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  1. #1
    Sam Carlson
    Tutorial Bot euromobile900's Avatar
    Join Date
    14 Aug 2010
    Medford, MA
    '90 900 LPT with a flat-nose conversion

    VDO Extra Gauges - c900

    SPGs often come with a set of three extra gauges, most commonly oil pressure, engine temperature, and either volts or outside temperature. These are made by our favorite instrument company, VDO.

    Difficulty: 1/5

    Tools Required:
    -10mm wrench (socket with swivel will make it easier) for knee bolster removal (optional)
    -Phillips screwdriver for center console removal (mostly mandatory)
    -1/8-27 NPT tap for oil pressure sender adapter bushing (if you determine you need to make a bushing)
    -Electric drill and cutting oil (if you determine you need to make a bushing)
    -Bladed screwdriver for miscellaneous prying
    -Good wire-strippers/crimping pliers for doing wiring

    Parts Required:
    -3 gauges
    -Their associated senders
    -Hydraulic fitting to adapt, or adapter bushing for sender, if applicable
    -A gauge holder of some sort (not one that goes on the A-pillar...please...for the love of god )
    -Some wire of various colors (or one color if you are Rain Man)
    -A terminal block (optional to neaten installation)
    -Some female spade connectors and either crimp connectors or solder/iron/shrink tube
    -Possibly some electrical tape to bundle wires together to make a gauge harness. Not good form to use this as an insulator.

    When choosing what three gauges you want, think about what you are most paranoid about. In my case, this was oil pressure and charging state (voltage), because I've run out of oil pressure on some very cold starts, and my alternator has been wonky in the past. If you're a tuner, you might like a boost gauge or an air-fuel gauge connected to a wide-band lambda. My third gauge is a clock because my clock under the tacho doesn't work and I'm too lazy to replace it.

    Once you've got your three gauges picked out, you will need a place to mount them. Some people like to mount them in the radio DIN or lower DIN. Again, weigh your priorities. You can also mount them under the lower DIN, in a special SAAB gauge holder that makes your central cubby smaller. These pop up on ebay from time to time, or post a "Wanted" in the classifieds. I have seen c900s that have the central cubby chock full of gauges, with a custom aluminum or plastic plate fabricated to hold them. I think you can fit at least 5 gauges in here. I will be using the lower cubby gauge holder, as I managed to score a broken one for cheap and repair it with Gorilla Glue and bits of a Folgers coffee can lid.

    Procuring a set of gauges can get expensive. The closest current equivalent to SAAB VDO extra gauges is the VDO "cockpit" line, in 2-1/16" diameter. These are easy on the eyes and blend in with the rest of the c900 interior. They're available cheapest in North America on amazon.com and eguages.com.

    Or, you can go scrapyard hunting. They are to be found on some Porsches and VWs (and even some planes) as well as SAABs. An easy test to see if a scrapyard-sourced gauge is good is to connect its power (+) and ground (g) terminals to a 12v source, leave the sender (t) terminal disconnected, and see if its needle "pegs" or shoots to the top of the scale. Modern gauges have a softening circuit so the needle will slowly peg; this is normal. This will not hurt the gauge. If you can get gauges, but not senders, I know the oil pressure ones are standard VDO range, 10-180 ohms, and usually 5bar or 80psi (both use 80psi sender). It's useless to get an oil pressure gauge that goes higher than this, because our relief valve begins to open around 60psi anyway.

    If you can get the sender from the junkyard as well, go for it. These have variable ohms, so they're easier to test. More ohms means higher pressure, temp, etc. For coolant temperature, you can probably use one of the extra plug holes in the inlet manifold (like where the NTC sensor goes). For oil pressure, you'll need to be adaptable if you want to keep your idiot light and have a gauge as well (I recommend this because a big red light above the speedo is a hell of a lot more noticeable than a gauge with a low reading off to the side). Some people swear by the "sandwich plate" which goes between filter and filter mount:
    Name:  oilsandwich.jpg
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    This will allow you to keep your stock oil pressure sender (for idiot light) and put in a single-contact pressure sender, as seen here.

    If you want to do it a bit more elegantly, you can buy a VDO sender with a warning contact for your oil pressure light and a terminal for the gauge as well. It will look something like this:

    C900s from before 1983 use 1/8 NPT thread for their oil pressure light sender, so you will need VDO part number 360-009 and you're all set. If you have a later c900 (which most of us do), you can use either VDO part number 360-028, which will screw right in in place of your stock sender, or you can use VDO part number 360-009 (which is usually cheaper) and make an adapter out of a hydraulic fitting. Do not use the adapter bushing from eGauges. It's brass and doesn't seal well. Ask me how I know! I elected to make myself one out of a hydraulic fitting, which resulted in a beautiful adapter bushing that I would trust with my life. To fabricate this, I needed a 1/8" NPT tap, which cost me all of $4, a file, and a drill. Easy.

    Adapter bushings more suitable for oil pressure senders are available on Amazon.

    If you want to save yourself all the trouble of sourcing parts from these diverse sources, you could just order a gauge kit from SAS (North America) but they command a premium, and this thread contains all the knowledge to do it for substantially cheaper. It depends what your time is worth to you.
    Last edited by euromobile900; 11 November 2010 at 07:17.
    Ask me a question about your c900! I promise I either can answer it or know someone who can

  2. #2
    Sam Carlson
    Tutorial Bot euromobile900's Avatar
    Join Date
    14 Aug 2010
    Medford, MA
    '90 900 LPT with a flat-nose conversion
    Ok so at long last, here is the installation:
    Now that you have your kit of gauge material together, it's time to start installing them. You'll need the wire, spade ends, and the terminal block for this step, along with your crimping pliers and wire strippers.

    The first step is to make a wire harness for the gauges.

    The rear of each gauge should look somewhat like this:

    Name:  ClockandTempGauge.jpg
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    On the L is a clock, and on the R is a VDO Cockpit gauge.

    The oil pressure, engine temp, and outside temp gauges will have three terminals on coming from the white plastic. One is marked (+), one (-) and one T. The T terminal is the one that goes to your sender. The positive is obviously for 12v and the negative is obviously ground. A word of note now: the gauge works by measuring ohms between sender wire and ground, so it's often beneficial for troubleshooting if your gauge doesn't work after installation to run a ground lead directly to your engine block (I use a screw on the intake manifold) to eliminate any stray ohms that may screw up your reading. Like this:

    The Volt gauge has only hot and ground, obviously. All gauges come with backlights. Older ones use a small bulb, and newer ones use a bigger one. The new one is a 194 wedge base, if you want to order LEDs to fit. Green LEDs would match the c900's backlighting, but I find that since the main gauges aren't really lit up green, it doesn't matter that the extra gauges aren't either. You could always change the main gauge bulbs at the same time.

    So your wiring harness will need at minimum 3 positives, 6 grounds (3 for lights and 3 for gauges), 3 switched positives for lights, and however many specialty wires (for senders or a constant 12v for a clock) you want. Since the positives and grounds can be consolidated, you're looking at 3 terminals in your terminal block to start out with. Then add one for each sender.

    To put your wire harness in, you'll probably need to remove the knee bolster. You could maybe do it without doing this, but it's significantly harder.

    You can run your signal and engine-block ground wires through the hole on the left side of your car, beneath the knee bolster, which goes right into the engine bay. I just snaked them through next to a vacuum hose, and between the brake booster and coolant tank, like so:

    I got the 12v for the backlights from the cigarette ash tray light, the 12v constant (clock) from the radio (gray wire), and the 12v switched and ground from the cigarette lighter. At least this way I know which fuse my gauges are on.

    Once you've got your harness built, CHECK IT with your voltmeter and/or a test-lightbulb!! Removing the knee bolster is a lot of work, so you don't want to fit it and then find out your wiring is wrong.

    Here's a final picture of the terminal block with wires coming in on one side, and wiring harness coming out on the other. You can see positive feed (red, splits to three red), floating ground to engine block (yellow, splits to 3 brown), and oil pressure sender (blue, to green). Clock constant positive is the other green one that hasn't been plugged in yet, and backlight wiring is not run through this terminal block, because the terminal block I had only had four terminals. If you are buying a terminal block, count how many you need before you order.

    All VDO gauges mount to the gauge panel either by a ring screwing on from the back, or a bracket mounting them to the back. I find it easiest to mount the gauges to the panel, connect the wires...

    ...and then stuff the panel into the DIN slot or cubby. When you're done, it should look and perform just like stock!
    Last edited by euromobile900; 11 November 2010 at 06:57.
    Ask me a question about your c900! I promise I either can answer it or know someone who can



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