Vacuum guage installation - PeachParts Mercedes-Benz Forum

benmack1 · #1 02-12-2012, 09:54 AM

I'm guessing this has already been covered many times but not able to find what I want so i am asking.

I am installing a boost guage on my 93 300D, 2.5T. I would also like to put in a separate vacuum guage to monitor my vacuum pump workings (had that fail last year).

Anyone got a suggestion on what scale guage I should get (no idea how much vacuum I should get at idle and cruise for instance). I don't want to max out a guage or have one that has too big of a scale. I assume maybe a 30 mm Hg guage full scale or so but really haven't got a clue about this.

Just to be clear, I'll be installing 2 guages, one for manifold boost pressure only and one for measuring the vacuum line pressure (vacuum). Kind of hard to find guages too locally. All they have are the bidirectional vac/boost guages and obviously on the diesel that isn't what I need as those systems are totally unrelated unlike the gasser situation with a turbo.

2nd question.

Anyone got advice on mounting guages in the ashtray area. Can I pull the whole flipper door on the tray and disconnect the lighter or how to folks do this. On my W124, there isn't much room other than the ashtray that I can think of. (anyone ever mount on the left side of the dash above the vent or on the door pillar and if so, got any pics?)

funola · #2 02-12-2012, 11:12 AM

0-30" Hg vacuum gauge is perfect for both. Ashtray is perfect location for the gauges. remove the housing (2 phillips) and unplug it.

charmalu · #3 02-12-2012, 11:47 AM

I found this thread doing a search.

Boost Gauge

a good picture of 3 gauges in the ash tray

Charlie

Jeremy5848 · #4 02-12-2012, 11:59 AM

that "vacuum" and "gauge" are two of the most frequently misspelled words in this forum?

But anyway . . .

Almost all vacuum gauges sold into the automotive market in the US of A are 0-30" Hg. The typical MB diesel vacuum pump operates at 20 to 25 inches of mercury so you'll be fine with a 30" gauge. Vacuum gauges aren't much fun to monitor as they never change (unless there's a problem) so you forget to look at them.

Another option is to get an gauge cluster from an MB gasser. Swap the left-hand 1/3 (with 4 gauges) for your diesel's left-hand 1/3 (with 3 gauges). The fuel, oil pressure, and temperature gauges, in my experience, use the same meter movements in diesels as they do in gassers. I saw no difference when I installed the gasser gauges in my diesel (does not apply to tach and speedo -- they are definitely different).

The fourth gauge is called "Economy" but is simply a vacuum gauge connected to the intake manifold of the gasoline engine. You can connect it to one of lines from your vacuum pump and monitor the pump that way. The cluster in the following picture is actually from a 1984 300SE -- I wanted a panel without the "Unleaded fuel only" markings, see discussion, below.

The "Economy" gauge is not calibrated but you don't really need to know the exact reading, just use the gauge to monitor for unusual changes. If the needle suddenly shifts to a new place, you will know that you have a leak or a problem with the pump.

In my car, I have connected the "Economy" gauge to the line that goes to the transmission. If the tranny suddenly develops hard shifts, I should be able to see a change in the vacuum gauge, providing it is a vacuum problem.

This is the finished cluster before I installed it in my '87 300D Turbo. Note that you should get a panel from an early 124 or 126 gasser or the gasoline pump symbol will have an "Unleaded fuel only" note that you will have to paint out. The earlier cars didn't have that note.

Note: The red and amber LEDs are warnings for low oil pressure and high coolant temperature that I added. They blink when the oil pressure drops below 15 psi or the coolant temperature rises above 100°C.

Jeremy

mach4 · #5 02-12-2012, 12:05 PM

Quote:

Originally Posted by benmack1

I'll be installing 2 guages, one for manifold boost pressure only and one for measuring the vacuum line pressure (vacuum). Kind of hard to find guages too locally. All they have are the bidirectional vac/boost guages and obviously on the diesel that isn't what I need as those systems are totally unrelated unlike the gasser situation with a turbo.

I'm thinking of doing the same thing on my project. In my opinion, what you want is a dual boost/vacuum gauge, something like this...

And if you use a cheap and readily available switchover valve (solenoid-activated vacuum switch) you can monitor boost in the default and using a momentary switch mounted next to the gauge, switch to vacuum.

Checking vacuum is more of a diagnostic and monitoring function so having a separate gauge seems a little overkill.

Here's a switchover valve

And most people probably have one sitting around from when they disconnected their EGR. Maybe still under the hood if they used the BB technique.

benmack1 · #6 02-12-2012, 01:43 PM

I did realize I misspelled gauge and have been searching on 'Guage' for 2 days. Turns out it still brings up a fair amount of stuff, I guess others are products of the public school system like me

.

The full dash panel gauge swap is interesting as is the switchover idea. I think I really like the idea of a dedicated vac gauge anyway as long as I can find a place to mount it. Great ideas that folks have though. Alot of creativity from this community.

toomany MBZ · #7 02-12-2012, 02:47 PM

Nice work Jeremy.

benmack1 · #8 02-12-2012, 08:42 PM

I see some 0-15PSI boost gauges. I was planning on a 0-30, but would a 0-15 be better or likely to max out on my model turbo/car?

Jeremy5848 · #9 02-12-2012, 09:02 PM

0-15 psi should be fine. The stock turbo is supposed to max out at 13 psi. 0-30 psi tends to be easier to get but 0-15 will give you better resolution. Try eGauges "Vision" series at this link. The one I put in my '85 (and moved to the '87 when I sold the '85), eGauges #150-101, still works great after 5+ years.

Some folks have also had success connecting the gasser "Economy" vacuum gauge as a pressure gauge by moving the needle. You get only about 10 psi of range so you have to set it up to measure from 5-15 psi, which is the important part of the range anyway. I think Sixto experimented with that installation. The main advantage is for those people who don't want to sully the looks of their ride with obviously added-on gauges. If you have an artistic bent (not me!) you can relabel the gauge face from "Economy" to "Boost," which I think shows a lot of class.

Jeremy

sixto · #10 02-12-2012, 09:27 PM

Documented here - 87 300D gauge cluster innards and boost gauge mod

I only get 7 psi between posts so it's set for 5-12 psi.

My eye scans that corner anyway checking the temp gauge.

It is an economy gauge since the ALDA's in place. Black = no/low boost = economical, red = boost = not so economical.

I have a place to keep change.

I've since inked over the 'premium unleaded fuel only' text but I didn't go a good job of it. I only burn premium unleaded Diesel fuel anyway

Sixto
87 300D

benmack1 · #11 02-13-2012, 09:11 PM

Quote:

Originally Posted by Jeremy5848

0-15 psi should be fine. The stock turbo is supposed to max out at 13 psi. 0-30 psi tends to be easier to get but 0-15 will give you better resolution. Try eGauges "Vision" series at this link. The one I put in my '85 (and moved to the '87 when I sold the '85), eGauges #150-101, still works great after 5+ years.

Some folks have also had success connecting the gasser "Economy" vacuum gauge as a pressure gauge by moving the needle. You get only about 10 psi of range so you have to set it up to measure from 5-15 psi, which is the important part of the range anyway. I think Sixto experimented with that installation. The main advantage is for those people who don't want to sully the looks of their ride with obviously added-on gauges. If you have an artistic bent (not me!) you can relabel the gauge face from "Economy" to "Boost," which I think shows a lot of class.

Jeremy

Thanks Jeremy, exactly the info I need. Not an originalist here, I just want a good reliable guage to look at. I am not sure I can pull off the dash replacement without making a mess

. I also like the price of the VDO guage. Thanks big time.

GregMN · #12 02-13-2012, 09:21 PM

Quote:

Originally Posted by mach4

Here's a switchover valve

That valve is good for 8 psi max.

I use the "economy" gauge as a fuel vacuum gauge so I can tell when a filter is clogging long before it becomes an issue. I noticed this photo was taken about 35k miles ago.

Jeremy5848 · #13 02-14-2012, 12:46 AM

Quote:

Originally Posted by GregMN

I use the "economy" gauge as a fuel vacuum gauge so I can tell when a filter is clogging long before it becomes an issue. I noticed this photo was taken about 35k miles ago.

An interesting concept. How is the vacuum gauge "wired" to show filter clogging?

sixto · #14 02-14-2012, 01:48 AM

If it's upstream of the lift pump you can only monitor the small filter. You might need a check valve so you don't get fuel in the cabin when you park nose down with a full tank. Or is there enough air in the line to resist backflow? Are such gauges jittery being so close to the lift pump?

Sixto
87 300D

GregMN · #15 02-14-2012, 10:41 AM

Quote:

Originally Posted by Jeremy5848

An interesting concept. How is the vacuum gauge "wired" to show filter clogging?

First off the car runs on WVO so the entire plumbing of the fuel supply to the lift pump has been changed.

Original diesel fuel flow: tank > inline filter > fuel heater > lift pump > diesel filter > injection pump.

Modified diesel fuel flow: tank > inline filter > fuel heater > diesel filter > fuel selector valve > lift pump > injection pump.

VO flow: tank > short heat exchanger > flat plate heat exchanger > heated fuel filter > hose in hose fuel line > fuel selector valve > lift pump > injection pump.

There is a "T" in the fuel line between the selector valve and the lift pump. The vacuum line goes from that "T" to the gauge. Since the line terminates at that gauge, no fuel can flow up the line because there is no place for the air to go to get out of it's way.

With unrestricted filters the needle is barely off the peg. On a long road trip I get several hundred miles notice that the filter needs to be changed. I can then change it at a place and time that is more convenient to me rather than spending "store bought fuel" while I look for a safe place to do it.

The needle not jittery at all.

A write up of the conversion is here: 1991 350 SDL Conversion to WVO