No RPM Signal

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No RPM Signal

Postby patbarber » Mon Jan 15, 2018 9:58 am

I have been unable to get my Microsquirt to pick up an RPM Signal on OPTO+-.

I am using the uS as Fuel only and connected the Opto+- wires to the Coil

Bosch Distributor using Pertronix Ignitor 1 ilo breaker points.

I have to admit that I did initially have the system wired backwards. It is not intuitive to place the opto+ on the - terminal of the coil and the opto- on the +12v of the coil.

I initially had something that looked like a tach signal, now I get nothing. I am afraid I may have popped one of the elements of this Opto Isolator Circuit.

I will be upfront, I don't know how wiring it backwards could have caused the problem. I don't believe the Pertronix Ignitor could have created the problem either, but I do not know. Interested to hear your thoughts. Thx.

Is there a way that I can drive this circuit with 12V (or 5V) and determine if the unit is working?
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Re: No RPM Signal

Postby hybrid » Mon Jan 15, 2018 10:28 pm

Yes, a 5v or 12v square wave (on/off) signal should trigger this circuit.
A stimulator (JimStim is best) would be a good investment although you would need to make an adapter for a Microsquirt.
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Re: No RPM Signal

Postby DaveEFI » Tue Jan 16, 2018 2:26 am

The input of the opto IC is a form of LED. Check using the diode test function on a DVM. It is possible to make it fail with too much drive current - same as any LED.
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Re: No RPM Signal

Postby patbarber » Tue Jan 16, 2018 7:13 am

Thanks for the info. After further digging, there seems to something wrong with this circuit. I have attempted to generate the square wave with no success. I have been able to get the VR1 circuit to trigger so I am certain it is not a software issue. I have ruled out the wire harness as well. I will try the diode test when I get home.

I am thinking there are only 3 primary elements of this circuit, the Schottky diode, the 680 ohm resistor (R20) and the opto isolator. I am thinking I can probe the circuit with the diode tester connected and determine which element is suspect. Your thoughts?


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Re: No RPM Signal

Postby patbarber » Tue Jan 16, 2018 4:12 pm

So, I think I found the culprit. Using the Diode Test Function on my mutimeter, I have found that the resistor, R20, has failed.
https://photos.google.com/share/AF1QipM_STBYOkqpvVlvBlvU6hEs7oszRTryYE9XDdsSTrr7Tm93CZkp7WWndh1Hgqgi-g/photo/AF1QipM0QU0THt_S-Cane3kLMam4qloLTtBH2x2U19XM?key=SUlWUDhKWTFCR2VHM1VCVjNVblJHQ01LTEx2Z0lB

I used the diode tester on the entire circuit and it read open circuit in both "directions" indicating that something was wrong. I used the diode tester on D4 and on the input pins of U4 and found that both diodes functioned (voltage drop in one direction, open circuit in the other). Placed the diode tester on R20 and it read as an open circuit. Resistance measurement indicated open circuit.

I dont have any 680 ohm 1/4 surface mount resistors lying around, so I dug into my supply of through hole resistors and made a 680 from a 470 and a 220 in series.

https://photos.google.com/share/AF1QipPtVcZ0chIh3nM1nTSwFpPuuVXmP637oJyAXXgDfoHqGKI9Hcl96R0hgYN2RmLI7g/photo/AF1QipPkWlysADJoD-LMe1CzhAkgUT9JZsKhNfaWZgpk?key=RWVoTTFNRHZwNDNteWpEZUstLXZCYXlSX1hIZUZR

Looks crude, but it will suffice to prove to myself that the R20 was the problem. We will let you know how it goes.
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Re: No RPM Signal

Postby patbarber » Tue Jan 16, 2018 4:56 pm

Success. It seems that adding a 680 ohm resistor over the top of the existing resistor did the trick.


My data acquisition system now functions as I had intended. Here are the traces.
https://lh3.googleusercontent.com/Oef6eaEqfuFracN5VpHByWi5rfoJALkk1BFfqG6HvQblHGGh497kNSutFo-CTS2s9tOGfZJbPSF-kZ56fAbw8ud5kCN6NBcTEj7dfxuAAPpGt6T97SXJvNrQLdveVcmTLsbRDPH4xg=w2304-h1440-no

I now have to find a 1/4 watt 680 ohm surface mount resistor. to replace the bodge that is in there now. Are these standard parts or is there a specific part number I should be looking for?
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Re: No RPM Signal

Postby hybrid » Tue Jan 16, 2018 5:03 pm

Hmmm... how do you think that resistor fried? Or do you think it never worked?
Any part that meets the specs should be fine.

Edit:
Since you mentioned pertronix, be very careful with the version 1. They have left quite a few people stranded because they burn out or burn a coil out.
I think the issue is with where the engine stops. Since it's just a magnet that makes a contact, if the engine stops at the right place and you leave your ignition on, it can be grounding the coil the whole time.

This may have also caused the issue with your Micro. Maybe you can put a current limiting resistor inline or something to help.
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Re: No RPM Signal

Postby patbarber » Tue Jan 16, 2018 5:41 pm

hybrid wrote:Hmmm... how do you think that resistor fried? Or do you think it never worked?
Any part that meets the specs should be fine.
.

That thought clearly crossed my mind. Seems more like an infant mortality failure than a user overload problem. To me it is not a big deal. I just need to find that 1/4 watt resistor and get it properly replaced so the bodged resistor I have in there doesn't cause any further issues.

Edit:
Since you mentioned pertronix, be very careful with the version 1. They have left quite a few people stranded because they burn out or burn a coil out.
I think the issue is with where the engine stops. Since it's just a magnet that makes a contact, if the engine stops at the right place and you leave your ignition on, it can be grounding the coil the whole time.

This may have also caused the issue with your Micro. Maybe you can put a current limiting resistor inline or something to help


I have version 1's in two of my other British cars. I have not had a problem with them. You are correct in that they will melt down if left powered on for a significant period of time. I think the Ignitor II's have a timeout circuit which prevents this. Turn the power off and all will be fine. If I work on my cars and need the key on for some reason, I will pull one of the pertronix wires from the coil to prevent it from burning out. I often forget to reconnect it and wonder why the car will not start.

I don't think having the unit powered up was the cause of the resistor failure. The opto+- circuit has been disconnected up until yesterday. The car that I have uS install on is a Formula Ford 1600 race car. I am using the uS as a data acquisition converter taking engine parameters and transmitting them via CAN to my Aim Solo DL data logger. I disconnected all the ignition system while performing all of my development and troubleshooting. Like I said, not a big deal. I will replace the resistor and move on.

Thanks for the feedback!
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