Tachometer Output is putting out an incorrect signal?

[Doorgoo]

Hello everyone, I have an early WhodWho MS unit and everything was fine running wasted spark and suddenly one day the tachometer died. Long story short I bought an oscilloscope and the pulse looked fine so I proceeded to buy a new cluster and one of those SI board capacitor replacements despite my board metering okay. Still no dice. Finally I spoke to Greg from Bavarian Restorations and he told me that my signal voltages were incorrect. I’m at a loss with this car, sadly I only have the one so I don’t have another to test this stuff on. Thanks for any insight.

[varg]

The MS Tach output jumper is supposed to be a 12V square wave output, so +12V pulled low (to zero). Is this "whodwho MS unit" microsquirt or megasquirt based? I'm just guessing it's microsquirt based. You are probing your tach out pin and referencing that oscilloscope to the ground plane of the ecu, correct? It looks like you have 10.27V getting pulled down to 7.25V instead of 12V getting pulled down to zero. The microsquirt circuit for tach out is the following:


Based on what you've presented me with I'd suspect that Q1 is on the fritz.

[McGyver]

I bought an oscilloscope

Off topic, what oscilloscope is that? are you happy with it?

[Doorgoo]

The MS Tach output jumper is supposed to be a 12V square wave output, so +12V pulled low (to zero). Is this "whodwho MS unit" microsquirt or megasquirt based? I'm just guessing it's microsquirt based. You are probing your tach out pin and referencing that oscilloscope to the ground plane of the ecu, correct? It looks like you have 10.27V getting pulled down to 7.25V instead of 12V getting pulled down to zero. The microsquirt circuit for tach out is the following:


Based on what you've presented me with I'd suspect that Q1 is on the fritz.

I will take a look at that thank you! It is indeed microsquirt based

[Doorgoo]

Off topic, what oscilloscope is that? are you happy with it?

It’s one of those DIY kits you can get everywhere, the DSO 138. It’s definitely hobbyist grade, I only bought this because I can’t get to my work one due to COVID. It’ll get the job done but it’s less than intuitive and the signal won’t be completely pure but it clearly was enough to reveal my issue!

[Doorgoo]

varg do you think it’s worth trying to fix or am I better off buying a new microsquirt module? Like obviously a component is cheaper but am I just asking for more trouble down the line

[varg]

I don't see any reason to replace the whole unit, the failure could be a fluke or it could have been caused by something like a wiring problem or internal short that damaged the transistor. That's if it's the transistor, it's just my guess based on what was presented. In any case inspect it and probe the InjLed pin, that should be logic level with the opposite waveform to the tachout waveform, also unplug the cluster and test the resistance between the tach out pin and ground. It should be open.

[Doorgoo]

varg you were right you beautiful bastard! It works again I could cry