Test an elec vacuum pump on my 85 300D - Page 2 - PeachParts Mercedes-Benz Forum

funola · #16 10-05-2015, 01:01 PM

Finally got the videos sorted and some of them uploaded.

https://www.youtube.com/watch?v=oZ8XQAew-Ok

I pulled over 1/2 a block from an on ramp, shut the engine off with the elec vac pump running then started the camera to show the sound of the pump running (gauge at 15" HG {0 to 8" HG is green, 8 to 15 is yellow, 15 to 28 is red}). Started the engine and merged onto on ramp traffic with impatient drivers behind beeping. The gauge in front of the vac gauge is a boost gauge.

As you can see, during cruise it is at around 10 to 12 " of HG, hard acceleration drops it down to 8" of HG. Shifts were smooth with no clunks or flaring. Braking was normal and does not show on the gauge (unless I pump it many times in succession).

I have since routed the cable from the pump inside with a cig. lighter plug so I can manually turn off/on the pump. During a steady cruise, if I turn off the pump, the reservoir vacuum drops from 15 to 10 " HG in around 15 seconds. At 10" of HG, shifts are still normal. When it goes below 6" HG is when the shifts get rougher. If I set the vacuum switch to 11" HG to turn the pump on, it may limit the duty cycle of the pump to be a practical system IMO.

I did this elec vac pump test because mach4 says it is not viable for automatic transmissions just to see for myself if true.

My results says otherwise. I dunno what's different in his setup. Different vac pump? Different reservoir? Differences in VCV and transmission modulator? Different leakage rate in the system?

mach4 · #17 10-05-2015, 02:44 PM

Quote:

Originally Posted by funola

I did this elec vac pump test because mach4 says it is not viable for automatic transmissions just to see for myself if true.

My results says otherwise. I dunno what's different in his setup.

Your tests tend to confirm my conclusion. The design vacuum for a 617 is 22" at sea level. The system should hold 22" at all times except for agressive braking where the vacuum will drop momentarily (or high altitudes). With the VCV continually bleeding vacuum, the duty cycle of a typical automotive electrical vacuum boost pump designed for brake booster duty, is quickly be exceeded. This might be different perhaps with a large capacity continuous duty vacuum pump. My tests were done using a VW vacuum pump that just didn't have the capacity. I know I could have pursued a belt driven pump, or a big honkin' electric one and upgraded the alternator, but that put things well outside my design parameters.

The trade off of designing a large custom vacuum system vs the minuscule chance of a mechanical pump grenading just did not pencil out in the end - particularly since vacuum pumps are easy to inspect and rebuild should bearings start to fail.

I don't deny that it's [I]theoretically possible[/] to run at 15", but the safety margin is just too thin for my taste.

For me, your test does nothing but reinforce my decision to abandon the concept of an electrical vacuum pump for automatic transmission MB diesels. As others have demonstrated, for a manual transmission MB, it's definitely a viable solution.

Of course, everyone is entitled to a different view and different design parameters and the risks associated with operating at different safety margins.

This is good data - thanks for sharing. Definitely helps us all learn and understand what's going on.

dude99 · #18 10-05-2015, 03:02 PM

Quote:

Originally Posted by Mxfrank

I'm pretty sure that every engine that has a VCV has a vacuum pump with two isolated ports.

The pump you have pictured is an early one that had the vacuum outlet plumbed into the air filter. Later designs eliminated this and just vented internally to the crankcase. Both designs have only one vacuum port.

dude99 · #19 10-05-2015, 03:02 PM

I might go the electric route if the mechanical pump in my E290 ever goes out. Since I have the 722.6 transmission I don't have to worry about vacuum bleed for the transmission.

funola · #20 10-05-2015, 03:05 PM

Quote:

Originally Posted by mach4

Your tests tend to confirm my conclusion. The design vacuum for a 617 is 22" at sea level. The system should hold 22" at all times except for agressive braking where the vacuum will drop momentarily (or high altitudes). With the VCV continually bleeding vacuum, the duty cycle of a typical automotive electrical vacuum boost pump designed for brake booster duty, is quickly be exceeded. This might be different perhaps with a large capacity continuous duty vacuum pump. My tests were done using a VW vacuum pump that just didn't have the capacity. I know I could have pursued a belt driven pump, or a big honkin' electric one and upgraded the alternator, but that put things well outside my design parameters.

The trade off of designing a large custom vacuum system vs the minuscule chance of a mechanical pump grenading just did not pencil out in the end - particularly since vacuum pumps are easy to inspect and rebuild should bearings start to fail.

I don't deny that it's [I]theoretically possible[/] to run at 15", but the safety margin is just too thin for my taste.

For me, your test does nothing but reinforce my decision to abandon the concept of an electrical vacuum pump for automatic transmission MB diesels. As others have demonstrated, for a manual transmission MB, it's definitely a viable solution.

Of course, everyone is entitled to a different view and different design parameters and the risks associated with operating at different safety margins.

This is good data - thanks for sharing. Definitely helps us all learn and understand what's going on.

Not so fast! My vacuum gauge is tee-ed at the VCV input. Where did you put yours? I swapped back to the mechanical vac pump and guess what? It behaves almost exactly the same as the elec vac pump per the vac gauge! Idled at 15" HG, drops at about the same rate with throttle input and cruise at 10 to 12" HG. The only difference I noted was vacuum recovered a little faster after a drop with the mech pump. The gauge tee is after some restrictor orifices, maybe that is why I am not seeing anything higher than 15" HG. I think if I measure vac at the pump, the numbers will go up. I may just try that.

This is just a test to satisfy my curiosity. I am not ready to pull the mechanical pump and replace it just yet unless it decides to grenade and the engine survived.

mach4 · #21 10-05-2015, 03:22 PM

Quote:

Originally Posted by funola

Not so fast! My vacuum gauge is tee-ed at the VCV input. Where did you put yours?

Ahhh, that makes a huge difference! As part of the research process I teed into the VCV to verify it was working properly, but my final testing was in the main line to to the brake booster.

Sorry I missed that little detail

funola · #22 10-05-2015, 03:36 PM

Quote:

Originally Posted by mach4

Ahhh, that makes a huge difference! As part of the research process I teed into the VCV to verify it was working properly, but my final testing was in the main line to to the brake booster.

Sorry I missed that little detail

On which side of the main line did you tee in? Pump side or brake booster side?

funola · #23 10-05-2015, 03:41 PM

Quote:

Originally Posted by mach4

........I know I could have pursued a belt driven pump, or a big honkin' electric one and upgraded the alternator, ........

My pump is rated at around 7 amps @ 12 V, however when I measured current, it was 3 amps. Maybe my ammeter was off, but even at 7 amps, it's not a big honkin pump.

mach4 · #24 10-05-2015, 04:11 PM

Quote:

Originally Posted by funola

On which side of the main line did you tee in? Pump side or brake booster side?

Pressure is pressure....no difference which side.

I tapped in by temporarily disconnecting "other consumers (c)". EGR is disconnected as is "blue saucer", so this would be a direct port into the main line. My current vacuum gauge is tapped in there too.

funola · #25 10-05-2015, 05:49 PM

Quote:

Originally Posted by mach4

Pressure is pressure....no difference which side.
e too.
.......

There is a check valve between the brake booster and pump in my hard vac plastic line. Are there no difference in pressure on either side?

I moved the tee for the vac gauge to the inlet of the vac reservoir. At idle, I am seeing 17" HG now, 2" higher than when the tee was at the VCV. Reving the engine does not drop the vac now, whereas vac would drop when the gauge tee was at the VCV. I have not taken it for a drive yet with the new gauge location.

mach4 · #26 10-05-2015, 06:14 PM

Quote:

Originally Posted by funola

There is a check valve between the brake booster and pump in my hard vac plastic line. Are there no difference in pressure on either side?

There are actually two check valves in the main vacuum line between the pump and brake booster. The one on the pump prevents vacuum from bleeding back through the pump. The one in the line is to prevent vacuum loss from the brake booster if vacuum is suddenly lost. Under normal operation there will be essentially the same pressure on both sides of the check valve.

A relatively small vacuum leak can have significant results. When one of my hvac vacuum pods tore, it made shutdown difficult.

funola · #27 10-05-2015, 07:17 PM

Quote:

Originally Posted by mach4

There are actually two check valves in the main vacuum line between the pump and brake booster. The one on the pump prevents vacuum from bleeding back through the pump. The one in the line is to prevent vacuum loss from the brake booster if vacuum is suddenly lost. Under normal operation there will be essentially the same pressure on both sides of the check valve.

A relatively small vacuum leak can have significant results. When one of my hvac vacuum pods tore, it made shutdown difficult
...........

Well, I am measuring essentially between the 2 check valves in the main line (same as you?) and I am getting 17" of HG at idle. I'll take a video during the road test.

mach4 · #28 10-05-2015, 08:00 PM

Quote:

Originally Posted by funola

Well, I am measuring essentially between the 2 check valves in the main line (same as you?) and I am getting 17" of HG at idle.

Here is where I'm currently taking my vacuum reading. It's a little tough to follow in my rats-nest of wires and vacuum lines, but I've shown the routing with a yellow line.

The first gauges screen is one I took just now showing vacuum at idle and the second one is one I took some time ago on a hard pull. Note - 1241 F EGT and 9.9 psi boost. Also note the fuel pressure differential - the first one is after stretching the overflow spring. And finally the 146 intake temp is just heat soak from having sat for 10 minutes after getting back from a trip around town.

funola · #29 10-05-2015, 08:51 PM

Quote:

Originally Posted by mach4

Here is where I'm currently taking my vacuum reading. It's a little tough to follow in my rats-nest of wires and vacuum lines, but I've shown the routing with a yellow line.

The first gauges screen is one I took just now showing vacuum at idle and the second one is one I took some time ago on a hard pull. Note - 1241 F EGT and 9.9 psi boost. Also note the fuel pressure differential - the first one is after stretching the overflow spring. And finally the 146 intake temp is just heat soak from having sat for 10 minutes after getting back from a trip around town.

Are those vac numbers from the mech. pump? I'm more interested in the elec pump numbers.

mach4 · #30 10-05-2015, 09:07 PM

Quote:

Originally Posted by funola

Are those vac numbers from the mech. pump? I'm more interested in the elec pump numbers.

Those are indeed the engine driven pump numbers.

Once I determined that the VCV continually bled vacuum (and not just bleeding vacuum down to the level required to send the simulated throttle position to the transmission) and the capacity of my electric vacuum pump was not capable of maintaining vacuum within a reasonable duty cycle (30% or so) I abandoned the project.

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