Joined: Dec 10, 2017 Posts: 337 Location: Southern Maryland
Posted: Sat May 22, 2021 1:38 pm Post subject: M38 Vacuum Pump
I have the dual Vac/Fuel pump on my M38, but don't have the wiper motors installed. Is it ok to cap the vacuum lines at the pump until I get the wipers installed to make it quieter or should I leave it uncapped? I'm running it uncapped now but it's pretty noisy.
Thanks,
Mike B _________________ Mike B
1953 M38A1 Brush Truck
1952 M38
1951 M100 Trailer
Are you also not using the stock military distributor? _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Dec 10, 2017 Posts: 337 Location: Southern Maryland
Posted: Sun May 23, 2021 12:15 am Post subject:
That is correct Wes, The PO converted it to 12 volt.
The pump currently isn't doing anything other than making noise, it's not connected to anything.
I guess capping the suction side would be no different than having the wiper motor switch turned off now that I think about it. What about the pressure side? Any harm in capping it also?
Mike B _________________ Mike B
1953 M38A1 Brush Truck
1952 M38
1951 M100 Trailer
If you still are using the waterproof military distributor you should still allow the vacuum side of the pump to function. The sealed distributor does not breathe (vent) well on it's own. The ionized gases that can build up inside are explosive and must be forcibly vented. That is why the vacuum side of your pump is plumbed to the wiper "T" and then from that "T" to the distributor housing and the wipers.
You do not mention your fording system plumbing but that plays an active role in what you are doing with the vacuum side of the pump as well.
If you intend to eventually get the vac wipers installed and working then I suggest simply capping the "T" at the leg that leads to the wipers and not disturb the plumbing itself.
Another issue is by plugging up the vacuum pump's inlet and outlet the next time you fire the engine up the vacuum pump will rupture it's own diaphragm trying to do it's designed job, pumping air.
Study these vac and vent system illustrations and if you wish post some similar photos of your current configuration.
Joined: Dec 10, 2017 Posts: 337 Location: Southern Maryland
Posted: Sun May 23, 2021 1:54 pm Post subject:
Wes, thanks for all the diagrams.,
My M38 is a driver and the PO set it up more like a CJ-5. It has a non-military 12v Autolite distributor with no vent connection. The transfer case has a mushroom vent, the bell housing vent port is plugged, the PCV is connected from the side cover to the intake with the check valve in line but no valve. There are no lines run for the vacuum system at all, but I do have the carb vent piped to the air intake tube. Maybe I should replace the Dual Pump with Civvy fuel pump and just not worry with it...
I'm on the fence about wipers period, I never drive in the rain and if I do put wipers on at some point they may be electric.
Mike B _________________ Mike B
1953 M38A1 Brush Truck
1952 M38
1951 M100 Trailer
Good choice. Sell the dual gas/vac pump to a needy M38 owner and stick a standard single action gas pump for a CJ on it. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
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