Greetings fellow enthusiasts,

I'm suffering a two fold issue with my vacuum wipers. They have always been very reliable. Something has changed and I can't figure it out. The wipers stall every time I put my foot on the gas. Usually they will come back to life if I release the accelerator but not always. Sometimes I need to nudge them physically to get them moving. At idle they are very strong. I've always kept them well lubricated. The strange thing is that the diff still shifts easily which is also vacuum operated. I put a vacuum gauge on the line and I'm getting around 18-20 at idle which is normal. I have the backup electric vacuum pump that would occasionally blow it's fuse but that was a minor nuisance because I only ever had to use it in the driving rain when going up steep hills. Now it has run out of steam too. I took it apart. Everything looks good and the windings passed basic tests. Ran it on the bench and it too was creating about 18 psi. Reinstalled it won't run the wipers and quickly overheats. I'm chasing ghosts. Any helpful advice greatly appreciated.

 

You may have a leak/worn motor. There are countless ways of varying effectiveness from using smoke machines to propane (or other fuels). You can try to revive your motor seals with ATF. There are specialty shops that will tend to your existing vacuum motor. A forum favorite is https://rebuildingtricowipers.com/what-we-rebuild

 

I appreciate the reply. As I said in my post, I've always kept the motor lubricated. I know many people restoring these trucks have this issue with dried out seals but I doubt that's the issue. When I bought my truck 20 years ago that was the case. It had sat for many years. I lubricated the motor and it's been great ever since. Until now. At idle I can hold onto the bar that connects the two and it pulls very strong, as in, you can't hold it back. This only happens when the engine comes under load.

 

It is normal for vacuum to drop when engine is under load. It seems that isn't new to you. However, you perceive that the wipers behavior has degraded substantially, so something has changed. If it has been a bit, it may be worth while to just start from scratch and check the vacuum lines, check the motor and it's components and really service it up. I wonder if under enough vacuum, a leak isn't sucked closed, but with less vacuum, opens up and leaks...

And another pondering, how is your vacuum advance system?

 

The pivot shafts on my original wipers got very hard to rotate. I took them off and flushed the shafts with brake cleaner and shot them with penetrating oil, then motor oil, and they were like new. You might try that.

 

Good suggestions. I did just that. I had 3/16 hoses to the wiper motor, auxiliary pump and speedo shifter. I never had clamps on them because they were very tight. Seemed too tight so I bought new 1/4 inch hose and pinch clamps. No difference. I would have no problem tearing into the wiper motor like I have with literally every other piece of this truck but it just doesn't seem like it's the culprit. As I said, at idle it's powerful. I'm wondering about the vacuum distribution gizmo on the firewall. I've sprayed a little propane on it and there's no change in RPMs. I believe I'm getting full vacuum advance but it does seem like isn't developing quite as much power as it did after the engine was first broken in. Nothing dramatic. Just a little less. Wish I had a floor dyno!

 

When you say 'pivot shaft' are you referring to the shaft that the wiper arms are connected to? I've never dismantled them.

 

I forgot to mention that I ran across these folks years ago when I was just getting started. I thought I'd go back and see if they had some good info but their site is unreachable.

 

How does one lube the vaccum motor,and with what?
51 M3 here,mostly stock,engine and wiper motor are pure stk.
Gw

 

Yes. Once you disconnect the linkage, you should be able to tell if the pivot assembly moves freely.
They can't be disassembled, but you can remove them from the truck by disconnecting the linkage and removing the nut on the cowl. The outer end of the shaft is covered up by the knurled hub, so cleaning and lube'ing has to be done from the inside.

 

Normally you disconnect the hose running from the firewall to the wiper motor and stick it a container of your favourite oil. 3 in 1 or Marvel Mystery are ideal. Move the wiper arms back and forth and it will suck the oil up into the wiper motor. Remove the hose from the container and continue to move the wiper arms and any excess will run back out the hose. If the motor is completely stuck you may have to inject the oil and then lit it sit for a couple of days.

 

Been there done that! As far as dismantling goes. The only part of this truck that hasn't been previously dismantled is the flawless transmission. The wiper assembly moves very freely but if I end up removing it it wouldn't hurt to give it a cleaning as you've suggested.

 

Well I thought I should share my temporary fix. When I was diagnosing I found that when the wipers got stuck all I had to do was pull slightly on the crossbar and they would come back to life. It was raining this morning and I had to go fetch a load so I grabbed a bungee cord and looped it over the crossbar and down to the heater control. Worked like a charm. Now the wipers are strong even going up steep hills with a full load! Can't explain why this works.

 

Interesting! Would make me question the motor/seals condition.

 

I'm thinking more like you may have a burr or wear spot in the pivots and the linkage is hanging up in itself. You may need to take it apart and make sure the pivot points are all round and smooth, clean and freshly lubed. Any wear grooves or ovaling in the holes could be enough to hang up the linkage. Downward pressure like the bungee is adding may be enough to bypass the rough spot.