megasquirt resistor trouble

[ultimatepoghero]

All right for some time now i've been having trouble getting consistent spark from my number one cylinder (erradict and slow blink from timing light) i figured out that the optical wheel in my 84 z31 dizzy has a large square hole just for the No. 1 and it was throwing off the megasquirt, so i swapped it out for one out of a 83 280 zxt (it has identical slits all around) which seemed to really improve the the way the car sounded and revved didn't skip and pop alot like before. i cranked the car several times with no problems but before it could really even warm up completely the car died I assumed the changes required some sort of adjustment for warm up, fuel, spark or something. i gave up after that for the day because My buddy had let someone borrow the timing light (i work on the car at his house where the car stays) so i had to wait till the next day to check the spark and more likely correct the timing. So i get the timing light but never use it cuz the car wont crank and i see i'm not getting any rpm signal. I replace the 12v 25watt flameproof 1K ohm pull-up resistor i have on the CAS signal with an extra one i had that was 1.2k assuming maybe it burned up or something the car cranks up upon doing this but dies quickly again, like less than a minute. I assumed because the replacement resistor wasn't very secure this happened so i connected it properly soldering the wires to it and such. But after that again no rpm signal no crank. i have another similar resistor thats 1.5k that i try but it doesn't work at all. Can someone tell if i need some kind of heavy duty resistor better than the ones i had and where i can get them. the ones i got didn't have like a heat sink like the 25 watt 3K ohm resistors i put on my injector for low-impedance, but they were coated heavily in some stuff that i guess made them flame proof like the packaging said. Or is it now that i'm getting a proper signal from my dizzy it's causing this because maybe my setup is wrong. In the beginning i tried to use the power transistor but it wouldn't spark however i did not use a 5v resistor before the power transistor like seen here http://www.blozup.com/ms.php only a 12v resistor on the CAS. I assumed that when i bypassed the power transistor and it cranked maybe i didn't need it or it was bad i don't know for shure. In my setup the signal pretty much goes from the CAS thru the 1k resistor to the Megasquirt to the coil to the dizzy no relay board or GM HEI like seen in a diagram here http://forums.hybridz.org/showthread.php?t=88246, can someone help me

[Tony D]

WOAH! 25 WATT?

i think my pullup is a 1 watt. maybe 1/2 watt.

You may not be doing much current limiting with a 25 watt resistor. I don't know that it needs to be that big to pullup the optical signal from teh dizzy.

[ultimatepoghero]

so more wattage is bad, please elaborate so i can understand why if you could. i originally had a 1 watt or 1/2 watt resistor on there but the lead connectors broke one day when i touched it to check it after i noticed i wasn't getting rpm signal like now. So i'm gonna say perhaps i should focus on low tolerance % if i want to look for a good resistor for this application. apparently i don't know as much about resistors as i would like, and thank you for your reply.

[Xander]

wattage has nothing to do with it. It doesn't matter how high or low the wattage is. A pullup or pulldown resitor only has a couple of milli amps running through it. If it is getting hot than you have wired it wrong.

 

Wattage is amps times voltage. given that voltage is about 12 volt and the resistor you used was 1k ohm. you are only running 12 miliwatts through the resistor. power (wattage) is amps times voltage. 12 miliwatts times 12 volts is only 0.144 watts.

 

so basically is doesn't matter what the wattage is on the resistor for pullup functioning. a tiny 1/4 watts 1k ohm resistor will work the same as a 25 watts 1k ohm resistor.

 

 

you should also keep in mind that the 25 watts resistors that you used for injector dropping resistors are not 3k ohm. They are 3 ohm. that is a thousand times less resistance !!!. Maybe you got it wrong because afaik they don't even make 1k resitors at 25 watts.

[ultimatepoghero]

thank you for your feedback xander and your explanation on wattage, but its not that the i know for sure the resistor is getting hot it seems that when i check it with the multi meter when it's not working i get less resistance than i should (about 750 ohms from the 1K) and weird voltage readings out of the end going to the ecu like 5 or 8 when its all hooked up not 12 like it should and it does give 12v out both ends when its not hooked up, but after i left it alone and tried using the exta resistors i had i checked it again and it was reading 1k ohms again. if you think i have it hooked up wrong why did it work so well before when my optical wheel was causing misfires, i think maybe the proper and consistent signal is causing more stress on the resistor now what do you say. Should i try wiring up the power transistor again like in the blozup diagram or use a gm hei like the diagram i found in this forum, or is it the lack of the second 5v resistor before the coil after the ecu. I just find it so weird that this has become a problem all of a sudden now that i finally got that nice wheel with the identical slots which i thought was the only thing standing in my way of me properly tuning this car. When the number one cylinder was misfiring the car would run for so long that once the car started to run hot from revving the engine too much while parked while i was playing with MS settings and such.

[Xander]

if you think the optical system is the culprit maybe you can try to modify the slot for cylinder one with a bit of tape. make it as long as the rest.

 

I have used the 4 pin hei module to run the car and it worked well. So that is also an option.

 

I am now running an multispark ignition box by mallory.

 

When you get stuck like this it is often best to start from scratch again. Check everything is working right. Check the signal from the optical system. is it high when the slot is on the sensor or is high when the sensor is blocked. Try reversing the output to the coil.

 

I am using a hall setup. I have three magnets spaced 120 deg apart embedded in my flywheel. The sensor is placed in the plate between the block and the gearbox. when the senor "sees" a magnet it grounds a pin. this pin is directly connected to the opto isolator inside the megasquirt. because the signal from the hall sensor is very clean I can connect it directly to the opto isolator. no puldown resistor required.

 

I would imagine that the optical system in the megasquirt would work the same. If the output signal is high (voltage present on the output line) when a slot is pressent maybe you can trigger the opto isolator directly from that.

 

I''ll have to look into this CAS system some more.

 

 

I know that you have probably read the sticky that Moby wrote but I would like to point you attention to this part.

 

*Pullup/Pulldown resistor quick course

You use a pullup resistor to get your line(or pin,connection) to some voltage (5,12, etc). Suppose you want a pin to have 5volts. If you just add a wire to 5 volts there is nothing to limit the current and if the pin happens to be an input to a transistor that is switched to ground then you have a short to ground from 5 volts and you burn stuff up. So instead you add a resistor to the 5 volts and the resistor will limit the current according to ohms law. Current is voltage divided by resistance. If you put a 1k ohm resistor to 5 volts then if the pin is switch to ground by internal electronics it can only draw 5 milliamps max(the resistor current-limits the path).

 

A pulldown is similar except that you want a pin/wire connected to ground and you want the path to be current limited.

 

To size the pullup or pulldown correctly you should know what the circuit is that you are adding the resistor to. But as a general rule you can use a 1k resistor for 5volts and a 2k resistor for 12volts. That limits the current to a few milliamps which is conservative. Sometimes the circuit may require more current to operate properly.

 

For the pullup resistors I am just using 1/4 watt resistors.

 

I=E/R I=12v/1k I=.012 or 12 milliamps

 

P=I*E P=.012*12v P=.144watts little over 1/8 watt

 

I=current

E=voltage

P=power

R=resistance

[ultimatepoghero]

i've already tried the tape thing that is why i got the wheel out of the 280zxt because the tape idea noticably improved the situation just not 100% i figured it needed to be ultra percise and the 280zxt wheel was perfect identical slits all around and they even line up perfecty when i hold it next to the other wheel. everything seems to work perfectly when the resistor is doing its job im positive its just giving out in some way cuz thats the only thing i have to change to get it to work. I'm gonna try a lower wattage resistor because i was using one like that before and it worked for a loooong time before i figured out about this optical wheel thing the lead connectors just broke on it from me physically fiddling with it alot i would have re-used it if i could. If that doesn't work i'll try a 4 or 7 pin module and/or adding that second resistor and/or power transistor i'm not ready to start from scratch again just too many other things to at least give a try first, i just wanted someone to give me a point in the right direction, but Thank you for your help and effort.

[ultimatepoghero]

used the cas out of the 280z had to flip the optical wheel over everything works great!! and i'm off to tune!