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Old 01-10-2003, 03:58 AM   #1 (permalink)
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What does this valve do, and what would be signs of it going bad?

I don't know if this related, but most of the time when I mash the pedal, my C4 doesn’t down shift when in 2nd or 3rd gear. I don't think it's related; so if you could comment on both topics I would appreciate it.

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Old 01-10-2003, 05:27 AM   #2 (permalink)
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The modulator valve is connected to an intake manifold vaccum source, and is used to control the throttle valve in the transmission. The strenght of the vaccum tells the transmission how fast the engine is spinning, and the transmission uses this information in conjunction with the speed of the governer valve to determine when to shift the transmission. If it were going bad, your transmission would not shift well on up shifts OR down shifts.

Massing the pedal to the floor and having it down shift to 2nd is usually a function of the kickdown lever. At slower speeds, the throttle valve (remember ... controlled by the vaccum modulator) will kick it down at lower RPM's but will not kick it down at higher ones as it thinks your going fast enough to be in 3rd, so sees no need to make a shift. The kickdown arm is mounted on the side of the transmission where the shift linkage attaches (and where the neutral start switch is), and is a mechanically controlled device. In other words, you must make sure that your kick down linkage is all in tact, and adjusted correctly. When it is, stomping the pedal to the floor will pull on the kickdown arm, which in turn opens a valve in the valve body and causes the downshift.

HTH.

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Old 01-10-2003, 11:30 AM   #3 (permalink)
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Pull the vacuum line that attaches the modulator to the intake and look for tranny fluid.
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Old 01-10-2003, 01:44 PM   #4 (permalink)
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What does this valve do, and what would be signs of it going bad?

Controls throttle pressure, which interacts with governor pressure to control shift point and firmness in relation to engine load and throttle position. When it goes bad, shifts get late and harsh.

when I mash the pedal, my C4 doesn’t down shift when in 2nd or 3rd gear.

Johnpro covered downshift linkage details; in the interim, you can do like I've always done (I don't use downshift linkages)....downshift manually when in a passing or heavy acceleration situation.

Good luck!

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Old 01-10-2003, 02:56 PM   #5 (permalink)
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Hmm, Shifts are harsh at times even with out much throttle. They could be late to. I seem to notice it when the engine/trans is cold, but seems to be fine when it warms up.

I wish there was another way to tell.
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Old 01-10-2003, 04:44 PM   #6 (permalink)
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Disconnect vacuum line at modulator, plug line, and re-test, comparing to previous observations. Post results.
Also, note any ATF leakage from hose nipple on modulator...

Get back to us...
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Old 01-10-2003, 04:44 PM   #7 (permalink)
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For a start, make sure the Type F fluid level isn't low.
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Old 01-10-2003, 06:46 PM   #8 (permalink)
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I won’t be under the car again till tomorrow at the soonest, but your saying to plug the vacuum line from the intake, drive the car, and compare?

Last time I was under there (last weekend), I noticed there was fluid around the modulator. There's fluid all over the place though; the pan gasket is cracked. I’m Going to replace it this weekend.
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Old 01-10-2003, 08:45 PM   #9 (permalink)
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Remove rubber hose at modulator; plug hose (bolt works); test as indicated in my prior post.

Post back the results of your driving and visual inspections....

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