NissanDiesel Forums

I am in the process of replacing the alternator, but it looks like the shaft of the rebuilt alternator is about 1/16" too long, so when I bolt on the vacuum pump, it locks up. I learnt this by almost setting the V-belt on fire when I restarted the truck. There are some size discrepancies, so I am wondering if I have a stock vacuum pump, or something else. Here is what the pump looks like



On the back side, there are 3 tiny screws that attach the back cover to the pump, which is different from the picture shown in the FSM. When I looked at the specs in the 82-83 FSM, my pump seems to be way larger.

Vane length "A"": 15.24mm vs 13-14mm (FSM)
Vacuum pump housing ID: 59.75mm vs 57-57.1mm (FSM)

It seems unlikely that the pump wore down 2.65 mm from the specs.

Zoltan wrote:I am in the process of replacing the alternator, but it looks like the shaft of the rebuilt alternator is about 1/16" too long, so when I bolt on the vacuum pump, it locks up.

TTBOMK, the automotive diesel vac pumps are all interchangeable. Did you read this thread?

I did. I did not remove the rotor from the pump, so there's no possibility that I put it back in the wrong way. I only removed 2 vanes, and I measured the ID of the house through the gaps where the vanes go.

Um, was Philip in that last post? Gotta stop doing that, Philp!

Zoltan, I have a working assy. on an '82 SD22:


If you really need the info, I can pull the vac pump off and measure the rotor's protrusion from the back face of the alternator. May not be able to do that until Sunday though.

zoltan wrote:I did read it. I did not remove the rotor from the pump, so there's no possibility that I put it back in the wrong way.

There is nothing preventing you from TRYING assembly with the rotor installed the other way and then see if it still binds up when turned by hand.

It's most peculiar... the reason why the vacuum pump was locking up was that I tried to use an O-ring that was 0.7mm thicker than the original. The new alternator shaft was the perfect size.

First, I went with Philip's suggestion and tried to flip the rotor... however, in my vacuum pump, one cannot put the rotor in the wrong direction. One side of the rotor is machined out (about 2 quarter coins deep) and the back cover has a corresponding circular protrusion. So if one tried to put the rotor in the wrong direction, the back cover would not be able to go back on, it would stay 2-3mm ajar.

This picture shows the old O-ring on the left and the new one I tried to use on the right. As you can see the new one is almost a perfect match as ID is concerned.





Here you can see how the new O-ring fits the groove on the vacuum pump (I am still hoping that some one recognizes what kind of vacuum pump I have).




However, when one looks at the side profile, the old O-ring is much flatter than the new one (1.72mm vs 2.54mm).



I didn't want to believe, that such a small size difference would make the pump lock up, so I switched back and forth b/w the 2 O-rings twice to ascertain that THAT was the problem... But no doubt. I guess what might have happened was that when I bolted on the vacuum pump with the new O-ring, the uneven clamping force (the bolts are not 120 degrees apart, they are distributed somewhat unevenly around the circle) must have caused the housing warp a tiny bit, which led to the lock-up.

A valuable lesson learnt: "One should not screw with O-rings, because they will screw you back !!"

Zoltan wrote:A valuable lesson learnt: "One should not screw with O-rings, because they will screw you back !!"

The 1982 FSM page BR-15 does not show the three phillips screws securing the center plate to the pump housing. I know my truck's vacuum pump does not have them. But the 1984 FSM page BR-13 does show those screws. So ... you've got the later version.( )

Good sleuthing!

I'm guessing that the fatter o-ring caused the pump's backplate to distort into the rotor. Either that, or the whole assy. cocked when tightened. Either way, I'm glad you got it sussed.

the 1984 FSM page BR-13 does show those screws. So ... you've got the later version

So is it for a SD22 or SD25? I noticed oil dripping from the O-ring this morning so I will have to go back to the dealer to get the right one.

The SD is just chock full of weird-sized o-rings. I ran into the same problem with the oil cooler o-rings. IIRC, those o-rings were NLA from Nissan, and I didn't pursue the forklift supplier angle, but I'm sure they would have been available there.

I think I went to O-rings West and bought some. Min. qty was (50), so I may still have (48).

Zoltan wrote:

the 1984 FSM page BR-13 does show those screws. So ... you've got the later version

So is it for a SD22 or SD25? -snip.

What does "later" suggest? Ask the partsman to compare part numbers for an '82 SD22 vs an '84 SD25. Has to be <$50 for one of each.

Has to be <$50 for one of each.

Woah.... $7.20 ! I prematurely placed an order at Nissan to find out later tonight that it was leaking at the fittings of the VP connecting hose... Part# is 14688-61503, but the parts clerk couldn't tell if it was still available. We are having huge winds lately and his internet connection was down.

Unless you broke the metal fitting ends, just find a hydraulic hose shop and do what I did. You'll have it all back in a day or two and for a LOT le$$.

Philip, I think it's $7 for the o-ring to the alternator, not the hose, I think that's what Zoltan was saying.

Yep, it's $7.20 for the O-ring. I will put it in my safe if it comes in .
Meanwhile I am getting the hose repaired.