Can someone tell me what the gauge on my 77 F600 that reads "vacuum in HG" is for and what it is telling me? I am thinking it has something to do with the OEM governor? I removed the governor/spacer under the carb. and replaced it with a solid spacer. The truck runs great but the red light on the aforementioned gauge comes on under high RPM.

 

That just means that the gauge is reading in inches of mercury instead of water column. Hg is mercury in the periodic table of elements. If the gauge was calibrated for water it would read "Vacuum WC". Also, at high r/m there is little, if any, vacuum to measure upstream of the venturi because the throttle plate in the carburetor is open pretty wide.

 

The vacuum gauge's purpose is to indicate lugging conditions. On large diesels there is a BMEP governor that prevents it.

 

Thanks, so it is nothing to be concerned with then right? Exactly what purpose does it serve and why is the gauge even there at all? If the motor is lugging then the driver would know just like any other vehicle. With the red light it would seem logical that it was supposed to serve as some type of warning light that would require corrective action from the drivers part. Seems very odd to me.

 

Vacuum gauges were fairly common in prior years, especially for those who wanted to maximize fuel economy.

http://www.summitracing.com/expertad...ve/answer/3672

Consider it a "cool feature".

wrong info removed

See post 7, while vacuum gauges can be used for fuel economy, this is a brake vacuum gauge, as explained below.

 

Not odd at all if you know the purpose and know how to use it. Example: You could be pulling a load up a hill and you're only doing 35 mph, you're in 3rd under and your tach is reading 2,800 r/m and dropping. When the red vacuum light comes on you shift to 2nd over, etc. (although I wouldn't wait that long personally). Lugging is a serious condition and can do a lot of engine damage. There are many drivers that don't really know what lugging is, and wouldn't recognize it even.
.

 

That gauge is there to let you know that you have sufficient vacuum for the brakes to work properly.


Not anything to do with the engine lugging, at least how Ford installed the OEM factory gauge on hydraulic braked trucks. Insufficient vacuum will cause the brake booster to NOT function properly. And this is important to proper braking.


That is why the gauge has a RED light that comes on, normal vacuum gauges DO NOT have this feature, when that red light is on you DO NOT have sufficient vacuum in the braking system and the truck should NOT be driven until such time as the vacuum is back to the proper level.


FYI - This gauge is hooked up from the factory to the brake booster vacuum system, NOT the intake manifold. Which is why it does not function like a aftermarket vacuum gauge which is hooked to the intake manifold and reads direct intake manifold vacuum (Brake booster vacuum is protected by a one way check valve)


All this is covered in the factory owners manual, which come up from time to time on Ebay for very little money, and just like the factory service manuals, are a invaluable source of information for questions just like this.


Aftermarket vacuum gauges that are added on, are for engine lugging indication as mentioned, and are hooked directly to the intake manifold, and will go up and down rapidly if the throttle is stabbed. Your gauge does not do that, nor should it! At least as how it came from the factory.


This is a safety feature first and foremost!


David

 

Very good point David. The light would also indicate a failure of the vacuum system, like a large leak or broken hose. In that case I find it curious that my F-6 with Hydrovac brakes doesn't have this feature, nor is there any mention of it in the owner's and shop manuals which cover air brakes as well up to the F-8's. And FYI, the vacuum for the brake system DOES come from the manifold and it shares the vacuum source with the two-speed axle and speedometer shift circuits, and the windshield wipers. Where else? There were no belt driven vacuum pumps on board then. Also it would be highly unlikely that you would want to apply the brakes while pulling a load uphill.

 

OP can trace out where the vac gauge is sourced. Our '58 F600 and the '65ish C600 we had did not have vacuum gauges. Vacuum wipers and vacuum 2 speeds were long gone in '58; op has a '77.

OP has brake questions, so it would really be a good idea to trace it down--if it's brakes, best get that figured out.

 

Two different types of brake vacuum gauges (2557 & 2A036). 17360 is the tachometer.

 

To clarify, yes all the vacuum comes from the intake manifold, BUT unlike a regular vacuum gauge, the factory medium / large truck vacuum gauge sources its vacuum from the other side of the one way check valve, therefore it does not fluctuate up and down with the cracking of the throttle plates like a vacuum gauge does that is hooked directly to the intake manifold without a one way check valve. That was what I was trying to explain, and frankly needed to add more words to make it clear. I have never seen one of these trucks with a separate vacuum pump btw.


The function of this vacuum gauge is clearly listed in my 1970 and 1975 medium duty truck owners manuals. Not sure it was something that was done in the 50's though.


Vacuum wipers traditionally did NOT have a one way check valve nor reservoirs, so they would slow down / stop as intake manifold vacuum was reduced with higher throttle opening. Different system, cannot be compared to these brake systems.


Bill says it best - this is a BRAKE vacuum gauge, so it is not a intake manifold vacuum gauge. Hopefully that clears up the intended function.


David

 

Left: Scroll down to 2557 / Right: Scroll down to 2A036

 

I agree, and stand corrected. I'm looking at it from a '50's perspective, and it's given me some insight into upgrading my system.


Thanks.





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It is always hard, I look at it from a late 60's to early 70's perspective, and at times my an answers come up short for other decades / years


David

 

Thanks for that explanation and clarification. Good idea to show the driver if he has brake vacuum.