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1990 Merc Grand Marquis - Printable Version

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1990 Merc Grand Marquis - RenegadeRaven - 03-18-2011 11:17 AM

Completely stock 302, AOD

The problem I am having is shifting.
I have been working on cars for 20+ years and even I am stumped. Car starts perfectly fine and runs pretty good. As I am driving and picking up speed it seems that it completely skips 3rd gear and goes directly into OD. After it goes into OD, when I try to pick up speed, it shakes and jerks. The transmission in this car is brand new. Car has 110k. When I press the accelerator it does go right into passing gear, then right into OD after I ease up on the accelerator. Shifting is pretty normal otherwise, 1st - 2nd as it should. I can help the driveability by driving in just D, but it still skips 3rd gear. At around 45mph D seems to work well and there are no shaking issues. I took the car to a tranny shop (A top rated one) and the mech said that the tranny is in perfect condition. I asked about the kickdown cable and he said it was in great condition. Throttle Valve Cable is properly adjusted. I also got a 2nd and 3rd opinion, all say the same thing, which rules out the tranny problem. Even checked the shifter cable. I did give the car a tune up, which includes the following work: New plugs, wires, fuel filter, air cleaner, cap and rotor, and TPS. Cleaned the IAC and MAF. Checked fuel pressure and was right where it's supposed to be. All vaccume lines are in great shape and tested each one. Changed the coolant, oil and filter. There is no Check Engine light, and all instrument lights work. Car is in MINT condition other than this small problem. I have taken good care of this car, regular maint, and oil and filter every 3-4k. Any help would be awesome.

Could the Coolant temp sensor cause this?
Thanks in advance

RE: 1990 Merc Grand Marquis - Garner - 03-18-2011 09:41 PM

It sounds as though 3rd gear is missing as in either burnt out or a hard part broken. If you have an engine miss it will be amplified when the transmission is "Locked Up" and under a load and that would be the shaking and jerking. The engine is trying to lug in to high of a gear. A temperature sensor will sometimes keep lock-up from working, but it shouldn't affect 3rd. I know that you are hoping for an easy "out" by mentioning sensors but an 1990 AOD was pretty much mechanical. Sometimes dropping the pan and carefully inspecting what is on the magnet will help. I have also cut the filter apart and looked at what is in it for clues. Many times the fluid, pan, and filter will be clean and no burnt smell because a metal part shears clean. There aren't any parts in the pan and the clutches & fluid don't burn because it is skipping that gear.

That's my best theory. I hope it helps.
Please let us know what you find.

RE: 1990 Merc Grand Marquis - RenegadeRaven - 03-19-2011 05:14 AM

Thanks for replying Garner.

As I mentioned earlier, I had the transmission looked at by 3 different tranny shops and all said the transmission was fine. I suspected the same thing, that maybe 3rd gear was totally missing. This is the second transmission in this car. The first one started slipping a little in first gear, but after it got up a little speed it was fine. Perfect on the highway. To fix it would have cost more than to replace it. I opted for a BRAND NEW tranny from Ford since the cost was only about $100.00 more. I did drop the pan and found it to be clean with no metal shavings anywhere, even on the magnet. I also took the car to Voloskie, who has an OBD1 Diag. I figured that just because there was no "Check Engine" light, doesn't mean it didn't throw a code. But, alas, there were no codes stored. I'm wondering still if the coolant temp sensor may be giving a bad reading to the com? Or maybe the com is faulty? I do know that the coolant temp sensor is also the "Master Sensor", which means that all other sensors act based on the reading received from it. That can cause a lot of problems that most ASE mechanics oftentimes overlook, but the car doesn't overheat. Still stumped. I should also note that there is no miss at all in the engine. The Engine is healthy as an ox and runs perfect.

Again, thanks for replying. Your help is greatly appreciated, and any and all advice is acceptable. There are no wrong answers, only wrong solutions.

RE: 1990 Merc Grand Marquis - Garner - 03-19-2011 08:00 AM

I worked in a transmission shop for quite some time. We didn't do anything else, just transmissions. The "AOD" was NOT my specialty, but I did work on them. As I stated, if I remember correctly, it is a mechanical transmission. It is controlled by cables and vacuum. There is nothing external that would tell it to skip a gear. You could cut all of the wires on one that is working properly and it would still shift through the gears. The only thing that you wouldn't have is the torque converter lock up, reverse lights, and neutral safety. I would guess that the transmission shops that you checked with have younger mechanics working there and they probably don't have much experience with an AOD. They see an OBD-II, AODE or a 4R75W that is controlled by the computer and solenoids. No codes means no problem.

I think that I may be able to contact the guy that specialized in Ford transmissions at the shop where I worked. If I can I will ask his opinion and then get back to you. We also have a Ford guy here that may have some ideas. "way2old, jump in here and correct my mistakes."

Thanks for your patients. Please keep us posted if you learn anything new.

RE: 1990 Merc Grand Marquis - RenegadeRaven - 03-19-2011 05:22 PM

Thanks again for replying, and for the information.

I know this transmission is all mechanical. I've been working on cars my whole life, but I know engines. I am clueless where transmissions are concerned. The pro shop where I first took this is Gillece transmissions here in Pittsburgh. They are one of the best shops around and it's a bunch of older guys who've been doing since time immemorial. I know what you are saying about these new kids who know nothing. Kind of like going to an auto parts store with some pimply faced 17 year old who knows nothing about cars or car parts, but can use a computer to look things up. I totally understand and I hate that as well. These guys I went to are in the transmission business and have been doing it a long time. But I'm not ready to rule it out. I am also looking into what you said earlier about maybe being something of a power issue where it doesn't have enough power to hold in OD, and causes it to buck and shake. But, would it not shift into a lower gear if that's the case? I am not worried about the "Cheap Fix". I have 2 other vehicles and I make pretty good money where I work (Air Methods Advanced Aircraft Maintenance). This car is used only as a grocery getter for my wife, but I also want it to be safe for her, and I am going to give it to my son when he gets his license. I will take your previous advice and look into other areas as well. I really do appreciate all of your help on this issue. It's finally good to find a forum where there are "Real" mechanics who donate their time to those who know need it. Many thanks

RE: 1990 Merc Grand Marquis - Rupe - 03-19-2011 06:20 PM

I'm not the Ford guy but I have been around for a bit. What are the shift points under normal acceleration for the other gears and was there a time when all was well or is this a new problem since the new trany? (an installation issue?)

The reason for asking is I got to thinking about old fashion vac modulators and gov asseblies combined with the kick-down cable (rod?) adjustment. (if there is one) IOW, if something is out of whack and first / second gear are slightly delayed on the up-shift then when you reach say 40 mph the trany decides to go straight to lock up, which can only happen if it skips 3rd gear. Heck, perhaps the spring / plunger for 3rd gear in the valve body are the issue?

Again, I never followed Fords but I figured if I throw some old school logic out there it may ring a bell with someone.

BTW, if I recall correctly the only trany control on an OBD-I is for TC lock-up and that should be tied to road speed and temp, which is about the only thing you might find with a "live-drive test" on a scanner. IOW, perhaps the ECM doesn't see the correct road speed and powers up the TC lock-up too soon?

RE: 1990 Merc Grand Marquis - RenegadeRaven - 03-19-2011 08:32 PM

Thanks Rupe for your assistance.

This is a new problem. The transmission shifted fine for the 1st few years. I had it installed 3 years ago. Worked perfect. What you say makes perfect sense about other internals, like the GOV. I know it has one. I am checking all VAC lines to the tranny tonight. This Transmission has 3 cables going to it. One is from the actual shifter, One is the kickdown, and another is the throttle valve cable. A lot of people get confused when they think that a kickdown and throttle valve are the same thing. It is not. In some cases they do exactly the same thing. On the 1990 OAD Transmission, the kick down came from the accelerator pedal. The throttle valve comes from the intake manifold, and of course the shifter, and, if you don't know what the shifter does then you should not be posting on here LOL. Anyway, This setup has had many problems in the past. The throttle valve tells how much throttle is being given, and the kick down shifts the vehicle into passing gear. If only 1 of them is off then there will be shift problems. Even the gear shift selector. It may be that something is not adjusted properly, or, one of the problems you suggested, spring/plunger. What you are saying makes a lot of sense. Maybe it is going into lock-up. It does have a new TPS, which really doesn't need a computer to install. It may also be that the cables going to the tranny were adjusted while a bad TPS was present, affecting how the tranny shifts with a working TPS? These all makes sense, however with what we know so far, as with what Garner was saying, it is a mechanical transmission and only a few thing will affect it's performance. You guys are awesome. Thanks for posting. I'll keep you all posted on the things I find.

RE: 1990 Merc Grand Marquis - Rupe - 03-19-2011 08:53 PM

OK, that clarifies a few things and we know it did work at one point so something has changed and it's not an installation issue or "operator error" as some would say. If the gears are in the right place then the selector is adjusted right and if you get kick-down then that's probably ok too.

You sound like you at least have a grasp of what's going on so maybe Garner can throw out a pressure test to verify the throttle valve is working... and maybe come up with what the shift points should be so we can compare notes on the other gears.

RE: 1990 Merc Grand Marquis - way2old - 03-19-2011 08:57 PM

Has anyone checked the 3/4 shift servo? Sounds like it is going from second to OD. Does it go into lock up? The brand new trans from Ford is probably a rebuilt one. Those transmissions are also bad about the forward clutch pack failing. This clutch pack affects higher gears.

RE: 1990 Merc Grand Marquis - RenegadeRaven - 03-20-2011 03:35 PM

Thanks again.

@ way2old: I am not sure if anyone checked any servos. All that Gillese told me was that the transmission was new, and in perfect working order. I would think that if something was wrong then they would want to fix it, at least for business sake. Shops usually tell you if there is a problem so they can make money off you. Not so much to rip anyone off, but, you get the point. But 3 tranny shops? Makes no sense to me.

@ rupe: The car is just a little driver. My hot-rodding days are over, damn lol. So when I accelerate, it's usually just casual. I don't try to go 0-60 in an instant. I am going to take the car out today to see at what speeds the car shifts at under normal operation. I think the best thing to do is to post cold and hot speeds so you guys could see if something is amiss. I did check all the lines and seems as though they are in good shape. Vac lines look to be OK. If someone could post the normal shift points that might help also.

I am not sure how the clutch pack works. Is there a simple method for testing? I'm sure the 3/4 servo would need to be checked by a professional. It does go into lock-up, so maybe that would rule out the torque converter?

Thanks guys. This really is the best automotive forum on the web.