Posted: Sun Jan 24, 2010 5:44 am Post subject: Weak windshield wiper pressure
Have very poor wiper pressure on my M38A1. I have replaced most of the rubber hose, but not much difference. Could there be a leak where the pressure originates, or in the motor above the windshield, and is it easy to open up these without doing any harm? Your help appreciated, thank you.
They are not easy to open since in most cases you must grind off the small swaged tits that fasten them together. You will see rebuild kits pop up on ebay for them.
Basic system inspection and tuneup is simple. The vacuum (not pressure) source for the wipers is the intake manifold aft end. That tube runs to the bottom of the dual action fuel/vacuum booster pump. The manifold vacuum is boosted by the lower half of this pump and that line runs up to a "T" fitting near the firewall and just to the left of the rear end of the intake manifold. That "T" supplies vacuum suction for both the distributor vent system and the wipers. A hose attaches to the short tube off that "T" and goes to a shut/off valve on the windshield or dash. From there it goes to each wiper vacuum inlet port. If you have the complete vehicle based fording system there will be two tubes going from the vent air in ports of each wiper to the air cleaner via the right side of the windshield.
Any leak on the vacuum side plumbing will reduce wiper performance. Any plugging up of the vent air side of the wiper system will reduce wiper performance. The buildup of gum and dirt inside the wipers results in low wiper performance. Worn internal parts including the wiper vane will reduce wiper performance.
So where do you start? Start with a complete visual inspection of all external parts and plumbing. Then connect a vacuum gage to the vacuum in line on each wiper motor (one at a time) and run the engine. It should read 10" at idle and 12" at half throttle. If this is your readings then the problem is in the wiper motors. Spray some WD40 into the wiper motor through the vent air in port and try the wiper motor. If performance improves you can either accept that new performance level or rebuild/exchange the wiper motor. If the vacuum readings were too low then check all plumbing for leaks. If no leaks are found then the vacuum half of the pump is bad. While checking for leaks try disconnecting the distributor line from the "T" fitting and hold you finger over that port. You should get a slight rise in vacuum suction. If you get a big rise then the distributor plumbing may have a serious leak or the .040 restrictor that belongs on the distributor side of the "T" fitting may be missing.
One other point, the vacuum and vent metal tubes on the windshield are 1/4" OD but you do not use 1/4" ID rubber hose on them. You use a special size hose that is 7/32" ID with a 3/32" thick wall.
If all your plumbing is good and leak free and vacuum suction is good (10" to 12") and motor performance does not improve enough with the WD40 you can try to rebuild the motors yourself or you can have "The Wiperman" rebuild them for you. http://www.wiperman.com/ _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Apr 18, 2005 Posts: 458 Location: Rhode Island
Posted: Mon Jan 25, 2010 5:30 pm Post subject: wipers
100% on board with Wes's advice. First you need to check the vac system. No vacuum, no wipers.
Once you are sure thats all squared away, including the plumming for the wipers, then turn attention to the motors.
I have seen cases where the motors were full of dirt. Some light compressed air may help eject loose sand etc.
When all else fails the Wiperman does a nice job. He strips the units and rebuilds them. He is also very switched on to these jeeps and pointed out that my motors had the handles in in the wrong position because they were on an arctic windshield.
Tom
Posted: Tue Jan 26, 2010 3:15 pm Post subject: Allow me to show my ignorance
I am restoring a M38 and my son is restoring a M38A1. We have rebuilt the wiper system on both vehicles. At idle the M38A1 has 19" vacuum and the M38 has 17" vacuum. The wipers work fine at idle. However when both vehicles are accelerating or pulling a hill the wipers slow down considerably, sometimes stopping. Is this normal or do we still have a problem somewhere? I thought the vacuum booster pump would prevent that. When I was 16, I had a 1970 IH Scout with vacuum wipers that did the same thing and was told that it was normal for the wipers to slow down when pulling a hill, but I don't recall it having a booster pump. But that was 35 years ago and I may not be remembering correctly. Any advice will be appreciated.
Most cars with vac wipers had a boost system. Yes it is quite common for the slow down when you are accelerating. It's the low manifold pressure with the throttle plate wide open and the engine's RPM hasn't caught up yet. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Jan 27, 2010 Posts: 98 Location: Warner Robins, Georgia
Posted: Sun Jan 31, 2010 8:55 am Post subject:
Fill the vacuum motor with DOT 3 brake flluid and let it soak for a few days. Then work the wiper arms by hand. This is and old trick that antique auto restorers use on vacuum motors. Had to work the blades about 50 times on my 56 T-Bird, and then they came on so strong they would knock you down if you got in the way. You will get a lot of smoke from the engine when you start the engine if the flluid goes in the manifold, but it wont hurt the engine. Good luck.
I would stay with a petroleum based oil/lubricant. The Dot 3 if spilled or splashed on your paint it will really piss you off. The spray oil does just as good a job of freeing up tired rubber wiper parts. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
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