BillC3

My wife recently bought a '97 993 C4S that keeps lighting the CEL. The CEL only lights up while cruising on the highway, it won't do it while driving on "surface" streets.

I keep getting the same six error codes (as shown on my Durametric s/w). The codes are:

108 Supply Voltage
175 Unknown DTC: 175
209 Unknown DTC: 209
26 Oxygen sensing range 2 (cylinders 1-3)
34 Oxygen sensing range 2 (cylinders 4-6)
107 Supply Voltage

Can anyone tell me what those two unknown codes are, and what they signify?

Also, based on the advice of a well-respected local shop, I replaced the alternator last weekend. When they tested the car, they said the alternator voltage was too low -- they were seeing 11.5 volts at idle. Well, after the install, I'm still seeing 11.5 volts at idle, and 12.7 volts at 1400 RPM and higher (as seen on the Durametric).

Any idea what's going on? The shop said that the car should be seeing at least 14 volts, but two different alternators were putting out the same amount (which probably also means that the original alternator was okay).

Also, do I need new O2 sensors? And, are they the primary (before cat) or secondary (after cat) sensors? I watched the sensor output on the Durametric display, and both the primary and secondary sensors seemed to be cycling up-and-down pretty well.

Any help is greatly appreciated. This is driving me nuts (and bankrupt, too).

Steve Weiner-Rennsport Systems

Bill,

Was the VR replaced at the same time as the alternator?

Does the car have factory belts (fan & alternator) or aftermarket ones?

How many miles on the car?

Are the oxysensors original?

The answers to the above helps diagnose the car.

BillC3

VR means Voltage Regulator? If so, it was not replaced separately. I assumed the VR was built into the alternator, like most modern alternators seem to be. If it's a separate piece, where is it located? The alternator I installed is a genuine Bosch rebuilt-in-Germany unit.

The belts are in good shape and tensioned correctly.

The car has just over 40K miles. So, I suspect the O2 sensors are original.

Also, the car runs just fine when the CEL comes on. No difference at all in power, throttle response, etc.

Steve Weiner-Rennsport Systems

Sometimes they are included, othertimes not. Don't assume anything,....look at your paperwork and ask. Its located at the rear of the alternator.

This is a well-known issue; aftermarket belts create all kinds of problems on these OBD-II cars. Factory belts are the ONLY way to go.

They might be OK @40K so I'd check out the above items before making a commitment to buy new ones.

They always do,....

BillC3

The VR must have been included with the alternator. The new alternator exactly matched the old alternator, and I didn't swap anything between them except the fans.

The fan and alternator belts had been replaced by a previous owner with a multi-rib belt ("serpentine" type belt) with a spring-loaded tensioner. There's really no way for this thing to lose tension or to slip. And, if there was an issue with the new belt system spinning the alternator too slow, I would expect a low-voltage problem to show up at idle, not on the highway at 2500-3000 RPM.

The car did sit for a looong time before my wife bought it. I'm wondering if maybe the battery might be going bad. The battery was replaced fairly recently (within a year or so), but the car had to be jump-started a couple of times when we first got it because the battery went flat, but it's been starting fine for the last few weeks. I've tried putting a charger on it for 4-5 hours on several occasions, and the charging current never dropped below 3 amps (6-amp charger).

Kika

Is it possible that the old alternator was missing the regulator also? would seem very strange, but...

BillC3

Since it went in just like the p-car/diy article, I'm pretty sure it's there. According to the pics, it looks like the big wire attaches to the output post of the regulator.

BillC3

Here's an update:

On a hunch, I replaced the battery, and that seems to have taken care of the "Supply Voltage" errors. However, we're still getting the O2 sensor errors. So, I'm guessing this means I need to replace the O2 sensors. Anyone have any idea if it's the front sensors or the rear sensors, or do I need to just suck it up and replace all four? The local NAPA store quoted me nearly $600 for all four sensors. Anyone know of a less-expensive place to get the sensors?

TRINITONY

I've been on this forum for some time and I've never heard of all 4 sensors being replaced. Sensors are expensive - what some folks have done on MB is to buy the genric bosh sensor and use a crimp tool to connect to the original contact. Not sure if anyone has done it for a porsche though, just a thought.