Instrument cluster rpm signal

[üblemaschine]

I did a quick search and confirmed I am tapping the right wire, but I am just curious if anyone can tell me what the rpm signal from the motronic to the cluster should look like on an oscilliscope? I have an intermittently dead tach and its always dead when I go to read the signal. I am getting something that looks like a normal ac wave with a pointy looking square wave mixed in. It changes frequency with revs and is about 3v in amplitude. I'm having the same issue with 2 separate clusters that work in other cars ok, I think it is dash wiring or a motronic issue but I'd just like to have an idea of what the signal should look like.

thanks

[TobyB]

I thought the tach got a square wave from the ecu,

but I've never scoped it- the shift light, set to '5v'
just worked, so I never bothered.

t

[Gregs///M]

Yeah its a square wave, although dont expect a perfect signal from a 30 year old technology DME. Your tach issue is almost 100% related to a dying SI board. The SI board controls and powers the tach, temp, MPG and OBC as well as the SI LED readout. Any issues related to these gauges can be attributed to a failing SI board which is very common.