Ford NH 1520 It's the HST filter!

mrbrettbaker · Mar 8, 2011

I posted here about a friend's 1520 blowing the hydraulic filter seal. Well, I just went to look at it myself and it turns out it is the Hydrostatic Transmission filter. I downloaded a plumbing parts diagram and see that it is routed to the cooler and can't imagine there is supposed to be a great deal of pressure in that part of the system, as the cooler would not survive. I removed the housing and inspected the gasket surface for straightness and surface smoothness. I checked the check vave in the housing and it is free and in good shape. Does anyone know what in the trans could cause this much backpressure? I know nothing of this model trans but have worked on and rebuilt my fair share of others and they all returned cooled fluid to the sump, little or no backpressure. RickB? I'd hate to start pulling this trans apart and find I missed something simple.

tcartwri · Mar 8, 2011

The wrong filter.

mrbrettbaker · Mar 8, 2011

Yea, wish it was that easy. Last one bought at the dealer. 2 before that same part number as the one that was on there for years. Calipers on the gasket surface confirm it correct in size.

jinman · Mar 8, 2011

The 1520's HST transmission has a charge pump that pulls fluid from the reservoir and then sends it under pressure through the HST filter and cooler before it arrives at the inlet to the main transmission. The charge pump is designed to supply the HST with all the oil it needs so its pump won't cavitate. The normal charge pressure can be as high as 200 psi, sometimes much higher with cold oil. If that HST filter is not very, very tight, the gasket can blow out. The instructions for tightening the HST filter are not the same as the engine oil filter. The HST filter must be very tight. Also, any restriction in the lines can raise the pressure. If the cooler has some obstruction or a hose is crimped, it could cause pressure to increase to much higher levels than normal and put additional pressure on the HST filter seal.

The only way to know the actual pressure level would be to "T" into the line and put a pressure gage to monitor the charge pressure. If you are not sure what the pressure should be, it's a guess, but with very cold oil, I think you could be looking at 250 psi or higher.

tcartwri · Mar 8, 2011

mrbrettbaker said:
Yea, wish it was that easy. Last one bought at the dealer. 2 before that same part number as the one that was on there for years. Calipers on the gasket surface confirm it correct in size.

Filters of the same dimension are not necessarily the same spec. Try another filter.

mrbrettbaker · Mar 8, 2011

OK, thanks for that info about how the hydraulic system operates, that definitely steers me in the right direction for diagnosing this problem. I just found out another piece to the puzzle over lunch that may help you help me. The owner went to the dealer with the # RickB (and the partsfische) said was correct SBA340501030. when he got home, it would not thread on, I believe the filter bore was too large. when he went back to the dealer they gave him the same WIX filter that was originally on it 557088? In the parts drawing I see they have a superseded filter housing, could this tractor have one and this be the reason for the different number? the first SBA filter WILL NOT fit,and the 7088 was on for years.
I really don't want to just "try another filter" as besides having to hve the right thread and seal diameter, not all oil filters are designed to handle hundreds of pounds pressure, and obviously this one is. It worked in the past, just need to find what has changed.
Thanks everyone

RickB · Mar 9, 2011

SBA340501030 is the correct hydrostatic filter for a 1520, in fact just about all Ford and NH compacts with hydrostatic transmissions. If that filter won't fit, the filter head is either incorrect or has been damaged by prior installation of an incorrect filter. It's that simple. If you insist on using a Wix filter as a hydro charge pressure filter, I can't help you. You need to get the correct filter on that tractor and see if the problem persists. An overly restrictive filter will in itself cause a pressure problem which will not be present downstream at the cooler. If you prefer to spend time and money chasing a different problem that likely isn't there, it's your choice. If you need info on the correct charge pressure testing, I suggest that you get the applicable service bulletin from your local dealer.

For what it is worth, the Wix 57088 (they don't list a 557088) has the same 3/4-16 thread as the SBA340501030. If one fits, the other will. Which doesn't mean they both perform at the same level.

mrbrettbaker · Mar 9, 2011

Well if they are both the same thread then a damaged head wouldn't allow the wix on either. BUT you did open a possibility. The NH filter he picked up may have been defective. I just had my guy service my Duramax pick up and the first fuel filter we recieved had the right part number but the center hole was 1/4" or more too big!!
I'm staring at the filter NH dealer gave me and it says 557088, maybe not wix, but napa cross and app both show 7088 and typically you add a 5 to the front and you have wix, just as you pointed out. I agree 100% with using oem whenever possible, especially when there are problems, but in our case we tried and it didn't work. I will try to share what I find in this mess.