Recurrent TPS pot drop-out?

[Lance Morgan]

While troubleshooting an older Saab 900 w/bucking probs, one potential
fault (of a few) discovered is that the Throttle Position Sensor
potentiometer has a drop-out. This is a basic 3-lead pot: 5V V-ref,
variable output signal, and grnd

With my scope on it's slowest time/div setting w/o flicker, 1 ms, I
simultaneously lose both the Vref and variable pot output signal (input
to Electronic Control Unit) - so with a grid count, a minimum of 10 ms -
w/the pot at 1/3 open, which strongly corresponds to the car's harshest
bucking. The DMM just picks up some slight V and resistance wandering at
the pot's problem point. Voltage and resistance, on either side of the
problem point, seem okay.

What's seems peculiar is that this is the 2nd pot failure (?), near or
at exactly the same opening position as the recent previous pot

I've V-dropped all relevant circuits with both a DMM and scope - all
appears okay. Alternator output is steady with nominal AC ripple. The
car has never been jump-started with the recent/new/failed 2d pot

Is there anything else I should be looking at?

Thank you,
Lance

 

[Harvey White]

While troubleshooting an older Saab 900 w/bucking probs, one potential
fault (of a few) discovered is that the Throttle Position Sensor
potentiometer has a drop-out. This is a basic 3-lead pot: 5V V-ref,
variable output signal, and grnd

With my scope on it's slowest time/div setting w/o flicker, 1 ms, I
simultaneously lose both the Vref and variable pot output signal (input
to Electronic Control Unit) - so with a grid count, a minimum of 10 ms -
w/the pot at 1/3 open, which strongly corresponds to the car's harshest
bucking. The DMM just picks up some slight V and resistance wandering at
the pot's problem point. Voltage and resistance, on either side of the
problem point, seem okay.

What's seems peculiar is that this is the 2nd pot failure (?), near or
at exactly the same opening position as the recent previous pot

Sounds more like mechanical wear to me. I suspect that it might be a
model year problem, but it's hard to say. If you can replace it with
a conventional part, I'd suggest one of the best Allen-Bradley pots,
which appear in old military equipment, and are generally rather high
quality. No telling what Saab has stuffed in there, though.

I've V-dropped all relevant circuits with both a DMM and scope - all
appears okay. Alternator output is steady with nominal AC ripple. The
car has never been jump-started with the recent/new/failed 2d pot

I don't see how a jump start would affect the pot, but I can see how
it might affect the alternator if done improperly.

Is there anything else I should be looking at?

Not a Saab expert (although I do own one...).. but I'd look at
replacing the pot first, if possible, then the assembly...

You might want to ask on the Saab groups to see if this is a high
failure item on a 900.

Harvey

 

[Walter Harley]

While troubleshooting an older Saab 900 w/bucking probs, one potential
fault (of a few) discovered is that the Throttle Position Sensor
potentiometer has a drop-out. This is a basic 3-lead pot: 5V V-ref,
variable output signal, and grnd

What impedance is that wiper seeing? DC current flow through the wiper can
cause damage over time.

 

[Lance Morgan]

Sounds more like mechanical wear to me. I suspect that it might be a
model year problem, but it's hard to say. If you can replace it with
a conventional part, I'd suggest one of the best Allen-Bradley pots,
which appear in old military equipment, and are generally rather high
quality. No telling what Saab has stuffed in there, though.

The 2d pot is a new OE Bosch module, identical to the first

I don't see how a jump start would affect the pot, but I can see how
it might affect the alternator if done improperly.

It seems extremely unlikely - was just asking - as the TPS, ECU (altho
presumably protected to some degree), jump-starting, etc., can all be
connected

Not a Saab expert (although I do own one...).. but I'd look at
replacing the pot first, if possible, then the assembly...

You might want to ask on the Saab groups to see if this is a high
failure item on a 900.

Harvey

No TSBs or anything on the boards. I'll try a 3rd unit, to see if this
was just a weird coincidence. If it fails again, perhaps I'll try to
fabricate something.

 

[Lance Morgan]

What impedance is that wiper seeing? DC current flow through the
wiper can
cause damage over time.

1100 - 3000 ohms. The 2d new pot has less than 500 miles on it. The
strange part is the failure near/at the same spot as the 1st

 

[Harvey White]

The 2d pot is a new OE Bosch module, identical to the first

Hmmm. still could be a bad model run, but hard to say.

It seems extremely unlikely - was just asking - as the TPS, ECU (altho
presumably protected to some degree), jump-starting, etc., can all be
connected

You'd have to put a lot of voltage on the arm, or at least double the
voltage on the whole pot... I don't see how that could reasonably
happen.

Given the mechanical nature of a pot, I'm still going to shoot for the
mechanical damage aspect.

No TSBs or anything on the boards. I'll try a 3rd unit, to see if this
was just a weird coincidence. If it fails again, perhaps I'll try to
fabricate something.

I've got a 9.5, most I've done so far is to start to mess with the
phone wiring and a few other electrical things... I'm not so happy
with the engine mechanics.

Harvey

 

[[email protected]]

With my scope on it's slowest time/div setting w/o flicker, 1 ms, I
simultaneously lose both the Vref and variable pot output signal (input
to Electronic Control Unit) -

Hmm...Vref is typically generated by a 5 V regulator inside the ECU.
If you lose that, you're guaranteed to lose the signal coming back
from the pot. (I wonder if maybe the pot is shorting to ground at
that point, killing the Vref?) Can you sub a pot with a similar
value, twist it with your hand (not driving the car, just sitting
still), and have a steady value for Vref? Have you tried kinetic
diagnosis (smack it) of the ECU?

You might also try asking over on rec.autos.tech.

Matt Roberds

 

[Lance Morgan]

In sci.electronics.equipment Lance Morgan


Hmm...Vref is typically generated by a 5 V regulator inside the ECU.
If you lose that, you're guaranteed to lose the signal coming back
from the pot. (I wonder if maybe the pot is shorting to ground at
that point, killing the Vref?) Can you sub a pot with a similar
value, twist it with your hand (not driving the car, just sitting
still), and have a steady value for Vref? Have you tried kinetic
diagnosis (smack it) of the ECU?

You might also try asking over on rec.autos.tech.

Matt Roberds

Yes - the 5 Vref is from the ECU. Both the variable pot output, and the
5 Vref, are dependent on the same ground, which V-drop tests good (both
scope and DMM). The ECU's ground(s) both test good. The drop-out,
AFAICT, only occurs when the pot is in one position

I generally don't like to smack ECUs ;-), but might try a fabricated
pot. I typically don't have much luck with these type Q's on r.a.t.

 

[Pat Ford]

Yes - the 5 Vref is from the ECU. Both the variable pot output, and the
5 Vref, are dependent on the same ground, which V-drop tests good (both
scope and DMM). The ECU's ground(s) both test good. The drop-out,
AFAICT, only occurs when the pot is in one position

I generally don't like to smack ECUs ;-), but might try a fabricated
pot. I typically don't have much luck with these type Q's on r.a.t.

Where in the throttle range is the problem happening? near WOT? if so look
for a loose connector by the brake m/c.
Are you checking for Vref at the ecu?? if not try that.
Pat