94GrandPrix Posted January 19, 2015 Report Posted January 19, 2015 Hello forum. So my '94 Grand Prix has recently developed a problem that has really got me stumped. What happens is when I need to accelerate hard enough for the transmission to downshift into passing gear, it will have a rather (too?) firm down shift and a very harsh grinding, almost banging sound, that seems to be coming from the transmission. Sometimes, it seems that the car will slam into lockup, or it will otherwise be a rather harsh shift into lockup. If I baby the shifts, it acts just fine. No slippage or anything like that. If I need to accelerate a little harder onto the freeway, the transmission will occasionally produce the same grinding bang sound between the 3rd to 4th shifts, but it is not as harsh as when it needs to downshift into passing gear. If the transmission isn't locked up and I need to climb a hill, the same sound will happen if I accelerate a little harder. WTH? I don't understand this issue. Maybe torque converter related? Is this what they refer to as "Torque Converter Shudder"? Is my transmission on it's way out? BTW, fluid is dark, but not black or burnt. Just added a bottle of ZMAX trans lube hopping that would help, and it did seem to lessen the harshness of this issue, but it does still happen. 1994 Pontiac Grand Prix w/ 3100 v6 and 4T60E automatic trans. Quote
94GrandPrix Posted January 19, 2015 Author Report Posted January 19, 2015 Forgot to mention I have replaced the vacuum modulator with the adjustable one and I am running it with 4 turns on the adjustment screw. It otherwise acted just fine with this adjustment setting. Quote
l67ss Posted January 19, 2015 Report Posted January 19, 2015 Forgot to mention I have replaced the vacuum modulator with the adjustable one and I am running it with 4 turns on the adjustment screw. It otherwise acted just fine with this adjustment setting. Just for diagnostic purposes try capping off the vac mod and see how it does Quote
94GrandPrix Posted January 19, 2015 Author Report Posted January 19, 2015 (edited) Just for diagnostic purposes try capping off the vac mod and see how it does Can you define capping off? It used to have a very harsh shift from 1-2, and the modulator was blown inside. Replaced it and that fixed the problem, and like I said, it was just fine with the 4 turns. Edited January 19, 2015 by 94GrandPrix Quote
l67ss Posted January 19, 2015 Report Posted January 19, 2015 Can you define capping off? It used to have a very harsh shift from 1-2, and the modulator was blown inside. Replaced it and that fixed the problem, and like I said, it was just fine with the 4 turns. Plug the port on the intake. My old 4t60 had same issue....was a vac mod issue Quote
94GrandPrix Posted January 19, 2015 Author Report Posted January 19, 2015 Could I be running too high of an adjustment and the trans just can't take it? Would it be better to run a littler higher or a little lower adjustment? Quote
l67ss Posted January 19, 2015 Report Posted January 19, 2015 Could I be running too high of an adjustment and the trans just can't take it? Would it be better to run a littler higher or a little lower adjustment? Unsure on that one.....I 4t65 swapped b4 I got around to swapping vac mods lol Quote
94GrandPrix Posted January 19, 2015 Author Report Posted January 19, 2015 Well i'm a little reluctant to plug the port on the intake manifold.. i could be wrong but wouldn't that jack up the fluid pressure a little TOO high? Like.. teeth shattering shifts level? lol The grinding bang sound only happens in passing gear, when i'm climbing a hill when the trans in not in lockup and i'm on the gas a bit, or after the 3rd and 4th shifts if i'm really on the gas. Transmissions are a bit beyond me lol. Quote
l67ss Posted January 19, 2015 Report Posted January 19, 2015 Well i'm a little reluctant to plug the port on the intake manifold.. i could be wrong but wouldn't that jack up the fluid pressure a little TOO high? Like.. teeth shattering shifts level? lol The grinding bang sound only happens in passing gear, when i'm climbing a hill when the trans in not in lockup and i'm on the gas a bit, or after the 3rd and 4th shifts if i'm really on the gas. Transmissions are a bit beyond me lol. Im just saying do it a lil while to see how it does. Mine shifted better Quote
Nas Escobar Posted January 19, 2015 Report Posted January 19, 2015 Well i'm a little reluctant to plug the port on the intake manifold.. i could be wrong but wouldn't that jack up the fluid pressure a little TOO high? Like.. teeth shattering shifts level? lol The grinding bang sound only happens in passing gear, when i'm climbing a hill when the trans in not in lockup and i'm on the gas a bit, or after the 3rd and 4th shifts if i'm really on the gas. Transmissions are a bit beyond me lol. Wait, let me get this straight... You don't have the vacuum line that's goes to the vacuum modulator hooked up and you took out the stock vacuum modulator for one that can be adjusted? That's where your problem lays. You NEED to hook up that vacuum otherwise it shifts too hard. I accidentally disconnected mine when I did the LIMG the first time and I had that same issue. My fluid went dark and that's why. All those hard shifts are burning up the transmission. You need to hook that line back up and reset the modulator. Quote
94GrandPrix Posted January 19, 2015 Author Report Posted January 19, 2015 Everything is hooked up correctly, the vac line and everything. It recently starting doing this. I replaced the vac mod about 2k miles ago. There is no harsh shifting other than occasionally from lockup down to passing gear. The shifts are just fine. Just for this odd grinding bang sound. Quote
RobertISaar Posted January 19, 2015 Report Posted January 19, 2015 in terms of vacuum modulator adjustment, you need to run at least as much line pressure as necessary to keep clutch/band slippage to minimal amounts, otherwise you're going to find all of the friction material stuck in the filter or at the bottom of the pan REALLY quickly. however, pressure beyond that is going to be limited by preference or shock-induced failure. Quote
94GrandPrix Posted January 19, 2015 Author Report Posted January 19, 2015 in terms of vacuum modulator adjustment, you need to run at least as much line pressure as necessary to keep clutch/band slippage to minimal amounts, otherwise you're going to find all of the friction material stuck in the filter or at the bottom of the pan REALLY quickly. however, pressure beyond that is going to be limited by preference or shock-induced failure. Would you say 4 turns on the adjuster screw is too much or not enough? Could too little or too much adjustment be causing my grinding bang sound? Quote
RobertISaar Posted January 20, 2015 Report Posted January 20, 2015 depends on the modulator. some, that might be enough, hard to tell. if it were me.... I would use a known-good modulator for reference and set the adjustable to that and firm it up from there. or if you had access to the pressure gauges necessary, you could tap into mainline pressure and see what exactly you're getting vs what you're supposed to be at. I could find the reference pressures if necessary. it does sound like something is engaging too harshly during gear changes, the fact that they seem to be entirely based on throttle position would lead me to adjust the modulator a bit downwards to see if that changes anything. otherwise, you may have a broken accumulator spring(or the accumulator otherwise being problematic)? you only mention it happening between 3rd and 4th gears if I'm reading correctly. Quote
94GrandPrix Posted January 21, 2015 Author Report Posted January 21, 2015 I actually went and had it diagnosed today. It's a bad chain in the transmission. So looks like the Grand Prix will be going under the knife for a transmission repair. Quote
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