M38A1 wipers

Deadguy · Post by **Deadguy** » Tue Oct 02, 2012 10:51 pm

I installed new motors, new arms, and new blades. They are Trico vacuum wipers. The driver side sticks sometimes, which is annoying enough, but what I noticed today, was that they were both running fine, until whenever I hit the gas. Then the driver side one would die, and the passenger side one would run slow. If I let up on the gas, both would work. Any suggestions?

Wiltshireboy · Post by **Wiltshireboy** » Wed Oct 03, 2012 1:01 am

This is what they do. On my jeep anyway.
As you accelerate, the distributor demands some vacuum pressure, so you lose some on the wipers. Drivers side seems to be affected first.
Makes for interesting driving on hills in the wet!

Post by **wesk** » Wed Oct 03, 2012 7:12 am

All vacuum wiper setups have the same problem. When you reduce the intake's vacuum by opening the throttle plate during acceleration there is a serious drop in available vacuum. This is why vehicles with vacuum wipers usually have the dual fuel/vacuum pumps to help with the wiper's need for vacuum.

All you need to do now is get your manual out and read a bit on the fuel/vacuum pump. You will see the expected figures for available vacuum there. Get a vacuum gauge hooked up and see if you are getting the required vacuum at the two specified RPM ranges.

If you are producing the correct vacuum readings then check your plumbing for leaks. Avery common leak source is using the wrong size hose. The metal tubing is 1/4" OD but you must use 7/.32" ID hose not 1/4".

Finally, the wiper motors like a wee bit of lubricant now and then. A little squirt of WD40 does the trick in the inlet side of the wiper motors. This is the side that is sucking air in. The side away from the vacuum source. If you have the complete fording system plumbing then you must disconnect the right side hose from each motor to spray the WD40 in. If you have the later wipers with the squarish cover over the air in port then spray under that cover or remove it and spray into that inlet port.

There should not be any distributor loads on the vacuum system in the M38 and M38A1. They do not have vacuum advance pots. That vent line is only a vent line and the load on it does not change with throttle opening. The jeep wagons and trucks did have vacuum advance units.

RICKG · Post by **RICKG** » Wed Oct 03, 2012 7:59 am

Dont forget the fitting at the wiper tee from the dist.
The fitting has a restricted .040 orifice.

wilfreeman · Post by **wilfreeman** » Wed Oct 03, 2012 8:06 am

Hey Deadguy - I have the same problem, with new components also (including new fuel/vacuum pump). I traced mine back to the fuel pump lobe on the cam being worn down - not enough vacuum (or fuel for that matter, but it runs pretty good). I decided my winter project will be to rebuild my spare engine.

Anyway, something that I thought about doing to cure the passenger side running on acceleration/driver side stopping, would be to install a plastic 1/4" fuel shutoff valve before the passenger side motor. That way you can cut that motor off if the need arises. Temporary or permanent fix - you decide.

Bretto · Post by **Bretto** » Wed Oct 03, 2012 8:44 am

Theoretically speaking, if one had a worn cam lobe, couldn't you get away with removing the spacer between the pump and block to restore the flow. Or shim the pump off the block accordingly? I'm in no way saying this is the correct thing to do, just wondering.

wilfreeman · Post by **wilfreeman** » Wed Oct 03, 2012 9:04 am

Good point - maybe removing the 1/8" spacer (it's a civi pump) and use permatex instead. Like you said, it will move the foot closer to the lobe (about 1/16th" maybe), which SHOULD increase pressure and vacuum. That could be a fairly easy temp fix for my problem.

What's the word Wes?

Post by **wesk** » Wed Oct 03, 2012 8:44 pm

The word is proper troubleshooting. Doin't start swapping parts until you have a reason. Low wiper performance means 1st step is check vacuum reading with test gauge. It's all in the TM and easy to follow.