NOT Normal P Lock Malfunction - ONLY Happens Over 80 Degrees. Never When Cooler

Rock_Star · Jun 21, 2024

pasadena_commut said: ↑

correct

nope.

When checking for an open circuit between pins A and C the resistance between A and C are measured. Because the resistance from one point on a wire to another point on the same wire is typically very low, the spec calls for a value of less than 1 Ohm. Some specialized ohmmeters may give an underflow error on a value which is too close to zero, but the odds are that isn't what you are using. They usually just round 1 microOhm to 0.000 (or however many significant digits are available). 0L would indicate high resistance in this scenario too. If you suspect that somehow your ohmmeter gives 0L on a very low resistance circuit do a positive control - just touch the two leads directly to each other. You know that the resistance in that circuit is tiny so the result should be zero. If the meter keeps giving 0L when it shouldn't in resistance mode suspect a broken wire in one or both of the probes. Cheap probes often break like that. Also, there is a battery in the meter, be sure it is still good. Like a Prius, a multimeter can do odd things when the battery goes low. Although, thankfully, most put up some sort of low battery warning.

Summary that might be useful (cable unplugged at all ends, test only at pins A,C and at a ground):

A-C (no shorts, no opens) A or C to ground is very high resistance, A to C is very low resistance
A-B-C (short at B to ground, no opens) A or C to ground is low resistance, A to C is very low resistance
A-B1, B2-C (no shorts, open at B1/B2) A or C to ground is very high resistance, A to C is very high resistance
A-B1, B2-C ( B1 and B2 shorted to ground, open at B1/ B2) A or C to ground is low resistance, A to C resistance is at least
the sum of the A to ground and C to ground resistances

very high resistance will often display as 0L, it might also show as something like 1.999 10^9
very low resistance will usually display as 0.000 (the number of digits varies between devices)
any other value will be some number larger than zero
on auto-ranging meters also have look for a "k" or "M" or some other indicator of the multiplier to see if it is 1,1000, of 1000000 Ohms
on non auto-ranging meters set the appropriate range for the value being measured. (That, is don't try
to measure 50 Ohms in the 200k range, it will usually round down to zero even if it is really 50 Ohms.)

Luckily <1 Ohm is usually good enough for dealing with car wiring problems, and we don't care if it is 0.1 Ohms or 0.001 Ohms. Because accurately measuring very low resistances is very difficult. See for instance:

Accurate Low-Resistance Measurements Start with Identifying Sources of Error | Tektronix
Click to expand...

Thanks for the explanation. That all makes sense. So it seems that by my readings there's an open circuit in PCON and PPOS. Still have no idea what the fix is, unfortunately.

I actually found a Prius specialist about an hour from me. They know the P con error, but have never heard if it being caused by heat like mine does. I'm taking it there next Wednesday. They'll do a diagnostic for $180 and we'll go from there. They have a bunch of Gen2 parts and ECUs in stock for $50, so if it's one of those they should be able to swap it out for me. Hopefully!

Rock_Star · Aug 19, 2024 at 4:47 PM

SOLVED!!!!

I took it to a local Prius specialist and they eventually found a bad wire to the transmission resolver plug. They said it was very difficult to find, that they'd OHM'd every wire multiple times, and it took them a couple of weeks of testing to determine that was the issue. Once they fixed that, the P lock error went away.

But apparently, they fried the parking brake control module in the testing/fixing process, so when they turned the car on it would short out and fry an ECU. It took them a couple of weeks to figure out that was the issue, but once they replaced the parking brake control module then the car was all good.

I highly recommend autobeyours.com / Frontier Automotive in Scottsbug, Indiana. They were very fair with pricing, considering how much time they put into it. They're all great and friendly and they are an honest shop. They specialize in Prius, but work on all makes and models too.

pasadena_commut · Aug 19, 2024 at 7:47 PM

Glad you finally got it fixed.

I shudder to think what the shops near me would charge for weeks of testing.

ChapmanF · Aug 19, 2024 at 8:39 PM

Those are good folks. But are you positive they said "parking brake control module"? Pretty sure in a gen 2 that's not a thing.

Tombukt2 · Aug 19, 2024 at 9:11 PM

I've heard about this resolver issue before somewhere probably right here That's why for this PLock malfunction it is pretty much mandatory to have the manual or you've done this hunt before and you know what's involved because I'm sure that's like a 34 page to do for this p lock business It must be one of the longer reeds and testings.

NOT Normal P Lock Malfunction - ONLY Happens Over 80 Degrees. Never When Cooler

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NOT Normal P Lock Malfunction - ONLY Happens Over 80 Degrees. Never When Cooler

Rock_Star Junior Member

Rock_Star Junior Member

pasadena_commut Senior Member

ChapmanF Senior Member

Tombukt2 Senior Member