rear end locks up when shifting into 4x4

[yellowstoner]

Hi everyone, this is my first post on this forum. I don't own a Tercel 4WD yet, but I am looking around for one that my daughter can drive to high school. I have liked them ever since I first saw them in the 1980's. Our current Toyota is a 1978 FJ40.

I just drove a 1985 Tercel with 180k miles on it. It's supposed to have a new clutch & rebuilt trans. The Tercel drives fine in front wheel drive, but when it's shifted into 4x4, it feels like the emergency brake was suddenly pulled, and the car lugs to a stop. Could this be something to do with the emergency brake, the rear end or even the transfer case? Could this mean the rear diff is a different gear ratio than the front?

Thanks
Dave

[splatterdog]

Sounds like a ratio problem. I would guess it wasn't a rebuilt trans. Just a working used unit from the wrong year. Easier/cheaper to swap out the rear axle now. I would also question the workmanship of someone who put in the wrong trans.

Time to wheel and deal on that one...

[yellowstoner]

Thanks. So I need to determine front versus rear ratios and, if different, find a rear end that matches up. Would I need to replace the whole rear end or just replace gears?

[sdoan]

Thanks. So I need to determine front versus rear ratios and, if different, find a rear end that matches up. Would I need to replace the whole rear end or just replace gears?

Just replace the rear gear cluster (differential). Basically you pull the wheels, drums, unbolt the bearing retainers (4 bolts each side) and pull the axles out about 6". Then you disconnect the driveline from the differential flange (4bolts), and undo the nuts around the differential (about 10 nuts) and then you pull it out. Installation is is reverse of removal. Refill with good gear oil.

The rear differential is easy to get and not too expensive ($75?) from a wrecking yard.

Good point to wonder about the thoroughness of someone who installed the wrong ratio trans. Check the gear lube in the trans - read how to do it - it's not intuitive. Seems like most problems with transmissions are low gear lube or abuse.

[Petros]

This could be an opportunity to get it at a good buy. If the 4wd does not work (which it does not) than there cars at this age do not have a lot of other things going for it. If it was fixed DIY, this is not surprising, it a shop did it they should correct it for free.

But before you go swapping out parts there are few other things to check. Inspect that all the tires are the same size and properly inflated, if one is different it could act this way. Also, with the trans in neutral and in 2wd, e-brake off, jack up the rear (put jack stand under it) and see if the drive shaft and the rear end turns freely, no binding or clunking, make sure all of the u-joints are tight with no movement in rotation. From here you can mark the tires and drive shaft with chaulk and see if you can carefully count the turns of the drive shaft with one revolution of a back wheels (the wheels must turn together). You can determine which rear diff ratio is in it, if it was not changed it should be the 4.1:1.

If you put in 4wd and jack up both wheels on the same side, brake off trans in neutral, and then mark the raised tires with chaulk. Rotate one wheel a full turn or two, and see if the other one turns the same amount. If not it is the front and rear diffs have different ratios.

The front and rear differential ratio must be the same, they came with a 3.7:1 in '83 and '84, and 4.1:1 for all '85 and newer. All the different transmissions are interchangeable (that is they look the same and they will bolt into the car), but many will, not knowing, swap out the whole front trans axle assembly with the front diff attached. They could mix up the years with the different diff ratios.

So as suggested, if they are different, you can either swap out the rear axle center carrier assembly (or "3rd member") with one from the same ratio as the front. Or you can also find the old trans axle (or another one with the right ratio) and just swap out the front diff assembly from the transaxle. You want to do either one as an assembly, not just change the gear sets, because the lash has to properly set if you disrupt the relationship between the ring and pinion gears.

Where are you located? If you do not want the car I might be interested.

[yellowstoner]

Petros, many thanks for that info--very helpful. I will call the owner tomorrow and see if he replaced the front diff along with the tranny. I will certainly let you know if I don't pursue this Tercel, but it's in Utah. One other issue with this Tercel is that the PS Pump has been removed. My guess is that it was leaking and was just never replaced.

[Shannon]

I have been immersed in this ratio issue for 2 months.
The 3.7 ratio is from 8/82 - 8/84. So, if you are in a junkyard check
the metal tag inside the drivers side door (on the body, not the door).
It will have the month and year of production. After 8/84 they are 4.1.
This is important, because I just found a junker with a November 84, which
meant it was 4.1 not 3.7. My 83 has a free back differential, which means that
when I check the gear ratio I would turn my wheel 2 revolutions while counting
the drive shaft revolutions. The drive shaft revolutions will indicate the gear ratio.

[SynthDesign]

I have an 85 manufactured in october of 84. It has original front and rear 3.73 to 1 diffs. Not the 4.10 to 1 which should be listed for the year range. Just letting you guys know.

[Shannon]

The toyota dealer told me those dates from 8/82-8/84 for the 3.73 ratio. So, that is interesting to know that an October 84 has 3.73 ratio. I am glad I just changed my transmission. Then the only way to know for sure is to count the gears. I was told the difference is the number of pinion gear teeth on these axles.

[SynthDesign]

Then the only way to know for sure is to count the gears. I was told the difference is the number of pinion gear teeth on these axles.

Absolutely. I mark the teeth with a grease pen as I count.

[toughtercel]

Hi , i use to had same problem years ago , being dumb that time , first of all motor had a crack block so i bought whole wreck car and swap motor with tranny from 1984 into 1986 ,it drive great but on 4x4 does'nt move then put 4 jack stand on the car test the rear axle it was working which mean the i have to change the tranny or the diffenail to match , second all 4 tires MUST be the same size, thanks

[splatterdog]

To determine diff ratios- Divide ring gear teeth by pinion teeth.


Example- 41 ring gear teeth divided by 10 pinion= 4.10

[mikey]

Thanks, everyone on this thread, especially petros and sdoan. I just successfully swapped the rear differential on my 86.

I bought this 4WD six weeks ago, and I knew that something was wrong with the rear end right away. I had hoped that when my mechanic fixed the axle bearings (lots of noise), that the 4WD would be better, but he gave me the bad news: not the original transmission. Here is what I have figured out:

If you jack up either entire side of the car, and turn either the front or back tire, the end with the 4.1 ratio will rotate exactly ten times for every eleven times that end with the 3.73 ratio will. This will confirm the problem, and tell you what parts you need, and where.

If you jack up just one of the rear tires, and turn it exactly twenty times, the driveshaft will turn either 41 times, or 37plus times, telling you the ratio of that particular rear differential. This is what I did at the junkyard.

The gear ratio is stamped on the ring gear of the rear differential, once you have it out of the car.

I'm just an armchair mechanic, and the $400 or more that my toyota guy guessed it would take was out of my poor boy budget right now. The job I did: junkyard differential, toyota gasket, new gear oil and sales tax cost me about $65!!

Thanks again!