MSgpio.com read only forums (Jan.15,2018)

Has anyone reverse-engineered a GM digital dash? Or maybe even found some scematics?

I've decided to throw out the old speedometer in my 77 cadillac and replace it with
a digital dash from a 92 Brougham (Perfect fit, thank you GM R&D department).

However, the wiring is of course different. It took me a whole night, but I think I have sorted out most of them.
When one of the wires touches ground, I get a pulse to the speedometer. I suppose that will work
fine with megashift?
How many pulses for how fast, I have no idea. Is the VSS connected directly to the speedo on those cars?
Broughams had 4L60e's but I will install a 4L80e. Maybe the teeth count on the output shaft is the key to something.

Also (Offtopic)
There are 5 unknown wires left that (at least one) should go to the fuel level sensor. No matter how I connect them, I get no reading whatsoever. How does (more) modern fuel sensors work? My old one is a 100 ohm resistor.

Thanks.
Peter

Peter,

I don't know the answer for your vehicle. But on one of my cars, an 1984 Corvette (which was an early GM EFI system, a little later than yours) the speedo VSS signal goes directly to the speedometer in the dash. On my car (and its gear ratios, tire sizes, etc.) the VSS signal works out to about 96000 pulses/mile. The dash then divides the VSS frequency down to 4004 pulse per mile (apparently later cars use 2002 pulses per mile) to send to the ECU. I suspect it was done this way so that the speedo calibration for each vehicle coming off the line was done in the dash rather than the ECU (in the days before cheap flash memory). At least some later GM cars have the VSS signal go directly top the speedo and ECU (in parallel).

However, either the raw VSS signal or the 'divided down' 4004/2002 signal would be fine for MShift, if the appropriate parameters are used to set it up.

None of the above necessarily applies to your car, but given GM's tendency to keep things the same, it *might* be relevant.

My fuel level sender is a conventional GM sender (0 Ohms empty, 90 Ohms full), and works fine with aftermarket gauges, so I can't help you there.

Lance.

Ah, thank you Lance.
Very appreciated!

"GM's tendency to keep things the same" is something I seriously count on.

/Peter