Bench testing gauge cluster...mostly tachometer
#16
Rennlist Member
I though I had a square tach laying around to test but couldn't find it. I suspect the tach is looking for a square 12v cycle, the frequency of which increases with rpms. Again, guessing, but imagine it triggers on the rising or falling edge...
#17
Rennlist Member
I put my little pc scope on the tach signal and confirmed it gets a pulsed signal. If you follow the schematics, this is clear since the tach is fed the same ignition signal as the KLR straight from pin 21 of the DME. This is the same pin you tap to get rpm's on the zeitronix, etc. The 28.1Hz and 35.6ms shown in the screenshot passes the sniff test as well, since that is what you would expect at an idle of 840rpm's. By the numbers:
840 rpm is (840/60) 14 revolutions per second. At 2 ignition events per revolution, you would expect to see (14*2) 28 pulses per second -- i.e., 28Hz. If you divide 1 second into 28 pieces, you get 1/28 or about .0356, which is 35.6 thousands of a second or 35.6ms.
So, create this pulse however convenient, and you should see 840 on the tach.
840 rpm is (840/60) 14 revolutions per second. At 2 ignition events per revolution, you would expect to see (14*2) 28 pulses per second -- i.e., 28Hz. If you divide 1 second into 28 pieces, you get 1/28 or about .0356, which is 35.6 thousands of a second or 35.6ms.
So, create this pulse however convenient, and you should see 840 on the tach.
Last edited by Tom M'Guinn; 02-11-2013 at 10:55 PM.
#20
Rennlist Member
It's not just software, it is a simple/learner's USB o-scope from parallax. I'm sure you can get more hardware for less but parallax has really good educational documentation I haven't found elsewhere...
http://www.parallax.com/Store/Microc...me,ProductName
http://www.parallax.com/Store/Microc...me,ProductName
#22
Sorry for the lag on this. My son has been sick and my wife seems to think I should spend more time in the house right now. I'll drag the jimstim to work this week and see if I can set it up over a lunch in one of the labs.
#24
Rennlist Member
That question probably will get more response in the rennlist section(s) for those cars. I only have schematics for the 944 series...
#26
Rennlist Member
Well, I confirmed your connections and applied a square pulse from an Arduino (I just used the example "sketch" called "blink" and changed the speed to get a good reading on the tach). Worked like a charm. I also checked with a lower tech approach by applying 12 volts and ground to those terminals, then tapping the signal terminal with a wire attached to ground. The tach is driven by ground pulses, not voltage, so rapidly tapping the the signal terminal to ground does the trick. It doesn't make a nice clean rpm on the tach like in the video, but you can get it to jump around that way.
#27
Racer
Thread Starter
Join Date: Mar 2003
Location: MA
Posts: 438
Likes: 0
Received 0 Likes
on
0 Posts
Thats perfect! So I should be able to get the tach to move by the following...
+12v -- term4
ground -- term2
tapping ground -- term3
Ill give it a shot today and let you know. Fantastic work Tom... thanks!
+12v -- term4
ground -- term2
tapping ground -- term3
Ill give it a shot today and let you know. Fantastic work Tom... thanks!
#28
Addict
Rennlist Member
Rennlist Member
While you guys are in there futzting around can you please sort out why our fuel gauges only ever manage to read Full about as often as the Cubs win the World Series?
#29
Racer
Thread Starter
Join Date: Mar 2003
Location: MA
Posts: 438
Likes: 0
Received 0 Likes
on
0 Posts
I connected my +12v DC source to the A, B and C connectors in pic...
By tapping the wire connected to A to ground the tach moves.
Very easy test to do. Thanks again Tom.
Greg -- You sure its not full due to the price of gas? =) I believe that when the gauge sporadically shows full its a fuel sender issue. Not sure though.
By tapping the wire connected to A to ground the tach moves.
Very easy test to do. Thanks again Tom.
Greg -- You sure its not full due to the price of gas? =) I believe that when the gauge sporadically shows full its a fuel sender issue. Not sure though.
Last edited by Teddy952; 03-11-2013 at 11:17 AM.
#30
Addict
Rennlist Member
Rennlist Member
You may be onto something with the price of gas. I have pulled the sender, done the diagnostics and it appears to be functioning properly. I 'may' have even replaced it, it's been awhile. Regardless, it generally reads 3/4 when I fill up, although it will on occasion read 4/4 (or close to it). Not an uncommon problem from what I read. Seems like the sweep of the gauge just doesn't coincide with the signal it receives from the sender.
Sorry to derail your thread, but I would love to figure this out.
Sorry to derail your thread, but I would love to figure this out.