1988 and 1990 AOD transmissions what is the difference?

[cgrocen]

Hi everyone,

I have a 1988 Grand Marquis LS with 64,000 miles on it. Original Arizona car with no rust. But, the overdrive is shot in the AOD transmission. I am pretty sure what did it in. The car had a class III trailer hitch on it and air bags inside the rear coil springs, so the car must have done some heavy duty towing in the past, and the little rubber grommet for the TV cable where it attaches to the throttle body was missing and the cable was hanging loose. So those two things probably burnt up the overdrive. Anyhow, I can get my hands on a decent used 1990 AOD transmission out of a Crown Vic. On Car-Part.com, the interchange for the transmission for my car is only 1988-89 Grand Marquis and Crown Vic. Does anyone know what the difference is for 1990? The seller says it should fit, but I want to be certain before I lay out the cash.

Thank you in advance,

Chris

[gadget73]

The shift lever on the side is probably different. I believe the 1990 used a cable shifter vs the rod linkage, and those got a different shift lever on the trans. Not a big problem to change it, but you will need to remove the trans pan. Otherwise the transmissions should be identical.

[sly]

the 90 is a beefier case too (changed in 89 IIRC)

[VicCrownVic]

I swapped the trans from my 89 MGM into my 91 MGM when I bought the 91 back in 2012 (October).

Yes, the 1990 should use a cable like 1991 instead of the rod linkage like 89 and down.

The shift linkage is different between the 89 and down cars and the 90 and up cars.
Also the speedometer drive gear that is machined into the trans output shaft has 8 teeth on 89 and down trans and 7 on the 90 and up trans. (There's a chance I could have the 8 and 7 backward, but I think I got it right.) This is not a problem but your speedometer will likely be off unless the two cars have the same rear gear ratio (in which case each VSS would have different driven gears that must be swapped) and/or you can try to find a driven gear for the VSS that will work. The VSS gear is super easy to swap.

The biggest issue is the shift linkage, if you have a chance to compare the two side by side you'll see that one arm is in park further back than the other. On my 91 with the 89 trans once I got the car out of Park I could not get it back into park. I had OD,D,1 and couldn't even get it into neutral, linkage setup would not allow things to physically move through the whole gear range. I didn't try the 91 trans in the 89 MGM before swapping things but I would have to imagine that you would only be able to get the car into P,R,N and maybe OD but probably not D or 1.

You will have to drop the pan on the trans to swap the linkages, start with the bum trans and take a look at this thread that was posted some time ago:
http://www.grandmarq.net/vb/showthre...=shift+linkage
There's some good info on swapping the linkage in that thread.

[cgrocen]

Looks like if the donor car has the same gear ratio diff and I can get the speedo cable gear (or whole speedo cable) and I change the shift linkage arm, I should be good to go. I was going to change the front and rear seals, as well as the shift shaft seals, so I would have to take out the shifter arm anyhow.

I will check out that thread from above.

Thanks for the information guys!

Chris

[VicCrownVic]

For the speedometer on 89 and down panthers the speedometer uses a mechanical speedometer cable but on 90 and up the speedometer is electronic. The VSS is interchangeable but the port on the VSS where a mechanical speedometer cable would plug in is blocked with a removable plug on 90 and up panthers.

Now that I think about it a little more it really doesn't matter what rear gear ratio the 90 has unless it is the same as your 88 in which case the gear from the 90 should be correct. No matter what you will need a driven VSS gear that has more teeth than your current gear. With a 90 trans in your 88 your speedometer will read faster than your actual speed without the new gear.

What is the rear gear ratio on your 88?

[cgrocen]

I am not sure what the ratio is. My axle code on the door jamb sticker is "5". Can you tell me what ratio that is?

[VicCrownVic]

That should be 3.27 with an open differential as long as nobody has changed the rear gears in the car.
If it does in fact have the 3.27 the gear currently on your VSS would have at least 18 teeth (17 teeth is correct for 3.08 on 89 and down). Someone here might know for sure what VSS gear is correct for a stock 88 box with 3.27, if it is an 18 tooth gear you will need either a 19 or 20 tooth gear, unfortunately I don't actually know for sure I only know that you will need a gear with more teeth. If the 90 also has 3.27 then you should be all set if you get the gear from the 90.

If you keep your stock VSS gear you will probably only be off by about 5MPH on average (more at higher speeds and less at lower speeds).

GoodSamaritan has a chart with box panther gear ratio codes in his signature:
Axle codes
Open/Lock/Ratio #
-----------------------
G / H / 2.26
B / C / 2.47
8 / M / 2.73
7 / - / 3.07
Y / Z / 3.08
4 / D / 3.42
F / R / 3.45
5 / E / 3.27
6 / W / 3.73
2 / K / 3.55
A / - / 3.63
J / - / 3.85

[cgrocen]

Here is a chart I found on my master parts CD. It is for 1987. It shows you could get an AOD with either 7 or 8 tooth output shaft, with 215/70R15 tires, and 3.27:1 gears. Not specified which cars those came in. I checked the driven gears, and they were either 18 or 20 tooth. I added the text in red to the chart. So, it looks like all I would need is a 20 tooth driven gear. Click on the image to see.



Thanks!

Chris

[miamibob]

Here is a chart I found on my master parts CD. It is for 1987. It shows you could get an AOD with either 7 or 8 tooth output shaft, with 215/70R15 tires, and 3.27:1 gears. Not specified which cars those came in. I checked the driven gears, and they were either 18 or 20 tooth. I added the text in red to the chart. So, it looks like all I would need is a 20 tooth driven gear. Click on the image to see.

[ATTACH=CONFIG]42226[/ATTACH]

Thanks!

Chris

Great info for those that need it - GOOD JOB!!

[VicCrownVic]

Well with that information I very well may be wrong about the output shaft drive gear. Weather it has 7 or 8 teeth may not depend on model year (but I read it on the internet... it must be true).
It may be more dependent on what axle ratio the car has. My 91 has 2.73 and if you look on your CD I would imagine that if 2.73 is an option for that year that it would show only one option for the output shaft drive gear.

Now that you know that 18 teeth is correct for the 7 tooth drive gear and 20 for the 8 tooth drive gear all you need to know is how many teeth the output shaft drive gear has. The only way I know of finding that out would be to remove the tail shaft housing and count teeth, there might be a better way though.

[cgrocen]

My cd is only for 1980-1989. For 1989, there is only an 8 tooth output shaft listed for 2.73:1 gears and 215/70R15 tires. I checked the driven gear and it is a 17 tooth gear.

I guess the number of teeth on the output shaft of the 1990 donor car may be dependent upon the rear gear ratio. Regardless, it comes down to the driven gear, either 18 or 20 tooth. After installation, things can be quickly checked by driving down the road and using a GPS that shows you your speed and compare that with what the speedo says. If it is the same, good. If not, change the driven gear.

Here is the chart for the 2.73 rear end for 1989:



Chris

[VicCrownVic]

That is what I expected to see, only one drive gear option for 2.73. It looks like I may have been backward about which of my transmissions have 7 and which has 8 though.
Looks like you have gathered the information you need for this swap as far as the possible problems go.

• Shift lever needs to be swapped
• VSS driven gear may or may not need to be swapped requiring either an 18 tooth or 20 tooth gear

[cgrocen]

Yes, I think this 1990 transmission will do the trick, with just the lever swap, and right driven gear.

Thanks for your input!

Cheers,

Chris

[VicCrownVic]

Oh, one other thing, 1991 has an aluminum drive shaft and 1990 may or may not. If it does and you can grab it, the aluminum drive shaft would be a nice upgrade from the steel drive shaft your 88 has, if you want it.