LS7 tacho feed

[C Lee Farquar]

My SPA shift light and tacho have stopped working, they were running on the GM ecu tacho output

I've spliced into the loom near the ecu and still can't get them to work. The tacho, an analogue Smiths unit jammed at 3000 revs and the SPA unit stopped showing the revs shortly afterwards. The SPA unit hasn't been fitted long. The SPA unit is showing my peak revs at over 10000, but it may have come like this as a factory default, I've never reset it. If I plug a code reader into the loom it shows the revs. The only confession I have is that I did some welding on the car and forgot to disconnect the ecu, not sure if this is exactly when the Spa unit stopped working.

I was planning to bin the Smiths unit anyway and just have the SPA unit and obviously I can send this back to be checked by SPA. Seems a bit too much of a coincidence that both units have failed.

I don't have an oscilloscope, which I believe I need, to check the tacho feed from the ecu so was wondering if there's any other way to run the SPA unit? I found an MSD unit via Google but on some links it says it doesn't work with an LS1.

A solution before my race on Sunday would be handy!

[spatz]

sounds odd, typically these outputs do not break if shortened so most likely you have a short somewhere in the cable. when you said you went into the loom right after the ECU did you cut the cable ? You still may use a TRUE RMS multimeter, it should show some value if the output is still alive.

[C Lee Farquar]

Thanks for the reply. I cut the tacho feed wire where I had previously joined it about 150mm from the ECU, thus isolating the wire that runs through the car.

I don't have a true RMS multimeter and they're quite expensive so I guess I'll send the SPA unit back and ask them to check it.

[Storer]

I think there is a lesson here for all owners. Arc, mig or tig welding on a car with computers (ECU, Dash, etc) is not advisable unless you disconnect them. The minimum you should do is disconnect the battery and alternator before welding and the earth should be as close to the weld as possible.

I know this is no help to Steve but it is a warning to others.


Paul

[C Lee Farquar]

I've done a fair bit of welding in the four years since fitting the LS7 and this was the first time to forgot to disconnect the ecu

I do have a battery cut out which was off.

[C Lee Farquar]

Finally resolved. It needed a live supply with a 1k ohm resistor in line to the tacho feed wire.

No help from Spa on this, other than a bill for telling me the unit is OK, just a few hours getting intimate with Google

[Storer]

As you had a working tacho before the welding I assumed you had the resistor fitted.

I (SR) had to fit one in the feed from the ECU to my tacho when we fitted the dash.


Paul

[C Lee Farquar]

It seems the Smiths Tacho I had fitted had a built resistor. When I fitted the engine I just wired the tacho up and it worked. When that stopped working so did the shift light. But not knowing about the resistor I checked everything else first.

Still, I'm happy now.