by Richard Atwell
Before jumping on the bandwagon and installing a set of VDO gauges be aware that the senders used have a limited accuracy (most of the gauges themselves are fine). Many people refuse to believe this is possible after spending a lot of money on them. Hoover once wrote, "You should also be aware of the fact that Volkswagen issued a number of Service Notes with regard to the VDO instrument cluster installed as an after-market item by many VW dealers, warning them that the instruments did NOT accurately reflect the state of the engine".
Use them mainly for noticing big changes such as temperature spikes (maybe a plastic bag got sucked into the intake vents and has plastered the grill protecting the fan). How will the "idiot" help you when there is a massive oil pressure drop while driving on the highway? Installing gauges will hopefully help prevent these events from turning into a costly repair assuming you are watching your gauges at the appropriate time. If you try to drive by them you'll go crazy watching the needle move.
The three most popular gauges, cylinder head temperature (CHT), oil temperature (OT) and oil pressure (OP), all have accuracy issues but they are useful for trending and preventing meltdown. Here's the skinny on why they aren't accurate:
The oil is heated by the friction of the contact surfaces within the engine and cooled by the fan directing air through the oil cooler. In order to provide adequate lubrication, the oil must be maintained at a suitable temperature. If it's too low, it will not burn off contaminants that have settled in the crankcase. When the temperature is too high it the oil thins and breaks down becoming a less effective lubricant (it loses its shear strength). The main reason to check your oil temperature is to ensure your oil is not overheating because the stock engine is designed to warm up the oil appropriately. When it does overeat you will also notice an oil pressure loss. This is less of a problem for synthetic oils that can be heated to 300F but pure petroleum based oils have a rated viscosity at 212F and begin to break down at 250F where synthetic oils are still comfortable.
1. The senders are not mounted in an ideal place to take the oil temperature which varies throughout the case. The oil pressure sender is located after the oil cooler so that's not a good place to tee into. Hoover says the best place to mount one is right before the oil pump using the oil gallery plug behind the oil pickup tube. This is because the oil that has just been through the engine and is on its way through again.
Unfortunately this means removing the plug and tapping the case which is beyond the skills of most VW hobbyists. (Factoid: some of the early Type 4 cases had a threaded plug which was removed and replaced with a temp sender on the industrial engines. Most Type 4's do not). Either way, that location is blocked by one of the engine mounts, which would require you to modify the mount as well.
The most popular location on a Type IV engines is the inspection plate cover just to the right of the oil strainer cover (see link at end). Folks drill the plate precisely with a 9/16" bit or weld a nut on the backside for the M14x1.5 oil temp sender.
914 engines had a inspection plate that was designed for this very purpose. VDO sold it at one time as 240 827. The price was $50-60 at the Porsche dealer, somewhat less at your VDO retailer. It's sometimes called the taco plate because of its shape and it replaces the stock plate. The problem with this location is that is still at the corner of the case and does not get a good supply of oil. Therefore the oil temperature is not indicative of temperature of the recirculating oil.
The sender in the photo is the longer style 914 sender. It's designed for a 400F factory gauge and can't be used with the aftermarket VDO gauge. Even with the shorter sender you will find that the dipstick will hit the sender and you will have to file 10mm off the end of your dipstick. Since the dipstick marks are further up, this is a safe modification to make.
These are the various Porsche part numbers involved (courtesy Dave Darling). The items marked in yellow are all that you really need.
The VDO part numbers have never changed:
Some people like the Berg Dipstick but I think it suffers from the same problems and you lose your oil pressure warning light and the light going on every time the oil temp hits 220F would be annoying. VDO makes a dipstick sender for their gauges but the reading is no better than the sump mounted sender. There is a Type 4 version of this dipstick sender (VDO50003 in the Just Kampers catalog) but be aware it's very long and designed for the 411 engine layout with it's overhead dipstick, unlike the short rearward bus dipstick.
2. If you drill into the inspection plate then the sender will be in the airflow below the engine. Some folks who've found the 914 plate often lack the accompanying cover plate that keeps the airflow from reaching the sender. The result is the same either way: the sender is overcooled and this throws off the reading so you must have a deflection cover in place.
3. The VDO sender has a very slow rise time. Once the oil reaches 250F it may take 3-5 minutes for the gauge to read that value. It's a problem with the sender not the gauge and partly due the fact that the sender is attached to a big piece of metal which acts as a heat-sink. When the temperature falls, the gauge responds instantly which is some improvement. This behavior is due to its thermistor based technology. If the senders were thermocouple based the response would be instant but thermocouples have other problems that have to be considered...
Another popular gauge is the Cylinder Head Temperature (CHT) gauge.
This gauge measures the head temperature. Some folks think that reading the oil temperature is enough to determine the engine temperature but it's not. Head temperatures are related to the work that the engine has to perform and will increase more quickly than the oil temperature. As the engine pushes the bus through the air at faster speeds, all of the frictional forces including the friction of the rotating engine parts resist that forward motion. Head temps increase further when you climb hills as try to maintain your speed.
The aircooled engine requires a steady flow of air for cooling and a working thermostat system. When the thermostat breaks, there is a failsafe mechanism to ensure the engine doesn't overheat. However, it's simply a spring and if not checked to make sure it's working you may end up with an overheated engine anyway. If you are missing the flaps all together, although the heads will be cooled, the oil cooler will not be so you must at least have the flaps in place!
The #1 problem with Type 4 engine is a sunken valve just like the Type 1 engine. This is due to overheating the heads because the driver was unaware of the problem and over several hundred or thousand miles the head slowly failed and the final result was lost compression on one or more cylinders or worse, a broken piston because the head of the valve fell off.
The CHT senders consist of a K-type thermocouple that is placed under the spark plug. The thermocouple itself is a pair of two dissimilar metals in the form of wires that produce a mV signal when heated. The junction near the spark plug is called the hot junction and the other end is called the cold junction. The ring under the spark plug is a simple ring terminal and the metal that it is made from doesn't affect the thermocouple according to Seebeck's law of thermocouples: it merely exists to transfer the heat from the plug to the thermocouple.
The voltage produced in the thermocouple is based on the temperature of the hot junction near the ring terminal relative to the temperature of the cold junction at the end of the connector that leads to the CHT gauge. The gauge and cold junction are usually calibrated around 72F. What matters is not what length of gauge of wire is used between the thermocouple and the gauge or which cylinder the ring terminal should be under (although #3 is ideal) or where the thermocouple should be located (although under the spark plug is the only correct location for VWs) but what temperature the engine compartment is at.
Because the thermocouple is calibrated for 72F, the readings will only be accurate at that temperature in the engine compartment. If it's colder, the CHT temperature will appear higher giving a false impression that the engine has fully warmed up. If the temperature is higher, say 100F or more, then the reading will be lower by 30F (the difference between the engine compartment temp and the calibration temp). This is unfortunate because the very time when we want to know that the engine is too hot, it is actually low by 30-40F because of the outside temperature.
The reason this happens is that the VDO CHT wiring harness contains a 1 ft. length of thermocouple wire and the rest of it is regular wire so the cold junction will be in the engine compartment. If you manage to run the thermocouple portion of the wiring through the engine tin and expose it outside of the bus you have similar temperature calibration problems because of the airflow and exhaust heat, etc. Even if VDO ran thermocouple wire all the way to the dash, then the cabin temperature at the cold junction would still affect the the readings (hot day, low reading). You shouldn't have to install air conditioning to have accurate readings!
This is the nature of thermocouples and in this form they are basically unsuitable for automotive applications. The cure is use a system that has a cold-junction compensating thermistor. What this does is measure the temperature at the cold junction so the gauge can adjust the reading. Very simple but for some reason you can't buy this type of system from the low budget gauge vendors.
Dave Bolen recently benched the VDO gauge and sender against an expensive calibrated probe. The results were even worse than the results the math predicts under a controlled environment
I finally got out my new CHT gauge and sender over the weekend and did some comparisons on the temperatures using the oven and the freezer.
Here is what I found out. First of all, the sender was "mis-wired", red went to the negative terminal on the gauge rather than the positive one.
The major thing that I noticed was that the gauge reads about 20 degrees difference (higher) when tilted upward at an angle like it might be mounted in a dash(rather than horizontal).
Placing the gauge in the freezer for 15 minutes while the sender was held at a steady temp raised the temp shown on the gauge by about 20 degrees.
The senders were placed between two pieces of sheet metal in the oven I set a pyrex dish on top of the sheet metal pieces in order make sure nothing moved. Gauge temps based on "house" ambient temperature(70 degrees).
All temps are in fahrenheit, and numbers that aren't "on the mark" on the CHT gauge are approximate.
The meter I used was a fairly expensive METEX with a type k thermocouple probe. The meter probe in boiling water reads about 210 at 1100 feet above sea level.....which is pretty much exact. The probe for the digital meter was ordered with the meter when I bought it.
So, if you graph these, what becomes apparent is that the difference between "real" temps and CHT shown temps becomes wider and wider, until at a true 400 degrees, the CHT gauge is off 60 degrees.
In other words, this gauge will not help you prevent a meltdown because it will force you to drive at such a low speed you'll never overheat the engine anyway. This may not seem like a problem but consider how you will keep up with highway traffic speeds? With variations in outside and engine temperature the VDO gauge is 100% unreliable.
Invariably everyone asks me, "if measuring CHT is so important to monitor what do you use?". I use a Fluke 179 multimeter with custom thermocouple wire I built for $15. There is a ring terminal under spark plug #3 and the Fluke has the CJC circuitry and digital display for dead accurate readings. Unfortunately I can't compare readings with anybody using VDO gauges but I suspect two VDO owners couldn't do that anyway.
Recently I made an awesome discovery in Dakota Digital. I have no idea why I haven't seen their name mentioned during the past 5 years while I've been online but they are heaven sent:
Read my post on Samba about them.
For either of these two gauges, I made my own harness consisting of some k-type thermocouple wire and a ring terminal to go under spark plug #3. The VDO harness is not suitable because of the wire they use and the fact is was never long enough for the baywindow bus anyway. It has always required you to extend it. The harness is easy to deal with because you can simply buy it by the foot from your local electronic surplus supply for much less than any instrument vendor sells. You will need 17 ft. of thermocouple wire for a baywindow bus.
Finding a 14mm ring terminal is much harder. I went through the entire AMP catalog and couldn't find anything with a big enough opening and a slim enough outer diameter to fit our spark plug wells. I actually located the VDO terminal in the AMP catalog and determined that it was a 13mm terminal that was drilled larger. I eventually found a suitable terminal on the net and I was surprised when told that it has been stretched in a die because they couldn't find the right size either!
I contacted Dakota Digital to help them locate a suitable ring terminal to provide with a VW length harness which they now provide in 18 ft. length. As of Nov 2006, you can now order their SEN-11-8 CHT harness which is 18' long for the bus and includes a 14mm spark plug terminal.
The oil pressure sender is the most accurate of the bunch.
I bench tested it against an expensive oil pressure test gauge recently and it read between accurate and 10 psi lower for every test I performed that gave a consistent reading on the test gauge. Unfortunate results but even so it's a worthwhile gauge to install to use as a replacement for the idiot light which only tells you the engine is hosed after the fact.
When you lose your oil, the idiot light will only come on when the pressure is 0.15-0.45 BAR. This is approximately 7 psi and this situation usually occurs on the highway. When this happens, you have to turn the engine off immediately and coast to the side of the road. In most cases, by the time you've noticed the light the damage has already been done to the engine. By installing a gauge you can detect the oil pressure loss but it still requires that you monitor the gauge while driving.
The VDO oil pressure sender comes in two varieties: single and dual sender. The dual sender also contains a WK contact that you can wire up the original oil pressure sending wire so the light on the dash continues to work. However, the threshold of this sender is about 11 psi so the light may flicker at idle from now on.
If your oil light doesn't come on when you first turn the key (before you fire up the engine) you will never know you've lost oil pressure until too late so fix it asap!
There are other gauges available to measure voltage, current, vacuum, etc. They all have limited usefulness especially when you carry diagnostic gauges with you like I do. I feel that when the engine is running properly and you are familiar with the sound, it's simply enough to stop and check the engine from behind rather than clutter the dash with a lot of ugly gauges. These gauges aren't portable either so when your friend lacks a diagnostic tool you won't be able to help out if you rely only on the dashboard instrumentation.
Some folks insist on driving by these gauges but I find them more distracting than anything else especially since the readings are misleading. The false readings waste more online discussion bandwidth than any other modification. Every time I say this I get a laugh from some idiot who refuses to believe it probably because he's spent a couple hundred of dollars on inaccurate gauges over the last 5-10 years.
Spend your money wisely and invest in better technology from Dakota Digital or use the VDO gauges to trend problems with your engine only. They are not useful for tuning purposes but may save your engine from meltdown and unwanted costly repairs.
I recommend the Solarix Dakota Gauges with the black bezels (SLX-11-1-K and SEN-11-8 CHT). They are digital and can be programmed to warn you of high head/oil temps and low oil pressure.
02/02/05 - Moved from FAQ
02/21/05 - Added CHT benchmarks from Dave Bolen
03/19/06 - Added Photo from VDO catalog
02/22/07 - Added photos of parts bags from Porsche (thanks to Jasan Carsley)
09/07/11 - Fixed broken photos, added translate button, updated footer