MKIV Supra - Tach Signal

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CenTxE92
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Joined: Wed Nov 22, 2017 7:11 am
Location: Austin, Texas

MKIV Supra - Tach Signal

Post by CenTxE92 » Tue Jul 09, 2019 3:27 am

At a bit of a loss here. Car running no problem and rpm shows perfectly fine when car is running and Scal is open. Tach used to work perfectly fine then out of nowhere just stopped working.

I’ve already checked continuity from Ecu to factory harness all the way to cluster.

With key on, have 12v at the tach pin. As I understand it, the tach signal is a pulsed 12v but I’m not seeing it pulse while running.

Checked with paste cal to make sure nothing got accidentally changed and absolutely nothing related.

Only thing that has changed since working is the crank trigger wheel. But like I mentioned, rpm displays perfectly in scal while running.

Thoughts/suggestions?

Thanks in advance.

TimH
JT Innovations
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Joined: Wed Sep 01, 2010 2:51 pm

Re: MKIV Supra - Tach Signal

Post by TimH » Tue Jul 09, 2019 8:32 am

How are you measuring the tacho drive signal?

At 1000rpm it will be a 50Hz signal (or thereabouts) and a DC meter may not show any deviation in average volts at that kine of frequency - although I'd probably expect to see a small change in volts between the engine off and running. Setting the meter to AC volts might actually show a change in volts vs rpm however.

Ideally you would measure it with a 'scope.

If it was working before then, basically:

- something has changed. Your explanation suggests nothing that is likely to have caused this to stop working however.
- something has broken. Prime candidate is the tachometer itself! Can you either swap the cluster or bench test the existing one?

stevieturbo
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Re: MKIV Supra - Tach Signal

Post by stevieturbo » Tue Jul 09, 2019 9:34 pm

typically outputs are pulsed grounds...not 12v's

Does every other aspect of the dash work as normal ?

If you're reasonably competent with SCal, and know what output the tacho has been assigned to ( not hard to find in I/O assignments ), you could do an output test to see if that brings any life to the rev counter.

Output testing could be a bit more user friendly than it is though, especially for pulsed outputs. But it's still usable ok.

A multimeter with pulse width, or frequency tested between 12v and the ecu output would be another test option ( scope would be best )

TimH
JT Innovations
Posts: 605
Joined: Wed Sep 01, 2010 2:51 pm

Re: MKIV Supra - Tach Signal

Post by TimH » Tue Jul 09, 2019 9:55 pm

Tacho feeds generally need a pullup to 12V to get a 12V pulsing signal; hence I'm assuming this is why 12V is being seen at the "tach pin".

Using output testing is, as said, limited. I've seen the barest of a flicker on a tacho using this method.

Suggest 'scope (best); AC meter (should give a vague idea of what's going on). As said: something's changed or something's died.

CenTxE92
Posts: 4
Joined: Wed Nov 22, 2017 7:11 am
Location: Austin, Texas

Re: MKIV Supra - Tach Signal

Post by CenTxE92 » Wed Jul 10, 2019 1:23 am

Everything else on the cluster works fine.

I de-pinned the tach output from the factory harness and used a power pen (12v+ source) and tapped the pinned and am able to get the tach to bounce.

At this point I'm pretty confident that A) the tach is functional. (It's a pretty recent TRD 10k replacement) B) the wiring from ecu to tach is without fault.

I didn't have my Fluke with me at the shop. Only had a friends janky little analog meter. I'll test the frequency this evening.

I have several MKIVs in the shop right now so if the frequency looks ok, I'll try another cluster/tach just to see.

Thanks again for everyone's input.

CenTxE92
Posts: 4
Joined: Wed Nov 22, 2017 7:11 am
Location: Austin, Texas

Re: MKIV Supra - Tach Signal

Post by CenTxE92 » Wed Jul 10, 2019 2:43 am

With car running at idle, tach output has ~14vdc @ 0.0Hz.

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