ive got a strange rumble when accelerating + turning (both directions left & right), it doesnt do it all the time, it seems to do it after about 1/2 hour of driving, almost like its temperature dependant, the best i can describe is like driving over a very rough road and how it rattles the cabin. I was thinking rear diff or torque converter, but if it were a torque converter problem it should slip on steep inclines, i drove it up a hill at a very slow speed and it does not do it, maybe because i was going straight which doesnt make sense to me. Any ideas as to what to look at as a starting point?

99 Navi 2WD

 

The problem your discribing sounds like your limited slip rear end is slipping. YOu see in most limited slip diffs there are a set of clutch packs so the rear wheels can grab at same time. But they also need to alternate grabbing when turning. Had the same problem with my 03 with limited slip. turns out dealer was putting in wrong oil in rear end. so i changed oil out with some full synthetic and you have to add friction modifier to the rear end also so the clutch packs can grab when turning. for my 03 it uses 75w-140w synthetic and 4 ounces of friction modifier. I Just bought the friction modifier at a local auto parts store i thinks its called trans x.

 

yeah i was leaning toward and hoping that it was only the diff rather than torque converter. I did not know these cars came with LSD so it didnt cross my mind, it seems like its locking up too early around the turns when the oil gets warm, thats why i mentioned that its seems to be temp (oil) dependant. I just bought the car and i saw a receipt from the previous owner that the rear end oil was changed, probably used some cheap stuff in there. Im going to try a different oil. I also didnt know about the 4 ounces of friction modifier, thanks for the info

 

I had my rear end replaced on another vehicle once... had an Auburn Posi put in. When driving it around the next day, I had the same problem you did. I called up the shop and had them ask the tech if he put the friction modifier in... I could hear him slap his forehead over the phone. I stopped in 15 mins later and they added it. Problem went away in no time.

 

What kind of damages can I expect if its run without the friction modifier? I just looked at the receipt from the previous owner and it looks like the diff oil was changed 12K miles ago with Mobile 80W140, and i dont see any friction modifier in the receipt, I think ill try just adding the friction modifier stuff 1st and see if it improves.

 

The friction plates may have worn a bit. Keep that receipt handy... it might save you some money if your LSD does not grab properly.

 

I was just about to post a new thread for the same problem. My '97 does the same thing, but only when the wheel is turning left and only @ 45-65 mph. I thought because of those variables it was something wrong with the variable speed power steering assist.

 

That could be a wheel bearing at that speed. The LSD sound is usually at slower speeds and tight turning angles. Shifting the load to the worn side can cause a grinding/groan in a bad bearing.

 

This was a issue that we fought with when buying ours used. Someone here pointed us to the TSB on this, and we took that in to the dealer (not ford) and they did the repair and we have had no more issues. I believe it cost them about $400. So if adding the modifier does not work, it sounds like it should not be too bad of a repair at a shop that does this. For us it was the clutch pack, and it was on several 60K+ units we tested. Guess that is why there is a TSB.

 

Ours does this as well around corners and under acceleration while turning. We will be rebuilding the trac-lok this summer.

 

pardon my ignorance but what is a TSB. I just noticed my '99 Exp vibration today while turning. Incidentally, I changed my rear diff fluid about 25K miles ago and DID NOT put the friction modifier....

 

TSB= Technical Service Bulletein. Go to the top of this page and Click on Recalls/TSBs and then find the Full Text TSB.

also, check the manual and if it calls for modifier, add it...as a matter of fact, I'd add it anyway.....

 

thx for the reply. i dont see TSB's for Expeditions. Only for F series and Excursion...???

 

sorry, not in Full Text section for your truck...here it is

https://www.ford-trucks.com/tsb/tsb.php/m-FORD/y-1999

here's the direct link to your issue,

https://www.ford-trucks.com/tsb/tsb....y-1999/t-15123

 

Any Ford limited slip differential requires friction modifier