Re: 1951 200 Vacuum Wiper Motor

Posted by HH56 On 2023/4/29 9:37:46

Not really. The wiper motor is almost always working off manifold vacuum so what a gauge says at the manifold will almost always be what is read at the wiper motor. There may be a small contribution from the vacuum pump but unless manifold vacuum is abnormally low that is not the usual case.

On cars with vacuum wipers made prior to 55 the vacuum source starts at the manifold, goes to the top or vacuum section of fuel pump, thru pump, and on to wiper motor. Since the valve springs in the pump are relatively weak and manifold vacuum and flow volume is almost all the time greater than what can be produced by the pump, when you turn on the wiper motor the larger flow of the manifold vacuum just holds the valve springs open and pump simply becomes a flow thru portion of the line.

When engine is under a large load or the throttle is almost fully open the manifold vacuum and volume drops to a point low enough the springs in the pump can start working to close the valves in the pump. The pump then starts to work and becomes the stronger source to supply a limited amount of vacuum to the motor. While the strength may be similar to what is produced by the manifold, the volume is considerably lower. The pump is able to keep the motor running but without the volume, cannot keep it running at anything much more than slow speed. Typically is is just enough to keep the wipers from stopping completely.

A typical manual hand pump such as the Mityvac may be strong enough but not have the volume to work the motor -- and that is assuming there are no leaks. You should be able to see motor move slightly but it will probably take many squeezes to make it move an appreciable amount.