Installing VDO gauges

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While the E30 was without doubt the best performance vehicle of the Eighties, its Instruments were lacking some of the features of other manufacturers, especially those of the Volkswagen-Audi Group. To remedy that situation, here's how to add extra gauges to your engine.

Contents

Overview

The standard dash cluster will give you an indication of engine temperature, and this is taken from a basic sensor in the Cooling system. This is great for making sure you don't overheat, but it's useless for telling you the condition of your Oil, which is far more important when it comes to racing or other spirited driving sessions. Neither does it tell you the status of your electrical system or the boost flow of your turbo. All of this information is available in the form of additional gauges that you can fit to your car.

The gauges are driven by sender units. which are the measuring sensors that you fit to the engine; you can't use any of the existing sensors in your BMW engine. No matter what gauges you use, it is essential to pair the gauge with the appropriate sender, so if your oil pressure gauge runs from 0 - 5bar, you will need a 5-bar sender; a 10-bar sender will give completely false readings, and vice versa.

The standard set of gauges fitted to similar-period Audis offered Oil Temperature, Oil Pressure and Volts, all manufactured by VDO. Gauges from slightly earlier vehicles such as the VW Golf II can also be fitted, and differ only in the data range presented.

Gauges

Oil Temperature

Cold oil dramatically increases engine wear, so thrashing the engine before the lubrication is ready will do serious damage to your car's internals. Similarly, oil that gets too hot won't lubricate as much as it should, so even steady use will damage the engine more than it should.

For that reason, fitting an oil temperature gauge will let you know how your oil is really doing, and when it's ready to give it some beans.

The Audi Oil Temp gauge has a range of 0-170°C. VW gauges will show 0-150°C. It is also possible to find 0-120°C gauges; some will say Oel (oil) while others will show the standard Coolant image, but they are identical units underneath.

The oil temp gauge is driven by a single thermistor sender. This little unit looks like a bolt with a pin on top, and has no moving parts. Once screwed in and wired up, a basic current is passed through the sender, earthing through the engine. As the temperature rises, the sender's internal resistance is lowered, increasing the current and therefore increasing the power to the gauge. This makes the needle move more to one side, and therefore reading a higher temp. How simple. What's important is to use the right sender for the gauge, so here are the VDO part numbers to order the appropriate sender (if you haven't got it already):

M10 x 1.0 M12 x 1.5
120°C 049-919-501
150°C 323-804-006-005D or 323-804-006-006D 323-801-010-003K
170°C 049-919-563B

Oil Pressure

Knowing the current engine pressure will tell you immediately when something's gone wrong in the engine, whether it's a blown head gasket or a broken crank bearing. If you keep your eye on the gauge, you should be able to see the pressure drop and turn off the engine immediately, before any lasting damage is done. This is particularly important in performance engines that take a lot of punishment, but is equally valid on long-life engines that have been running for 20 years without a rebuild. The oil pressure gauge can save the life of your engine; by the time your standard oil pressure warning light glows, it's often too late.

Oil Pressure responds to heat too, but that doesn't mean it's a duplicate of the temperature gauge. Of course, from cold the oil pressure will read very high as the oil pump tries to move the thicker sludge around your engine, and this will drop as the temperature increases, but the Oil Pressure gauge will give you a much more dynamic view of what the oil is doing. As soon as you put your foot down you will see an significant rise on the gauge, and letting off the throttle will immediately make the needle fall. In this way, you can learn how your engine responds to being pushed hard.

Oil Pressure gauges from Audis usually run from 0-5 bar, while earlier VW gauges show 0-10bar.

The Oil Pressure sender is the most significant part of the system, and should be a fat metal canister with a threaded bolt at the bottom and one or two pins at the top. These top pins will either be spade connectors or threaded posts with nuts; both can be used in your E30. If your sender has two top pins, it means you don't need the oil pressure switch which is currently fitted to your engine, since that function is built into your new sender.

The most important part is the pressure rating of your sender; on the side of the bottom bolt face should be a pressure rating; 0-5 or 0-10. It is essential that this matches the range of your pressure gauge. The sender part numbers are:

1/8" NPT M10 x 1.0 M12 x 1.5
0-5 bar 360-081-030-010 360 006 360 007
30/10 30/2 30/6
0-10 bar 360-081-030-138 360 023 360 02
30/138 30/9 30/22

Volts

Some consider the Volt Gauge or Voltmeter to be filler; an extra dial to make the cockpit look more advanced, more technical. While it's certainly not as crucial for the engine as the Oil gauges, the Volt gauge is very important for assessing the health of your charging system, and lets you know how your alternator is doing, whether your battery is up to the job and whether you've got a serious power failure on your hands.

Unlike other gauges, the Volt gauge needs a simple connection to a switched 12V supply; there are no senders required to make the gauge work, and is therefore a very simple addition to the instruments.

Fitting

Gauges

Gauge mounting plate
Gauge mounting plate

The gauges you will be using are most likely the 52mm versions fitted to Audis and VWs. Sometimes these will come mounted in a plastic frame, but are easy to disassemble.

On the back of each gauge, you will see two threaded posts. These are the mounting arms for the mounting plate, which is a C-shape of metal with two holes to fit onto the mounting arms. The plate is held in place with two furled nuts. The idea is that the plate pushes against the back of the fascia, sandwiching the gauges in place. If you bought gauges without the mounting plate included, you will have to get creative and make your own.

Depending on where you want your gauges, you will have to fabricate your own fascia or buy one from our Zone traders such as Kos. To make your own:

  • Decide where you want to place your gauges, and take some measurements
  • Find a piece of plastic to use as a fascia, and cut it to size so that it fits the area you want it
  • Using a hole saw, drill holes the same size as the BODY of the gauge, but not as big as the face
  • Drop the gauge into the hole from the outside
  • Turn over, and fit the mounting plate onto the mounting arms. The feet of the plate should be pressing against your piece of plastic
  • Screw on the furled nuts until the gauge is locked in place tightly
  • Depending on placement, you will need to devise a method to hold the fascia in place

Placement

Tuning companies such as Hartge and Alpina offered additional pod units that sat on top of the dashboard; these were either extensions of the main instrument binacle, or standalone pods that sat on either in the centre or the passenger side of the dashboard. By placing the gauges on the same level as the other instruments, they gave the driver a more natural viewing position.

However, it is very hard to get hold of these pods, and aftermarket alternatives are of generally poor quality. They also necessitate drilling, cutting and gluing the dashboard, which is something you may be very reluctant to do.

Because of this, we recommend you choose one of these locations to fit your gauges. All three will necessitate a small amount of fabrication, but this can be done with standard plastic sheet rather than building a complete gauge pod.

Top Air Vents: This is where Alpina mounted their infamous Digital Dash Unit. By popping out and blocking up these central vents, you will be able to mount up to four gauges, or keep one vent and fit two gauges. This places the gauges in the most visible position, at the expense of losing an element of the air vent system, which is no real loss.

Ashtray: A very popular choice, and one of the Zone team sells fascias ready made for this location. The gauges are not obscured by the steering wheel, and no permanent damage needs to be done to the interior to mount the fascia. The downside is the loss of the ashtray, which is unacceptable for smokers and people who keep small change in their cars. Three gauges can be fitted here, or just two while retaining the cigarette lighter.

Under Heater Controls: Often referred to as the cubby hole, the area above the ashtray and below the heater controls seems perfect for an extra panel; so good that this is where Hartge often mounted their instruments. Placing your gauges here means you don't have to sacrifice any other features of the Interior.

Senders

The Oil Temperature and Oil Pressure gauges are driven by senders (not sensors), which need to be screwed in to the engine to take their readings.

This is the trickiest part of the installation. If you removed the senders from the donor VW or Audi, then the threads will be M10 x 1.0. If you bought them from a popular online auction site, they could even be 1/8" NPT thread. Either of these are incompatible with BMW engines, which use M12 x 1.5 as standard.

To solve this issue, we are going to use a series of T-pieces and hole converters to make your senders compatible with the holes in your engine. No drilling or tapping is required.

M12 long T-piece
M12 long T-piece

Depending on the senders you have, these are parts you will need:

T-Piece Converter
M10 x 1.0 LMA052/M LMA064/M
1/8" NPT LMA052 LMA064

All parts can be bought from LMA Performance or Demon Tweeks.

The T-piece will screw directly into the engine, and has one M10x1.0mm hole on the side to screw in one of your senders. It has an M12 x 1.5 hole at the end, which can accomodate either an M12-thread temperature sender, or can be converted using the hole converters listed above. When screwing in any new senders, remember to use the appropriately sized copper crush washers, to properly seal the mounting surfaces.

Oil Pressure Switch, to be removed
Oil Pressure Switch, to be removed

To fit these pieces, we'll be removing the factory-installed Oil Pressure Switch, and using that hole to mount our new senders. This presumes you have a two-pole Oil Pressure Sender, which can replace the original switch. If you have a single-pole pressure sender, you need to find a more creative solution involving more than one T-piece. But where is the existing pressure switch?

M10

On the humblest of E30 engines, the Oil Pressure Switch is mounted on the oil filter housing, at the front of the engine on the intake side. It should be poking upwards. By removing this and screwing in our T-piece vertically, the new Oil Pressure sender can be fitted to the side hole, and the Temp sender screwed in at the top.

M20

Senders mounted to an M20
Senders mounted to an M20

For the mighty M20 the Oil Pressure Switch sits at the bottom of the block, under the exhaust manifold.

It is recommended to mount the smaller Temperature sender onto the side of the T-piece, where it is closer to the block and therefore better at reading the oil temp. The Pressure sender will then screw into the end of the T-piece and face downwards.

M40/M42

The M40 and M42 engines are best suited to fitting these gauges. For these engines, locate and remove the oil pressure switch from the side of the oil filter housing, under the intake manifold. Here we can use the same T-piece solution as for the M20 engines, or we can take advantage of the blanking plug here.

M40 and M42 oil filter housings do have an extra hole, into which an extra sender can be screwed. Again, you will need to use one of the listed hole converters to make the hole compatible with your sender, but in this way we can use two hole converters instead of one converter and one T-piece.

M5x

The M5x Oil Pressure switch is a different design to the standard E30 unit, but its operation is the same. The oil filter housing is located on the intake side, just above the alternator, and the oil pressure switch screws into the back of it. This can be removed and replaced with a T-piece if using a double-poled Pressure sender.

Wiring

rear view of a gauge
rear view of a gauge

Once your gauges are physically mounted and senders screwed in, it's time to link them together. This is relatively simple, and needs little more than some basic wiring cable, some spade connectors and a soldering iron or cable crimper.

Power and light wiring looms
Power and light wiring looms

Look at the back of the gauges and familiarise yourself with all the connectors. At the top are the lamps, which may be a single spade connector, a two pin plug or a length of wire. At the bottom will be two or three spade connectors, and these will power and drive the gauge. They are marked:

Power

All gauges need power; your Volt gauge would be pointless without it. This means finding a 12V supply and a suitable ground close to where the gauges will be mounted. Presuming they'll be somewhere around the centre console, you can either use part of the existing stereo wiring, or take a power source from underneath the cigarette lighter; look for a red/green for permanent live, white/purple for switched live and brown for earth.

BE CAREFUL! If you've never played around with car wiring before, get someone experienced to help you. Playing with wires can result in electrical fires which will destroy your vehicle!

Once you've found out where to take a power feed from, you need to make a series loom (see pic). This is a length of wire with three female spade connectors in series, approximately 10cm/6" apart. The three connectors will fit one terminal on each gauge, and the far end will be connected to your power source. You need two of these looms; one for live power, one for earth. Make sure that all of your terminals are protected with heat-shrink insulation or electrical tape.

Lights

It'll be no fun driving at night if you can't see your newly-installed gauges, so each comes with a simple illumination bulb to light up the dial. These bulbs will have either one or two terminals or wires from the back of the gauge; one for power, one for earth. If only one terminal or wire is present, this will be for 12V power, and the lamp will ground through the standard power circuit.

Again, these lights need to be wired using a series loom; use insulated spade connectors to bridge your gauges together, and this time feed them to a 12V switched feed from the driving lights. On BMWs this is a grey/red wire, and there will be some running behind the stereo and under the cigarette lighter. Again, safety is paramount here if you are cutting into a power circuit; use common sense and be careful!

Data

The simplest part of the wiring. You need to run a cable from the terminal of your sender to the G-terminal of your gauge, and you can use whatever method you like. Some bodge it and use speaker cable, others use proper wire, but either way you need to find a path.

Start inside the car. Get down on your knees and look at the bulkhead in the passenger footwell (RHD cars). Under the glovebox you should see the main Loom going through the bulkhead via a big square rubber grommet. You need to get your own wires through that same hole to the engine bay. I don't care how, as long as you're careful and don't damage any other wires, but try.

You need to get two wires through that hole; one for each Oil gauge. Once that's done, find them in the engine bay (poking under the brake servo), and run your wires to your senders. Use cable ties to hold them in place, and keep them away from the engine block or exhaust, or you'll simply melt them and have to do it all over again.

Once your wire ends are where you want them, attach the appropriate ends; a female spade connector for the Oil Temp sender, and either another spade or a ring terminal for the Oil Pressure sender, depending on which style of sender you have.

The Oil Pressure Sender should have two terminals:

  • G - terminal for Oil Pressure Gauge
  • WK - terminal for Oil Pressure Light on the dash cluster

At this point, you should have a loose plug on the engine loom from where you removed the Oil Pressure Switch. Take a small length of wire (2-3") and add a male spade connector at one end, and either a female spade or ring terminal at the other. Use this new wire to connect the Oil Pressure Switch plug to the WK terminal of your Oil Pressure Sender, so that your dash light works properly.

With your senders wired up, your gauges should now be ready to go!

Common Problems

Oil Pressure Light On

Shut off the engine immediately and check for leaks! If you haven't tightened up your T-piece and senders properly, or didn't use copper crush washers, you are risking an oil leak. Running the engine will squirt out all your precious fluid, and you will kill your engine.

If you are sure there are no oil leaks, make sure that your little bridge loom (from the sender to the switch plug) isn't touching anything else metal such as the engine block.

Needles Don't Move

Check all the connections on the back of your gauges, and on your senders. It's very common to get the wires mixed up, meaning no power gets to the gauges.

If the connections are ok, check the fuses. You may have shorted out a connection while making your power wiring, and killed a fuse along the way. If a fuse has blown, replace it. If it blows again immediately, you have a more serious fault; you are either overloading that circuit, or your wiring has shorted somewhere. Under NO circumstances should you increase the size of the fuse to remedy the problem.

Needles Move Too Far

If this is your oil pressure gauge, then don't worry; oil pressure is highest when the engine is cold, so expect to see a maxxed out reading when first starting up. If this doesn't drop as the engine warms, then check your part numbers to ensure that you're using the appropriate sender for the gauge.

Inaccurate Readings

This is a common cause of mismatched gauges and senders, especially when it comes to the temperature gauge; many people have commented on oil temps of 130°C when the reality is more like 80°C. If this is the case, check your part numbers.