Just wanted to verify that this is indeed the problem.

MSII V2.0 on V3 board. MS controlling fuel only. The Jeep has been running for over a year, but have very seldom had it over 50 MPH. I have noticed some bucking at 2400~2500 and above but always assumed it was fuel as I have never taken the time to tune my upper fuel maps. Distributer is a 4 pin GM HEI with tach output.

Last night finally took the Jeep out on the highway and logged the run. To my suprise there are tach spikes all ove rthe place when you get above 2400 RPM. Nothing like the tach signal going to zero to make the car buck.

Attached is the MSQ, a screen cap of the log and the log file itself. What is the remedy for this.

Bernard Fife · Post by **Bernard Fife** » Thu Jan 26, 2006 9:33 am

hudsonhawk,

Assuming the ignition and wiring is in good shape, try lowering the next pulse tolerance for 'normal running' to about 25% or lower. If your code doesn't allow this (because that setting doesn't exist), upgrade it to the latest code (2.34 from here: http://www.megasquirt.info/ms2/code.htm).

Let us know how that works.

Lance.

hudsonhawk · Post by **hudsonhawk** » Thu Jan 26, 2006 10:04 am

All the logs below 2400 RPM are rock steady on the tach input.
I am running A DUI HEI that I bought new. It runs strong and I have not noticed any RPM oddities on my in dash tach. So I'm confident the Ingnition is working correctly.

I also completely rewired the Vehicle harness when I put the MS on. So I am fairly sure the wriing is in good shape. I am splitting the signal between the TACH and MS input, wonder if that has something to do with it?

I was planning on upgrading to 2.33 this weekend but guess I can go to 2.34. Will see how that goes. Thanks for the input.

Bernard Fife · Post by **Bernard Fife** » Thu Jan 26, 2006 10:16 am

Hudsonhawk,

Oh, I asusmed you were controlling the ignition - I neglected the part in your note where you said 'fuel only'. Sorry!

So that's a bit different. You you might try doing is putting a resistor inline with the dash tach (but not MegaSquirt). I'd try a 1K Ohm to start, and you can go as high as 100K Ohms (if it's too high the dash tach will stop working).

You can also try lowering the resistance of the R12 resistor - the default value is 390 Ohm, 1/2 Watt, but you could try 270 Ohm, 1/2 Watt - but you need to make sure it isn't getting too hot while running (as per the manual).

Lance.

hudsonhawk · Post by **hudsonhawk** » Thu Jan 26, 2006 10:53 am

I will try the tach resistor first. Its easier to modify. I can put a pot in there and hopefuly trim out the problem. THen replace it with a permanent resistor once I know what size I need.

John

hudsonhawk · Post by **hudsonhawk** » Sat Jan 28, 2006 4:36 pm

Okay, think Igot it fixed...

After thinking about wiring issues, I set in the driveway and reved it up to 2800 RPM. It ran rock steady and no issue with drop outs. So it seemed to only be happening under load.

Went back and checked my wiring. The +12v and tach wires to the the HEI had very little slack in them. So I cut and spliced about 6 inches of wire into them. Buttoned everything backup. Went ahead and upgraded to MSII 2.34.

Took it for a test drive. During acceleration, there is no hesitation at all. (maxed out at 3200 RPM cause I still dont have the upper maps done). Got the Jeep up to 60 (approx 2800 RPM) and maintained it for a couple miles. Again, no problems what so ever. So it looks like it was a wiring issue after all. Aparently under load the engine torqued over enough to cause a momentary loss of connection on the tach or power wire to the HEI. The analog guage on the dash is apparently not sensitive enough to see that drop out as I never saw any issues on it.

Thanks for the help.