Category Archives: Car repair

Adapting a Saab 9-7X for 2wd

One of the things we do a lot of today is not spending money on cars.  We push oil changes too far beyond their due date, and we really don’t want to spend money repairing cars if there is any way of getting out of it.  In this case a Saab owner’s front differential exploded, and the cost of repairs did outdo the vehicle’s value.

picture of Saab 9-7X
Saab 9-7X

What caused the diff to explode?  Hard to say, but I suspect the front universal joint failed, and the driver just drove on, ignoring the vibration until the top half of the diff came apart.  I don’t really know that for sure, as the driveshaft was missing when we got the car, but some violent act blew the top of the diff off, plus the fact that there was a big hole in the steering rack right below the the universal joint.

picture of exploded diff
Exploded diff

In the end, we left the diff in place, but took out the axles to keep it from flopping out of the car.  Getting the failed diff out would have required a lot of labor and it seems that if he kept all four wheels on the pavement, it shouldn’t go anywhere.  To hold the front wheel bearings in we needed to disassemble the axles and bolt the front constant velocity  joint stub axles back in.  Having the front driveshaft out didn’t compromise the transfer case, there were no fluid leaks. Then topped the whole thing off with a new steering rack.

picture of front axles
Front axles have to come out

The Saab 9-7X is just a Chevrolet Trailblazer with it’s own bumper skin.  It is full time awd, and it was unknown if the front driveshaft missing would be a problem for the awd electronics, and in the end there was no downside.  As long as you weren’t in the slippery stuff, you would never notice how the truck drove.  The repair was still expensive, but at least the guy can still drive his truck.  As for the next owner…. it’s never a bad idea to have a mechanic look over a prospective purchase.

picture of the stub axle
Reinstall the stub axles

What’s inside a Tire Pressure Sensor?

Tire pressure monitoring systems (TPMS)  have been made mandatory on new cars  in the US  since 2008 by the Tread Act.  Most manufacturers field their own systems, but all work in a similar fashion. Early systems had an Electronic Control Unit in the car dedicated to the sensors, receivers spread around the wheel wells, and a transmitting sensor in the tire that takes the place of the tire valve.  Almost all manufacturers have ditched the separate receivers and have put them in the ecu itself.

picture of tire sensor
Tire Sensor make by Schraeder, used in Chevrolets….

The tire sensor itself houses a microprocessor and enough memory to store tire pressure, temperature and rotation direction values.  There is also a radio frequency transmitter and an inertial unit that helps with power management.  The sensor doesn’t transmit anything until the car reaches 15 to 20 miles an hour, conserving energy as the built in 3V coin style battery is not replaceable, but is designed to last 10 years… some make it that long, some do not.

picture of tire sensor diagram
Breakdown of Freescale TPMS

Some sensors have to have their electronic id programmed into the car when changed out, some are learned automatically.  They have to be dealt with when new tires are installed, typically replacing the seals, but if corrosion has set in, it will probably need a sensor.  Learning a new sensor is not usually done by DIY’ers, as the electronic equipment is quite expensive.

picture of sensor layout

Replacing sensors with manufacturers original parts is pretty expensive, rarely costing less than a hundred bucks for a sensor… but some aftermarket types, like VDO’s RediSensor, has stuffed several manufacturers programming into a single sensor, enabling it to replace them directly and be programmed with factory tools.  The best thing is that they are available for $35-$50, saving a lot of the cost on new sensors.  It is important to use a brand that you trust, as some overseas knockoffs are just too cheap.


VDO RediSensor

 Freescale TPMS