Had codes E39 E52 E59 after car had been sitting all summer due to exhaust system falling apart and fuel leak had begun. Was told by my son who last drove it that speedo and cruise did not work either. Replaced all fuel lines and tank, fuel pump, sending unit assembly as well as all hard brake lines since All the lines run together and rear brakes were always poor, despite replacing MC and rear wheels cylinders that I discovered to be frozen, so figured if a line kinked or internal obstruction, that should eliminate that possibility too.

Car running and driving again now, cleared the E39 E52 and E59 codes since did not know when codes last cleared, probably years.. Took the car for a drive and SES light came on and OBD code now only shows E24. On the way out, speedo worked fine, on way back the reported speed dropped about 15 mph though going a constant speed, then later dropped to zero. I am guessing speedo head wornout at 181k miles. Reading 89 service manual (1/2 that book is electronic diagnostics..), read that code when activated also disables cruise control and VCC.

Cruise control being disabled makes sense if car can't tell how fast it is going, but what is VCC? Prior to car being parked, it was displaying SES light after driving 9 miles. Never figured that out and one of the other codes also mentioned VCC, but forgot which. It made no difference if car was driven fast or slow, just seemed to be triggered by distance. Car sat overnight and light was off, drove 9 miles and it would come on again. Car is harder on gas than previous 4.5 DeVille I had. The older one would get mid to high 20's mpg on the road, but this one has not run better than 18-19 mpg for several years. Torque convertor possibly not locking up?

So, I am wondering if speedo was running out of computer-acceptable range even back then and if VCC is torque convert lockup, whether speedo repair or replace will fix speedometer, VCC and cruise control all at once

Over my head here in all the electronic control interplay, but am I on the right track? Any electronics genius's on here?

 

VCC is the Viscous Converter Clutch. It is part of the torque converter. Instead of a regular, friction lined clutch in the converter, Cadillac used a viscous fluid type clutch assembly that allowed a much smoother clutch engagement on lock-up. No lock-up will definitely impact fuel mileage.
The most likely cause of your speedo problem is the speed sensor itself, or the wiring from it to the ECM. A worn speedo head would NOT set any codes, nor would it disable cruize or converter lock-up.
I would start by examining the wiring at the speed sensor, where it plugs in. The wiring can loose it's insulation, due to getting oil on it from the engine. If the insulation breaks down, or completely comes off, it can allow the wires to touch, killing the speed signal to the ECM. If the wiring looks ok at the connection, try unplugging it, and reconnecting. Sometimes that will correct a loose connection. If that does no good, then the next step, imo, would be to replace the speed sensor. The sensor is located on the passenger side of the trans, right where the axle comes out of the trans. On rare occasions, the reluctor wheel on the differential can come loose, and spin freely. This could also cause a no speedo signal.

 

Replaced the VSS and the wires and connectors were in good shape. Went for a drive and speedo was working, but after about 5 miles the speedo stopped working again, same as before and SES came on same as before. Now am thinking it must be VSS buffer, but parts stores say they don't show any such part. Nothing on eBay either. Speedo was and is working when car is cold, but after 5 miles or so it conks out. Only place to get the buffer a salvage yard?

 

Am now getting codes E24 and E59 as before, but now also getting an F48.