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Recurrent TPS pot drop-out?

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

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

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

Reply to
Harvey White

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

Reply to
Walter Harley

snip

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

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

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.

Reply to
Lance Morgan

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
Reply to
Lance Morgan

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

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.

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

Reply to
Harvey White

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

Reply to
mroberds

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.

Reply to
Lance Morgan

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

Reply to
Pat Ford

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