Fmvb 48re line pressure issue.

3

3rdgenmike

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Joined
May 3, 2022
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Took my truck for a test drive after installing new trans. Truck has a muldoons fmvb. Before the test drive at idle line pressure was 140-145psi in 1, 2, and 3. When I hit the lockup switch or overdrive it raised it to 190psi. 15 minutes into test drive I noticed line pressure was at 60psi when I was at a stop. Trans temp was 185°F. When driving at 1900rpm line pressure wouldn't go above 120psi in 1, 2 or 3. With lockup or overdrive overdrive it wouldn't go above 140psi. Let the truck sit overnight and rechecked line pressure at idle with trans temp at 65°F. 1, 2 and 3 all had 140-145psi again, with lockup and overdrive also going back to 190psi. So, I'd assume it's the pump right?
 
I had this issue when I didn't torque the bolts on the new pump and cover. I had no idea they got shipped loose.

The other thing I did twice in 50k miles was invert the Belleville spring and shoot the seal off the piston. I had to upgrade to a Georend spring.
 
biggy238 said:
I had this issue when I didn't torque the bolts on the new pump and cover. I had no idea they got shipped loose.

The other thing I did twice in 50k miles was invert the Belleville spring and shoot the seal off the piston. I had to upgrade to a Georend spring.
Click to expand...
That spring would be the one in the drum right? I can't remember which drum. Guess I'll have to pull the trans either way.
 
biggy238 said:
I had this issue when I didn't torque the bolts on the new pump and cover. I had no idea they got shipped loose.

The other thing I did twice in 50k miles was invert the Belleville spring and shoot the seal off the piston. I had to upgrade to a Georend spring.
Click to expand...
Also, which way does that spring face lol. Just to make sure
 
3rdgenmike said:
Also, which way does that spring face lol. Just to make sure
Click to expand...
Spring curves to the piston, somewhere around 200psi it flips toward the clutches, hold them applied and let's the piston overtravel, shooting the seal off the piston. I want to say it's the forward clutch grp but I would have to look. I know it's in my build thread.
 
biggy238 said:
Spring curves to the piston, somewhere around 200psi it flips toward the clutches, hold them applied and let's the piston overtravel, shooting the seal off the piston. I want to say it's the forward clutch grp but I would have to look. I know it's in my build thread.
Click to expand...
Pretty sure I got it in right then. Hopefully it didn't flip. I used a kit, so I'm not sure what spring they included. Hopefully Georend one
 
I had the issue also, went with a "billet" bell spring after and never had another problem with that.
 
jasonc said:
I had the issue also, went with a "billet" bell spring after and never had another problem with that.
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Now i'm paranoid about the spring. I bought a kit, so I emailed the company asking. Hoping my issue is just the pump. I have a spare one that I know is good as it came from my other trans.
 
The line pressure doesn't come back with the spring issue from what I recall. I'm almost certain that's how my pump acted.
 
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