Tach Spikes between 2500-3100rpm, no spark control

[fryea]

Howdy Howdy

I've seen alot of people having problems with tach spikes when running off of a hall sensor or VR sensor. Im running MS with fuel only, so Im getting the RPM signal off the - side of the coil. Whenever I floor it between 2500-3100 RPM, it spikes up to like 5800-6500 rpm, and the car will buck and go crazy and the AFR's will dip below 10:1. Its driving me nuts, especially when trying to pass people in traffic or going up hills, cause thats the main daily driving rpm's.

Ive heard of capacitors working, but that has been for people running with VR or hall sensors. My ignition is being triggered right now with just the stock k-jet ignition, whatever it is.

Could I add resistors inline with the tach wire to cure this? Capacitors?

[dirtbike]

I used to have this on my original MS installation, the spikes were usually when the car was idling and seemed clean when actually driving but I changed my wiring to accept the trigger input from the hall sensor in my block mounted dizzy when i went to MSnS abouit 4 years ago, as soon as I did this the tacho spikes dissapeared.

Are you using shielded cable as that will make a big difference?
Laters
Nick

[benflynn]

what kind of plugs/wires, try some resistor plugs/wires, also you can bring the tach signal back out to chassis insted of grounding to the system ground, can cause resets, do a lot of searching, it has all been gone over, and go msns!!!!!!

[qwkswede]

K jet distributors have VR sensors inside of them. At least the earlier ones do. But there is a module in between there and the coil, so really it doesn't matter. The megasquirt is getting the signal from the module to coil (-) connection. There is some filtering circuitry to build on the MS board for this type of setup if you have trouble getting a clean signal.

Here, look at this:
http://www.megamanual.com/v22manual/mwire.htm#ignition

The 'Dave' capacitor might help.

[fryea]

Yeah, going msns will be in the near future

For now though, need to get this sorted out, if only for the principle of it.

I think it has an older set of bosch something plugs in right now. I will check tomorrow if they are resistor plugs or not.

qwkswede: Im running a v3 board, which already has the dave cap installed (c30). Maybe a higher filtering capacitor (more volts?) could be put in its place.

What is puzzling about this is that the car has run just dandy without any of these spikes for the first 2 months, and then it slowly started getting worse and worse in the more recent 2 months.

Now i need to travel back to the parents house to dig up that lh2.2 block dizzy in place of the k-jet dizzy in order to start the spark control portion of megasquirt.

[benflynn]

in the instructions to build the boards they address this stuff, and how to isolate the trigger ground

[str8krewzn]

I have the same problem but from 2800-3500 but i think it could be cause by oily distro but not sure

[fryea]

upon looking at a graph after driving, the spikes only are intense after 60kpa (this is NA). By intense, i mean 13000 rpm spikes.

There are still minor spikes to 1700 whilst idling, but those do not affect me as much, yet may be an indicator

What weirds me out most about this issue is that is started developing over time, and I have not made any major changes to the car that would instigate such a change in tach spikes. Something might be wearing out, either mechanically or within megasquirt components. I just reflashed the firmware, so I know that is not the issue.

Spark control is coming this weekend, crossing fingers.

[740ATL]

are you using a usb-serial adapter? Check the log for the frequency of the spikes and see if there's a pattern. Every 5-10-15-20 seconds? If yes, that's your usb drive.

If not, do some reading on msextra for your setup. Throw a cap between the 12v+ on the coil and ground (a real ground, not the negative terminal on the coil). Got wasted spark? Put a cap on each coil. Mylar caps, something like 25uF or something.

[benflynn]

don't foget hi rez plugs, cured me after a good battle, i tried everything else first,also try to ground the head and the stock inj/sensor grounds(if that is what you used) to a diff point on the fire wall w/a strap

[fryea]

740atl: Although I do use a USB adapter, the spikes are definitely not at regular intervals. It is mostly after 60kpa, and always after 80kpa, in a super high frequency of spikes (2700-13000-2700-13000-2700-13000 within 1 second) and not in a regular pattern either. Even then, it occurs regardless if the adapter is plugged in or not.

benflynn: hi rez plugs? Is that the same as resistor spark plugs? I will try the additional ground (for added peace of mind after the +spark)


+spark this weekend hopefully

[fryea]

update: put a 22uf cap between the 12v wire to the coil and ground (used the strut nut closest to the firewall as a ground point).
No difference.

[benflynn]

yeah resistor plugs, i run ngk bpr8es

[fryea]

upgraded to +spark using the dizzy for input and vb921 as output. No more spikes. weeee

now i need to get a timing light and get the trigger angle right. Oh yeah, and find my timing belt cover so i can actually use the timing light :oops: