Rear Differential, Gearing, Towing & Speedometer Question

Apparently I managed to grind the rear differential in my '91 Roadmaster to bits while towing (noone warned me about using overdrive - thank you U-haul...).

Currently loooking for a used differential to replace the busted one with, if anyone knows of one... The last 3 or 4 my mechanic looked at were no good.

On that note, what would the impact be of going to a larger size gear? 4.15 is think is the other one we could get. How hard is it to change things so that the speedometer would show the correct speed again? What is the impact on towing? And on gas mileage? I believe the engine would be running at slightly higher RPM's at highway speeds?

While all of this is getting fixed, what else should I look at doing to prevent this from happening again? This is a non-tow package car origionally.

Thanks in advance for any help.

-Sven

While it's true that towing in OD is not recommended, this didn't hurt the diff. When in OD, the transmission is actually outputting 30% less engine torque which, in theory, would only help the differential last longer. Problem is, it causes a lot of added strain on the engine and transmission.

Perhaps just mileage put your diff out of commission. BTW, you didn't state what ratio you did have. A non-tow pack car may have been a 2-series ratio which you'd want to step up to a 3.23 or 3.42.

I believe it was a 2.7something. Any other advise on upgrades?

Instead of a used differential, consider a rebuilt one.
Then you get newer parts and new bearings!
There are many outfits offering these packages.
Re: a ratio change, yes if you plan to tow. But realize you then will
sacrifice non-towing gas mileage. I concur that a 3-series ratio would be your best compromise.
You can get 3.08:1, 3.42:1, or 3.73:1. Remember tho, higher numerically means
lower mileage.
Then you need to get the speedo gear changed at the transmission for correct calibration,
and it has to match the rear gear ratio, so you need to know that info first.
Many other variables to consider, but more specifics on what you are towing is necessary.
BTW, do you have a brake controller in the tow vehicle if what you tow weighs more
than 1500 lbs.? It is a requirement by law in many states!

No brake controller as of right now. Will add that once I find the time - not really an issue with a 6x11 uhaul trailor and the small stuff I currently tow.

Eventually I would like to a camper, at that time it will need to be added.

From what I have seen rebuilt is super expensive - I was quoted $60-100 used, $1500 rebuilt.

Jasper:
7726166 $2025.00 Limited Slip
7726167 $1679.00 Standard

s1l1sc wrote:

No brake controller as of right now. Will add that once I find the time - not really an issue with a 6x11 uhaul trailor and the small stuff I currently tow.

Eventually I would like to a camper, at that time it will need to be added.

From what I have seen rebuilt is super expensive - I was quoted $60-100 used, $1500 rebuilt.

Jasper:
7726166 $2025.00 Limited Slip
7726167 $1679.00 Standard

Don't know who you talked to and my records are buried right now but,
I got a rebuilt complete positraction center section from an outfit in Nevada for $199.00 --plus shipping,
and it came with brand-new bearings! All you then have to add is your ring & pinion gears--
and a shim kit!

@rkmdogs - If you can send me the info I would appreciate it - the shop did mention that the spider gears were bad as well?

O.K. Just for you, I went digging in my old files and found it.
I found them on eBay and made subsequent contact.
The fellows name is Erik Bertrand and you can reach him at:
ecbert@yahoo.com His company is BET Performance.com. They are in Carson City, NEV.
His company telephone number is: 775-690-2972
I bought mine in October of 2009, so I do not know if this information
is still current.
I bought an Eaton GOV LOC Posi 28 3 Series G80, and paid for it
via a telephone call credit card transaction!
He can address your specific needs, but I had to tell him the
spline count in my original diff, since there were two used at that time.

I really appreciate it. I will give him a call.

Here is a place that has the gears for the VSS.

http://www.transmissioncenter.net/

If you go about halfway down the page you will see Speedometer World. Click on the link, and if you look around a little, there is a formula for VSS gear ratios. The stock drive gear is an 18 tooth. There is also a chart for some gear ratios, so if you use one of them, you can just pick from the list.

If you need to change the drive gear, you have to remove the tailshaft housing from the trans. You can probably get it to work without changing the drove gear.

3.23 uses the 42 tooth green gear with stock 225/75/15 tires, and an 18 tooth drive gear. Tire size is also calculated into the formula.

Do not use a Gov-lock posi on these cars. They were intended for pickup trucks struggling with traction in mud/dirt, not on pavement.

I realize other types of limited slip differentials are more costly but I would rather have an open differential than a gov-lock in a wagon.

sherlock9c1 wrote:

Do not use a Gov-lock posi on these cars. They were intended for pickup trucks struggling with traction in mud/dirt, not on pavement.

I realize other types of limited slip differentials are more costly but I would rather have an open differential than a gov-lock in a wagon.

Fact statement not so! This is the same posi that the factory offered as an option!
Call them and ask!

If you mean "same" in that the G80 option code appears on vehicles with it, that is true, but they are physically different. The B-cars came with an always-on clutch-type posi made by Auburn. The Gov-Lock posi was made by Eaton and was used in trucks.

Info is here:
stock posi thread 1
stock posi 2

Gov-Lock posi threads here (and note how many times catastrophic failure is mentioned):
gov lock 1
gov lock 2
gov lock 3

No love for Torsen/TruTrac diffs here?

sherlock9c1 wrote:

....Gov-lock posi...

I affectionately call that G80 unit the "slam locker", when those units engage they slam-in pretty hard. Usually found in pickup trucks and would not recommend it for the anything else. They can be hard on the driveline. Very positive engagement though.

jimbeau wrote:

No love for Torsen/TruTrac diffs here?

I was making no post in favor of nor against those units, although if I had to do it again, that's the type I would get. Just wanted to put the caution out on the gov-lock posis. You can get them dirt cheap used, and I was just about to get one a few years ago, but after all of the research I did, I stayed away. Didn't want someone else to go down the same road and break something in their drivetrain.

From the links posted, I can't see why anyone would want one of those "grenade-lock" units.
Sudden engagement/disengagement would be irritating anyway.

These descriptions are extreme to say the least!
Mine does not have those symptoms!

Hey, we are just looking at pics of broken units. I imagine they are ALL nice until they come apart on you.

How things are suppose to work with these gov-lock diffs and what can happen are two different things. The idea behind it's operation is that when a 125 rpm difference is noticed between the left and right wheel the governor kicks in and locks the diff.

Our perception of the 125 rpm difference may seem like a whole lot of wheel speed difference, but what that equates to is about 1/8 of a turn of the wheel before the engagement of the locking device.

Take this for an example, we've all been here with the open diff rear ends, one wheel is spinning and the other wheel is stationary and the speedometer reads 40 mph. That only reflects about 1/2 of what the actual wheel speed is for the spinning wheel. So, the wheel may be spinning in upwards of 80 mph.

If you have ever had one of these diffs apart, there is a "starter drive gear and flywheel" type device in there that spins the governor. If we used a flywheel (diff spider gear), at 125 rpm, to spin (drive) the starter drive (governor) it would spin insanely fast in a very short period of time. (Of course, the overrun clutch in the starter drive prevents this, but the gear would still spin very fast.) Ideally, it should be a seamless engagement, but experience tells me otherwise.

The governor device is equipped with some flyweights that fly out due to centrifugal force causing engagement of the diff. The governor is meshed with the spider (side) gears inside the diff.

Hope this all makes sense and maybe provide a better idea of how it operates. Still forceful nonetheless.

gmtech wrote:

... The governor device is equipped with some flyweights that fly out due to centrifugal force causing engagement of the diff. The governor is meshed with the spider (side) gears inside the diff.

How do the flyweights cause engagement? By a 'pawl' of some sort?

Click this video link on the Eaton G80 Locker. I have it queued up to start right where it talks about the lock-up mechanism or watch the whole thing below.

^^^ Is that the Auburn style, or the Gov-lock?

Great instructional video, by the way!

Sorry, forgot to mention that little bit of info, but it is the governor lock type rear diff shown in the video.

So, (looking at video), the theory is good, but when that pawl slaps in at high speed, it's got the mass of a speeding bullet. Too bad. Fail.

That diff is suppose to lock-out the function of the governor lock above 20 mph. But is it error-proof? Only an diff explosion will tell.

[quote="gmtech"]Click this video link on the Eaton G80 Locker. I have it queued up to start right where it talks about the lock-up mechanism or watch the whole thing below.

GM Tech...Great video even for a non mechanic. So on my 92 OCC with towing package and G80 that is how it works, right?

COL

If so equipped yes, but pulling the cover to inspect would tell you for sure. Keep this in mind, of the RPO (Regular Production Option) G80 code, just because it shares the same alpha-numeric code, car vs. truck, does not mean that it is an Eaton Locker. Truck likely has the Eaton style locker and the car may have the conventional LSD (cones and clutches shown below). Two different animals. Actually laying your eyes on the unit will tell you for sure.