Supra Forums banner

1 - 6 of 6 Posts

·
handy with the steel
Joined
·
390 Posts
Discussion Starter #1 (Edited)
Having some trouble getting my tach working with my M150. I'm using one of the Half Bridge Outputs to drive the tach, but not having any luck. The HBO puts out a square wave pulsed between battery + and - , which is what I believe the tach is looking for. This output is also what the JRR Supra firmware/project comes configured - and I have used another HBO to drive the speedo - so I am pretty sure I'm at least going about this the correct way, but I'm stuck as to why it's not working. Here is the wiring diagrams for the tach. I've got the HBO pin hooked up to pin 29 on the II1 (orange) footwell connector:

249307


...seems pretty straight forward, although I don't have the igniter anymore (switched to smart coils) . That's the only thing I could imagine would be giving me trouble, but honestly I'm not sure why it would.

Anyway, I'm about at the point of pulling the tach out and making sure it hasn't gotten fried somehow, but figured I'd ask if anyone could point out something obvious I've missed before I go through that trouble.

Thanks.

EDIT: Also, the pin diagnostic shows 0 volts on the HBO pin. For reference, my speedo shows ~10V . Possibly open or short?
 

·
handy with the steel
Joined
·
390 Posts
Discussion Starter #3
Yes if you were wiring it to a typical lowside discrete output, but that's what the Half bridge output does. It switches to 12V+ and ground for you. Works for speedo. 🤷‍♂️
 

·
handy with the steel
Joined
·
390 Posts
Discussion Starter #5
Oh man, Maybe I misunderstood how the HBO works??? I'll try the pullup resistor. Thank you.
 

·
handy with the steel
Joined
·
390 Posts
Discussion Starter #6
cwtt

I owe you a beer my man. Pullup resistor did the trick. Thank you!

Now I am utterly confused about the Half Bridge output, as I thought it put out a square wave between 12V+ and 0V (battery negative)

Based on your comment about the dev software, it seems the firmware has to be set up to make the HBO either a high side or low side driver? Is that correct?

And if so, it seems silly that they didn't just assign one of the HBOs as a high side driver for the tach so you could skip all of this pullup resistor nonsense. Or maybe I'm still not understanding it correclty 😅

Either way, thanks again.
 
1 - 6 of 6 Posts
Top