[W126 Coupe] Tach Kaput

[jeffk at gsm.udallas.edu]

I was having the usual fuel-pump relay problems with my '88 560SEC. As a
temporary fix I got a relay from a 500SEC and it worked fine. Soon
thereafter (coincidence?) my tach failed. All fuses are OK, speedo, clock
work. Fixed the original relay, even replaced the EZL to no avail. The TD
signal must be active, (meaning the cps sensor is OK, right?) else the
N16/4 relay wouldn't work. This signal is routed through the X11 diagnostic
socket, but without a 'scope what should I look for? 
When it rains, it pours....odometer AWOL now. So I'm going to have to pull
the cluster out, I think. Any advice will be much appreciated. Thanks.
Jeff

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