Oil Pressure and Temperature (Part 4)

This really has been a ton of fun. We have a Porsche combination gauge mounted to the rear of my removed-from-bus dashboard. We have tested the VDO sensors against it and everything seems to line up. So, today we shift to installation.

Cable, Not Wires

inside belly pan

Rather than run another whole set of individual wires, increasing the spaghetti under the bus, I went a different route. In the TDI retrospective (See TDI install retrospective: Secondary Electrical), I mentioned a cable company. Shortly after that, I decided that I would do this gauge thing and use a cable to send the signals from the engine bay to the dashboard. I figured that I had the 3 for the oil system (temp, oil pressure gauge and oil pressure idiot light). I also want to know when the coolant is low and when the electrical system is tanking, like a normal car. So, I got a 6-wire cable, leaving one open for future expansion. Similar to most of the other stuff on this project, I ordered the cable in January before things started getting CoVid-interesting.

Snake-y

harness pops out here 

I want this install to be as clean as I can make it. So, I figured I would route the cable along the same route as the original main harness. Finding where it pops out inside the front of the cab of the bus isn't very easy: it is under the windshield-washer tank. This is much more obvious from underneath, hidden inside the center belly-pan between the main rails. I drilled a matching 11/16" hole an inch or so to the driver-side of the main harness pass-through. From inside the center belly-pan section, I pushed it up through that hole, inside the bus. I then took it from the inside, along the same path as the main harness over towards the fuse box, over the ventilation tubes and up behind where the dashpod sits. I gave myself an extra foot of cable so there wouldn't be stress on it later.

Sending the cable rear-ward was the easy part. From the center belly-pan, I threaded the cable from behind the front axle along the same route as the main harness, meeting it where it emerges from the steel pipe it travels to the rear of the bus. It ran along the main harness pipe, through a small pass-through in the rear cross-beam and back towards the starter. It passes right into the engine bay where the fuel does. I coiled the extra cable there. I had ordered 30' so I had more than enough to run it exactly where I wanted it. I cut off the excess, leaving almost 10 feet of cable, so the total need was just over 20 feet. For my future self, here is the wire color -to- purpose mapping:

Circuit	Color	Purpose
1	Black	Oil Temperature Gauge
2	Red	Oil Warning Light
3	Blue	Oil Pressure Gauge
4	Orange	Coolant Warning Light
5	Yellow	Alternator Warning Light
6	Brown	open

Pressure Sensors

splitter and stock sensor in

The TDI computer already has an oil pressure signal. It comes out of the oil filter housing. I decided that if that location was good enough for the computer, it was more than good enough for my gauge. So with a 1" spanner (that was the closest I had to the required), I knocked the stock oil pressure sensor free and applied the splitter (VDO 240 850). In order to get the splitter to point the right direction (away from the oil level tube), I added a washer between the splitter and the oil filter housing. If you do this, you may need to do the same. I put the original pressure sensor into the end and the new dual sensor into the split off the side. The dual sensor has a 17mm nut close to the threads, and tightening it can only be done from below, about 30* at a time. Fun.

Temperature Sensor Stuck?
Between the insertion of the original pressure sensor and the new dual-sensor, I went after the temperature sensor. Why? I realized that once I had the dual pressure sensor in-place, I would have my access and vision blocked by it. The plug in the oil filter housing was on fairly snug, but a pop with a ratchet holding a Torx socket did the trick. I don't remember which size; my apologies for not writing it down. Once free, oil seeped out of the hole, even though I had drained the oil before I started with the pressure sensor. No matter. It just made a mess. I first tried with the longer temp sensor (VDO 323 423), but found that it hung up on something inside the housing. I was barely able to get it back out again, and after looking at the sensor after I removed it I concluded that it was not a good idea to use that sensor here. I put a picture of it down below on the right.

Temperature Sensor, Take 2

temp sensor in

I cleared the hole, and checked for depth with a small screwdriver. It seemed like the sensor should have fit, but there is no doubting the compressed look of that sensor. So, grateful I had the shorter, but lower max temp sensor, I considered the impact. Consider that this is not an air-cooled engine, whereas the Porsche combo-gauge came from one. Also, the oil and coolant exchange temperatures, cooling the hotter oil as they do and that cooler oil is what is passing through this part of the housing on it's way back to the engine. I did a quick survey of posts about oil temperatures of TDI's and even the guys who run 'em hard say their temps don't get above 230*F. In fact, most of the folks out there pose that temps above 250*F would be cooking their engines.... and these folks have a sensor in this location, so the plan is holding, so far.

them bumps aint right

So.... I decided that this sensor is probably fine, especially for the short run, while I figure out if this is the long-term solution. Key to this thinking, is to remember that the gauge is to help identify patterns and trends. Also, to remember that this is the temperature as it is re-entering the engine. So, this will be a delayed signal of a climbing temperature event. In fact, it could be masked if the coolant is effectively pulling heat out of the oil. Concluding that I'd come this far and that there was not a better port to test temperature anyway, I continued. The shorter (250*F / 120*C max) sensor threaded right in with no trouble. I lightly torqued the sensors, filled the engine with oil and called it a day.

Wait a Minute...
One nagging thought I have about this is the numbering, or lack thereof, and the red zone on the Porsche combination gauge are completely out of alignment with what I would be concerned about as I drive a TDI. These numbers ("normal" between 194*F and 248*F) are way to broad. AND, regardless of which sender I use, the red zone starts outside the top end of it (at 302*F?). So, I may need to change that gauge at some point if it is really going to be useful long term. I mean, honestly, if normal is around 210 and getting too hot is 230, I will not easily see the difference on that gauge.

Well, that's it for today. All that remains is everything else. I will work on getting the engine-compartment end of the wiring done next. Then, I'll see about getting the dashpod wired and re-installed. Depending on how exciting and time-consuming those are, it could be one or two more posts before I have oil instrumentation.

Thanks, as always, for following along. Be safe, be patient, and if you can, patronize your favorite restaurants with a go order. Now more than ever, you can vote with your wallet for the businesses you want to survive. Personally, Amazon doesn't need my money; Discount Import Parts (now only on the east side) and the local mom-and-pop grocery do.