| squaw_ski_bum |
Has anyone seen this problem before?
Last weekend while visiting the snow in Tahoe, I noticed that the VTM-4 Lock wasn't working.
The VTM-4 itself was working fine, as I proved by taking the unplowed route over Donner Summit with no problems, avoiding the backup on I-80. But when I shifted into 1-2 and pushed the VTM-4 Lock button, the light didn't come on and it was apparent that the VTM-4 wasn't locking.
I checked it out last night, and found that the lock light would come on in R, but not 1-2. Then I noticed that the D2 light on the dash wouldn't come on. That suggested the shift position switch on the transmission was acting up. I checked for any codes using my OBD-II reader, and got P1607 from the PCM (power train control module which controls the shifting.) The manual says this can occur if the car has had to be jump started. (yes, the OEM battery died last summer.) The manual says reset it, and if it doesn't come back then you're ok. That worked for me.
I went for a test drive on a deserted stretch of road, trying shifting through all the gears. I noticed that I didn't get a downshift when going from 3 to 1-2. This was added evidence for the shift position switch theory. After playing around a bit, the D2 light started coming on again, I was getting downshifts as expected, and VTM-4 lock was working in 1-2.
For now, I'm inclined to believe that there was some oxidization on the contacts inside the shift position switch, and the shifting back and forth got it scraped off. I had a very similar problem on my Volvo 850 a while back, apparently it is well known on that car and the cure is the same - with the car stopped, shift back and forth through the gears a bunch of times.
Has anyone seen this, and any thoughts on whether my theory is correct? |
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| SuperTech |
| If the "2" light didn't light up, then you either had a bulb burned out in the cluster, or the gear position switch had an open contact. If it was the latter then mechanically...the transmission would have been forced in to 2, but the computer wouldn't have known. And since the computer has to see that the selector is in 1, 2, or R in order for the VTM to lock, that's why it wouldn't engage. |
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| squaw_ski_bum |
A burned out bulb for D2 in the dash display was one of the possibilities I considered, but very low probability since D2 gets used only very rarely.
The open contact in the transmission position switch is the likely culprit, since the computer controls the downshifts and until the contacts started working again, manually shifting to
D2 from D3 didn't cause a downshift. It also explains why VTM-4 wouldn't lock, since the computer didn't see that the car was in D2.
I am curious to know if junk and/or oxidation of the switch contacts happens often enough to get noticed, as was the case with my other car. |
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| SuperTech |
| So it did not lock and there was nothing illuminated for manual selecting gear 2...but what about 1? |
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| squaw_ski_bum |
Moving the shift lever over to 1 caused the D1 light on the dash to come on - but no VTM-4 lock.
Looking at the schematic, the transmission position switch (located on the transmission) contact is shared for 1 and 2. There is a separate switch for D1 located next to the shift lever that is activated by moving the shift lever to the 1 position.
So I think the PCM uses the position 1-2 contact from the TPS for the VTM-4 lock, and the D1 switch to prevent upshifts from 1st to 2nd. |
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| SuperTech |
| Ah that's right. Moving it from 2 to 1 is just a horizontal move. It doesn't actually move the shift cable attached to the trans. Yup, I'd be willing to say that you have a bad position switch. |
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