Diff fill/overfill?

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I changed out the gear oil in the front diff on the Burb on Saturday night to Sunday morning (pump broke Saturday night, couldn't replace until Sunday morning). It came with a conventional gear oil in the front and the appropriate fill level, per the OM, is to the bottom of the fill hole. The rear diff came with synthetic fluid and the appropriate fill level is 5/8 to 1 5/8 inch below the fill hole. However, my Haynes manual does not distinguish between the front and rear diffs in the "Differential lubricant change" section and simply states in the final step:

Quote:
Use a hand pump, syringe or squeeze bottle to fill the differential housing with the specified lubricant until it's level with the bottom of the fill-plug hole. If using synthetic axle lubricant, the level should be below the fill-plug opening by 5/8-inch to 1 5/8-inch on 1500 Series models, but no more than 3/8-inch lower than the hole on 2500 Series models."

I have a 1500 Series and filled it to the bottom of the fill hole. Should I extract some of the fluid from the front diff to bring it to 5/8 inch below the fill plug hole?
 
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I forgot to mention in my original post that I filled the front diff with a synthetic fluid, hence the question regarding the appropriate fill level and why I would deviate from the OM.
 
The reason for filling below the fill hole, Is so the fluid doesn't run out while checking them "Hot", Which most sane people don't do. (Yes, I've done it).

Leave it, It's not going to hurt anything.
 
Originally Posted By: GutsyGecko
Leave it. Can't see a few oz of fluid doing anything.


I would normally be inclined to agree.
However, there is some evidence that supports a lower oil level being correct in this application.
Although I don't get why there should be a different level for synthetic oil?

I believe the manufacturer in all likelihood has worked out the correct level for the application, and they should know best.
Typo's do occur as well.

Consider this.
If the level is down a bit, overall I can't imagine a problem in the grand scheme of things.
If the level is too high then it's "possible" the unit will develop a higher than normal operating pressure (under some conditions) and push the oil seals out.
That's the only legitimate reason why a manufacturer will spec a lower oil level than one would assume to be ok under normal circumstances.
If the seals get pushed out, this will certainly cause an oil leak and allow contaminates to enter the unit.

On the other hand I have seen where a manufacturer has revised the oil quantity upwards, and issued a TSB in that regard.
The revised oil quantity is 10% more(from 4 l up to 4.4 l) than the OEM spec for that same unit, as supplied to other car manufacturers.
That placed the oil level at well over the designers filler hole height.
There were no sealing issues in service with the higher oil level.

I would always err on the side of the vehicle manufacturer's service manual if in doubt.
 
Originally Posted By: Ducman
I would always err on the side of the vehicle manufacturer's service manual if in doubt.

As would I, but the problem is that I do not have a GM service manual, just the owner's manual, Haynes manual, and a Chilton "For Professional Technicians" GM manual. The Chilton manual includes drive axle fluid capacities and references filling the axles and checking the fluid levels, but never mentions the proper fluid levels measured from the fill hole. The OM specs a synthetic fluid for the rear axle [SAE 75W-90 Synthetic Axle Lubricant (GM Part No. 12378261) or equivalent meeting GM Specification 9986115], but does not mention synthetic for the front [just "SAE 80W-90 Axle Lubricant (GM Part No. 1052271 or equivalent)"], which makes it seem like there may be something to the Haynes instructions.
 
Originally Posted By: NMBurb02
I changed out the gear oil in the front diff on the Burb on Saturday night to Sunday morning (pump broke Saturday night, couldn't replace until Sunday morning). It came with a conventional gear oil in the front and the appropriate fill level, per the OM, is to the bottom of the fill hole. The rear diff came with synthetic fluid and the appropriate fill level is 5/8 to 1 5/8 inch below the fill hole. However, my Haynes manual does not distinguish between the front and rear diffs in the "Differential lubricant change" section and simply states in the final step:

Quote:
Use a hand pump, syringe or squeeze bottle to fill the differential housing with the specified lubricant until it's level with the bottom of the fill-plug hole. If using synthetic axle lubricant, the level should be below the fill-plug opening by 5/8-inch to 1 5/8-inch on 1500 Series models, but no more than 3/8-inch lower than the hole on 2500 Series models."

I have a 1500 Series and filled it to the bottom of the fill hole. Should I extract some of the fluid from the front diff to bring it to 5/8 inch below the fill plug hole?


I think the confusion is created by the discrepancies (seemingly) in the different manuals due to the wording. And some overthinking on your part.


The genuine OM is calling for the front diff to be filled to the bottom of the filler hole (as one normally would). They just happen to spec a mineral lube from factory.

You have filled the front diff with a synthetic oil.
That's where your getting confused, because you are extrapolating from the information given for the rear diff in the genuine OM and drawing a false conclusion from how it's worded in the Haynes manual.
Thus giving a false impression in your mind, that if one is using a synthetic oil, it should be set at a lower level. Your simply getting your wires crossed.

The rear diff just happens to be spec'd for a synthetic oil.

From what you are saying.
Both the manuals are in agreeance on the oil levels front and rear.
And it should simply be set lower for the rear.
I think this would be the case whether the oil was mineral or synthetic.

Don't read too much into it. You're over thinking it.
The Haynes manual has actually made a distinction between the front and rear diffs.
It's just clear as mud. It's given you the impression that there's a different oil level for a mineral oil as opposed to a synthetic for the rear diff.
Where the Haynes manual says "If using a synthetic axle lubricant" it's throwing you of track a little. The wording is poor.
That's just their way of making the distinction between the front and rear diffs.

This brings you to have a doubt about the oil level in front diff, simply because you've changed to a synthetic.

IMO, just set the level at the front and the rear diffs as per the genuine manual regardless of whether you have a synthetic or mineral oil in the front diff.
You'll be fine as it is.
 
Originally Posted By: Ducman
Originally Posted By: NMBurb02
I changed out the gear oil in the front diff on the Burb on Saturday night to Sunday morning (pump broke Saturday night, couldn't replace until Sunday morning). It came with a conventional gear oil in the front and the appropriate fill level, per the OM, is to the bottom of the fill hole. The rear diff came with synthetic fluid and the appropriate fill level is 5/8 to 1 5/8 inch below the fill hole. However, my Haynes manual does not distinguish between the front and rear diffs in the "Differential lubricant change" section and simply states in the final step:

Quote:
Use a hand pump, syringe or squeeze bottle to fill the differential housing with the specified lubricant until it's level with the bottom of the fill-plug hole. If using synthetic axle lubricant, the level should be below the fill-plug opening by 5/8-inch to 1 5/8-inch on 1500 Series models, but no more than 3/8-inch lower than the hole on 2500 Series models."

I have a 1500 Series and filled it to the bottom of the fill hole. Should I extract some of the fluid from the front diff to bring it to 5/8 inch below the fill plug hole?


I think the confusion is created by the discrepancies (seemingly) in the different manuals due to the wording. And some overthinking on your part.


The genuine OM is calling for the front diff to be filled to the bottom of the filler hole (as one normally would). They just happen to spec a mineral lube from factory.

You have filled the front diff with a synthetic oil.
That's where your getting confused, because you are extrapolating from the information given for the rear diff in the genuine OM and drawing a false conclusion from how it's worded in the Haynes manual.
Thus giving a false impression in your mind, that if one is using a synthetic oil, it should be set at a lower level. Your simply getting your wires crossed.

The rear diff just happens to be spec'd for a synthetic oil.

From what you are saying.
Both the manuals are in agreeance on the oil levels front and rear.
And it should simply be set lower for the rear.
I think this would be the case whether the oil was mineral or synthetic.

Don't read too much into it. You're over thinking it.
The Haynes manual has actually made a distinction between the front and rear diffs.
It's just clear as mud. It's given you the impression that there's a different oil level for a mineral oil as opposed to a synthetic for the rear diff.
Where the Haynes manual says "If using a synthetic axle lubricant" it's throwing you of track a little. The wording is poor.
That's just their way of making the distinction between the front and rear diffs.

This brings you to have a doubt about the oil level in front diff, simply because you've changed to a synthetic.

IMO, just set the level at the front and the rear diffs as per the genuine manual regardless of whether you have a synthetic or mineral oil in the front diff.
You'll be fine as it is.

There was a part of me thinking that as well. I do have a tendency to over think things.
 
the reason for filling them below the hole is so that when the fluid warms up in use it is not being whipped up by the gears!
 
Does it have the white vent cap on the front diff? I'm not sure why but gm thinks the black vent isn't compatible with syn fluid
 
Originally Posted By: JD4440
Does it have the white vent cap on the front diff? I'm not sure why but gm thinks the black vent isn't compatible with syn fluid

Dnewton talked about that when I was trying to select a fluid, but the only Suburbans included in that are 2500s and only those sold in cold weather climates; mine is a 1500 and is a California vehicle through-and-through.
 
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