Update and discovery on DME relay getting hot -another Warning!
07-04-2003, 08:56 PM
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Update and discovery on DME relay getting hot -another Warning!
I made some current measurements of the DME relays.
The 951 and 944 are running 5.6 amps and still run hot.
( Touching the top relay with relay housing cover removed)
The C4S fuel pump runs at 5.6 amps. ( Fuel pump only by
activating DME relay switch manually with ignition off)
However, under normal operating conditions, the relay runs at
10.2 amps during start up, which after a delay of a few minutes the
current increased to 13.2 amps!
However, here's the big difference: The C4S DME relay also powers
four O2 sensor heater coils. These coils measure 2.7 Ohms cold each,
and 6-8 Ohms semi hot. ( Typically various O2 sensors are 5-7 Ohms cold)
(The 993 schematic indicates 10 Ohms with no temperature specified.)
I measured with an oscilloscope the control voltage on the O2 sensor
heater coils. The DME uses pulse width modulation of about 30%
duty cycle to preheat the O2 heater coils, of which after a few minutes
it turns them on to full time heating operation.
This indicates there are peak current pulses of 17.8 amps for the
heater coils at cold start up, plus 5.6 amps for the fuel pump, this
totals 23.4 amps of current spikes with an RMS current of 10.2 amps
cold, and 13.2 amps hot -not good for a 15 amp rated relay that already
appears to be only 7-10 amp rated! (As many US made relays of this
similar size are rated in this smaller range.)
My speculation is that the original O2 sensor heater coils measured
10 ohms cold during the original design of the 993 series.
The Porsche engineers said O.K, and designed around it with this
known value. However, since Porsche did not manufacture this part,
the manufacturer changed or re-produced in error some low resistance
O2 heater elements. (Probably due to a supplier sending a batch of
resistance wire that was too low in resistance)
This type of problem is very common in any industry; and I have
encountered many similar problems in my years of engineering and
trouble shooting.
The purpose of the O2 sensor heater coils is to reduce the time for
proper O2 sensor operation, to improve the sensor operation in
a wide range of temperatures, and to aid in keeping them clean from
carbon contamination. I do not suggest that we disconnect the heaters
as this could result in fault codes, improper air/ fuel mix, and
contaminated sensors. This does suggest that all 993 series
equipped with four O2 sensors have greater current loads on the
DME relay. I don't think Porsche is responsible for a relay, of which
in IMO, is not capable of reliable 15 amps. Nor do I think they're
responsible for low O2 sensor heater resistance values.
They attempted to address the problem with an upgraded relay,
although it still falls short of curing the problem.
( Even the 944 series are having failures with just 5.6 amps!)
I encourage everyone to make measurements, I
would like to know if anyone gets different results...
For the measurements, I used a Hewlett Packard 3458A multimeter
( The most accurate meter) for the current measurements and a
Tektronix TDS 744A for the pulse width measurements.
All are N.I.S.T traceable and are used in a calibration cert laboratory. (Although, this isn't required to get useful measurements)
I will continue to search for a simple and permanent fix to this relay
problem...
The 951 and 944 are running 5.6 amps and still run hot.
( Touching the top relay with relay housing cover removed)
The C4S fuel pump runs at 5.6 amps. ( Fuel pump only by
activating DME relay switch manually with ignition off)
However, under normal operating conditions, the relay runs at
10.2 amps during start up, which after a delay of a few minutes the
current increased to 13.2 amps!
However, here's the big difference: The C4S DME relay also powers
four O2 sensor heater coils. These coils measure 2.7 Ohms cold each,
and 6-8 Ohms semi hot. ( Typically various O2 sensors are 5-7 Ohms cold)
(The 993 schematic indicates 10 Ohms with no temperature specified.)
I measured with an oscilloscope the control voltage on the O2 sensor
heater coils. The DME uses pulse width modulation of about 30%
duty cycle to preheat the O2 heater coils, of which after a few minutes
it turns them on to full time heating operation.
This indicates there are peak current pulses of 17.8 amps for the
heater coils at cold start up, plus 5.6 amps for the fuel pump, this
totals 23.4 amps of current spikes with an RMS current of 10.2 amps
cold, and 13.2 amps hot -not good for a 15 amp rated relay that already
appears to be only 7-10 amp rated! (As many US made relays of this
similar size are rated in this smaller range.)
My speculation is that the original O2 sensor heater coils measured
10 ohms cold during the original design of the 993 series.
The Porsche engineers said O.K, and designed around it with this
known value. However, since Porsche did not manufacture this part,
the manufacturer changed or re-produced in error some low resistance
O2 heater elements. (Probably due to a supplier sending a batch of
resistance wire that was too low in resistance)
This type of problem is very common in any industry; and I have
encountered many similar problems in my years of engineering and
trouble shooting.
The purpose of the O2 sensor heater coils is to reduce the time for
proper O2 sensor operation, to improve the sensor operation in
a wide range of temperatures, and to aid in keeping them clean from
carbon contamination. I do not suggest that we disconnect the heaters
as this could result in fault codes, improper air/ fuel mix, and
contaminated sensors. This does suggest that all 993 series
equipped with four O2 sensors have greater current loads on the
DME relay. I don't think Porsche is responsible for a relay, of which
in IMO, is not capable of reliable 15 amps. Nor do I think they're
responsible for low O2 sensor heater resistance values.
They attempted to address the problem with an upgraded relay,
although it still falls short of curing the problem.
( Even the 944 series are having failures with just 5.6 amps!)
I encourage everyone to make measurements, I
would like to know if anyone gets different results...
For the measurements, I used a Hewlett Packard 3458A multimeter
( The most accurate meter) for the current measurements and a
Tektronix TDS 744A for the pulse width measurements.
All are N.I.S.T traceable and are used in a calibration cert laboratory. (Although, this isn't required to get useful measurements)
I will continue to search for a simple and permanent fix to this relay
problem...
07-04-2003, 10:05 PM
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A very detailed report, David! Perhaps you've discovered a marginal area in an obviously critical electrical component. I'm curious if anyone here has ever had to replace their NEW DME relay (i.e. NOT the old 944 part). Let us know what more you find, David, and thanks for the efforts! 
Edward
Edward
07-04-2003, 10:41 PM
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Great stuff David! I think you do deserve the role of Chief Electrician at the HWFM Racing team if you accept it <img border="0" title="" alt="[Wink]" src="wink.gif" />
07-05-2003, 02:15 PM
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Chastened to have shown my own naivete by only measuring the DME and fuel pump relay loads ignition on but without a running engine, I went back and duplicated some of David's procedure on my car.
The fuel pump circuit, which draws 5.6 amps with the pump running but before starting the engine, does jump up to 10.5 amps briefly after engine start. It then sinks down a bit to a smoothed average of 9.8 amps, influenced only slightly by engine rpm.
The current in the DME circuit, quite unlike what happens in David's '97 C4S, never went above 1.6 amps with a cold engine start. It then fell to about 1 amp after 5 minutes of engine idling. It does not appear that the DME circuit is the source of the O2 sensor heater current in my 1996 C2, or the O2 sensor heaters in my car draw far less current than those in David's car.
These measurements were made with a Fluke Model 88 Automotive multimeter.
Chip
The fuel pump circuit, which draws 5.6 amps with the pump running but before starting the engine, does jump up to 10.5 amps briefly after engine start. It then sinks down a bit to a smoothed average of 9.8 amps, influenced only slightly by engine rpm.
The current in the DME circuit, quite unlike what happens in David's '97 C4S, never went above 1.6 amps with a cold engine start. It then fell to about 1 amp after 5 minutes of engine idling. It does not appear that the DME circuit is the source of the O2 sensor heater current in my 1996 C2, or the O2 sensor heaters in my car draw far less current than those in David's car.
These measurements were made with a Fluke Model 88 Automotive multimeter.
Chip
07-05-2003, 03:52 PM
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</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"> The fuel pump circuit, which draws 5.6 amps with the pump running but before starting the engine, does jump up to 10.5 amps briefly after engine start. It then sinks down a bit to a smoothed average of 9.8 amps, influenced only slightly by engine rpm.
</font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Chip,
Yes, the slight difference with your measurements
of 10.5/9.8 amps instead of 10.2/13.3 amps is likely the
result of your four O2 sensor heater elements having a
higher resistance. My O2 heaters measured 2.7 Ohms cold.
I'll speculate that yours will measure within 4-5 Ohms.
This O2 sensor variance is likely the cause for increased
DME relay failure. Also the variance in the number of
O2 sensors, with heater elements, used in the particular
model Porsche. (The relay section powering 1.6 amps to
the computer, which I also measured, is inconsequential.)
The O2 sensor heater elements can be measured by
unplugging them at the connectors. The O2 connectors
are located near the rear corner panels. There's four Teflon
wires. Measure only across the two white pairs of wire.
The lower the resistance, the greater the load on the relay.
Again, I want to put emphasis on the fact that this relay
is not worthy of doing the job -not even for the 5.6 amps on
the 944 series. IMO, it's way overrated as a 15 amp relay;
and if it's having to deliver 9.8 -13.3 continuous current with
up to 23.4 amps of pulsed current to vital components, it still
falls short of engineering standards.
</font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Chip,
Yes, the slight difference with your measurements
of 10.5/9.8 amps instead of 10.2/13.3 amps is likely the
result of your four O2 sensor heater elements having a
higher resistance. My O2 heaters measured 2.7 Ohms cold.
I'll speculate that yours will measure within 4-5 Ohms.
This O2 sensor variance is likely the cause for increased
DME relay failure. Also the variance in the number of
O2 sensors, with heater elements, used in the particular
model Porsche. (The relay section powering 1.6 amps to
the computer, which I also measured, is inconsequential.)
The O2 sensor heater elements can be measured by
unplugging them at the connectors. The O2 connectors
are located near the rear corner panels. There's four Teflon
wires. Measure only across the two white pairs of wire.
The lower the resistance, the greater the load on the relay.
Again, I want to put emphasis on the fact that this relay
is not worthy of doing the job -not even for the 5.6 amps on
the 944 series. IMO, it's way overrated as a 15 amp relay;
and if it's having to deliver 9.8 -13.3 continuous current with
up to 23.4 amps of pulsed current to vital components, it still
falls short of engineering standards.
