Vacuum pump and vent controls

[Jgreemo]

Hi,

I recently noticed that my A/C vents were not operating and when I checked the vacuum pump, the line running from the pump to the firewall was completely missing. My question is, what was the original line like? The pump appears to have a 1/4" fitting off the T, but the firewall line is only like 1/8" vinyl. I put in a piece of vinyl vacuum line to make the connection but don't have a good seal on that firewall line since it's so small. Any ideas as to what GM used to step the diameter down to fit that skinny line?

I also don't see any vacuum reservoir in the engine bay, which I imagine is under the dash, so for now I'm assuming I can just make the connection directly from the pump to the firewall.

Thanks!
Jon

[Robyn]

Factory equip was a rubber adapter that connected to the plastic tube and then the fitting on the pump.

You can get a small rubber hose at the parts house that will connect to the plastic tube, and then use a brass or plastic coupler that will allow you to adapt from the small hose to the pump using a larger hose.

You can if you like, install a vacuum reservoir with a check valve between the pump and the reservoir.

This is not needed as the pump pulls full 26" hg all the time the engine is running.
Reservoirs are used with a check valve on gas engines to store a vacuum supply during hard acceleration (Wide throttle) when there is near zero manifold vacuum.
No reservoir is used in the diesel system as it is not needed.

Depending on the tranny used, the vacuum is only used for the HVAC system.
If the rig has a TH 400, then there would be a vacuum control switch that sends vacuum to the modulator on the trans.
Even this is easily supplied by the pump and is not affected by engine RPM or throttle position.

Most parts stores will have a selection of plastic vacuum line barb fittings that will get you connected.

Good luck

Missy

[DmaxMaverick]

......Depending on the tranny used, the vacuum is only used for the HVAC system.
If the rig has a TH 400, then there would be a vacuum control switch that sends vacuum to the modulator on the trans.
Even this is easily supplied by the pump and is not affected by engine RPM or throttle position.

Most parts stores will have a selection of plastic vacuum line barb fittings that will get you connected.

Good luck

Missy

All correct, except for the emission system (EGR) and cruise control, if it has them. They will be vacuum operated, as well.

[Robyn]

OMG I forgot about those two.

Yup, the EGR solenoid and the EGR valve, also the EPR valve too.

YUP YUP, and the cruise.

This vacuum stuff just sucks :eek:

Must be old timers setting in eh :D

[Jgreemo]

So I pulled and replaced my dead vacuum pump but still don't have any A/C vent controls and no defrost. I have good vacuum off the pump but it's fluctuating, which seems to indicate a leak somewhere. My question is, the manual shows a vacuum tank, but I can't seem to locate it as the diagram in the service manual is worthless. Anyone know where this is located?

I've reattached the pump to the small line coming through the firewall and checked the connections of the interior lines. All seem soft and pliable still and everything looks like it's all there. I'd like to check the tank, since that's where the manual indicates the troubleshooting should start.

Thanks
Jon

[DmaxMaverick]

It shouldn't have a reservoir, and I don't know why your manual thinks it should. Perhaps the diagram applies to gassers as well.

If you don't find any leaking lines, then the selector valve is suspect. It's in the HVAC control head. It's a rubber rotary valve, and over time the material shrinks or becomes otherwise distorted. The connectors at the valve can also leak. I've extended the life of many of them by greasing them with dielectric grease. Use enough to seal it, but not so much to block the ports.

[Jgreemo]

Thanks for the feedback Dmax. I replaced the selector valve and the vents are still not moving, so I think I must have a leak up behind the distribution assembly. I'm not getting any vacuum at the controls, so it's clear now the problem is between the firewall and the controls. I had gone through the lines that were visible and checked the connections and they all look good but clearly somethings leaking.

[DieselDavy]

Are the vacuum motors working? Can you apply vacuum to them and see that they actually work? I think they do die once in a while!

[Jgreemo]

Thanks Dave, I'll be checking those asap as well. The new pump seems to be working well but showing zero vacuum at the supply to the controls. Getting the distribution assembly down from under the dash isn't something I'm looking forward to.

[scottm]

Is there an alternative to the mechanical pump, or is it still the best option?

[Jgreemo]

The pump on these years is the distributor style, which in addition to generating the vacuum, also drives the oil pump. I believe later years moved to a belt driven model.