Oil Pressure and Temperature (Part 5)

When I left off last time, I had a Porche combination gauge fitted to to the bus dashpod in the previously blank third-hole. I had wrangled the stock oil pressure sensor out of the oil filter housing and back into a VDO "T". Into the other side of that "T" I threaded the big dual-pole oil pressure sensor. Last, I removed a M10x1 plug near the oil pressure sensor spot and threaded in a sensor designed to capture water temperatures, topping at 150* (250*F). Today's post continues this saga with a detour into swapping out the temperature gauge innards with new guts from a new VDO gauge.

Wiring Rear

I started with the next obvious thing: wiring up the sensors in the engine bay. First was estimating where to cut the cable. I threaded it through the cable holder on the oil filter and then gave myself about a foot more. With a knife, I whittled the cable casing back to the cable holder. Then, I followed the color chart, starting with the black wire to the temperature sensor. This sensor is less than 4 inches from the cable holder, so while this was easy on the brain, it required some stretching and finger dexterity to cut, strip, attach a disconnect and heat-seal the connection. Once done, I cut the housing on the dis-connect so it could slip onto the stud on the end of the sensor. For the oil pressure related wires, I did not have ring disconnects. So, I will revisit those later, after the CoVid lockdown ends. In the meantime, I simply placed the stripped wires (red and blue for the warning light and gauge respectively) and threaded the cap down on the posts. This will work for testing purposes, but with the shaking that comes from a diesel plus the shaking of driving, one or both will fall off eventually, so I will need to apply the ring terminals.

I left the rest of the cable alone. Back when I was still suffering temperature issues, I removed the engine cover. I decided this was a great time to re-install it. So, with a couple M6 bolts and washers, the cover went back on.

More Gauge Fun

cut line on new gauge

temp gauge removed

The longer I thought about the suitability of the oil-temperature gauge in the combination cluster, the less enthusiastic I became about installing it without trying to do something about it. So, I decided I would hack-up a water temperature gauge and swap the innards with the innards of the combo gauge. My plan was for it to continue to look like the original gauge, but the top of the range would be 250*F, aligning with the sensor. This might have the opposite effect of giving me an "in the red zone" appearance when we're still down in the 230*F range. I think I can live with that, so long as the gauge gives shows a clear difference between 210*F (normal) and 235*F (my believed top end of normal).

So, how do we swap the innards? It starts with removing the temperature section from the combination gauge. It is held on with small slotted screws. Once removed, the unit pops right out. I had an extra new-old-stock (NOS), new-in-box (NIB) temperature unit I got off eBay before I got the combination gauge. I figured I would use that as my transplant target so I can revert back to the original later. I found a few differences between the original gauge innards and the new ones. I will highlight those differences as I work through this.

New VDO Gauge Destruction

old innards top
new innards bottom

At a high level, I followed the Dave Barton PDF process where you cut the housing in half to get to the inner workings of the new gauge. I found that a standard screwdriver was 1/4" thick, so I used that to mark where to cut. I threaded the collar up to that point and then traced around the gauge housing, using the collar as a guide. I taped over the gauge lens with blue painters tape and the cut the line with my hacksaw. As directed in the instructions, I cut just enough to break through and then rotate the gauge housing to extend the gap around it. The top part lifted off right where the gauge face meets the housing. Perfect instructions; nice job Mr. Barton. I also used the salad-fork method to remove the needle, using sparing force so the needle slowly lifted off the little pin. I encourage taking great care on this step.

With the needle off, I removed the small slotted screws on either side and then removed the gauge face. Then, I moved to the flip-side. With a 7mm socket, I removed the nut on the post. The tabs hold the gauge innards in by friction. So, I put the mounting collar on backwards, so I could stand the gauge on its (now missing) face, with the tabs pointing upwards. With a framing hammer, I gently encouraged the gauge innards down by tapping on the tabs and the bolt-post. If you do this take care that the pin the needle sits in does not get crushed or bent or in anyway really touched. It did not take much force to get the innards to move. I held the hammer about 4 inches from the head and tapped.

Old VDO Gauge Mount Destruction

gauge mounting plate

I started with the same process as I had done with the new VDO: salad fork on the needle. This was not as clean a process as it was with the new one. In fact, I would discourage anyone from trying to remove the needle this way if you intend to reuse the needle or the innards. In my case, the needle was damaged, and I was unable to tell how the original needle was connected to the pin. I resolved to using the new VDO needle and kept going. I removed the 2 slotted screws holding the gauge meter / face on. Underneath was a thin piece of dried up wax paper. I carefully separated that from the gauge innards.

With the gauge face torn down, I switched to the mounting side. The old gauge separates from the metal plate fairly easily. With a 7mm socket, remove the nut from the central post/bolt. With a slotted screwdriver, encourage the innards to pull away from the metal plate. It is held purely by friction.

Re-Assembly Preparation

testing old and new together

Now, we have a small pile of parts. From the new VDO innards, the tabs need to be removed from the rest of it. These simply slide out. Now, look at the 2 innards. They look very similar in some ways, but have one major difference on the not-gauge-face side. The inputs are 90* offset. If you look at the old metal mounting plate, you will see that the 4 holes around the central-post/bolt hole are 2 different sizes. The old gauge innards' holes line up with the smaller set and the new gauge innards line up with the larger set. The plastic thing that holds the tabs for the older model has 2 plastic nubs that line up with the larger holes. Those nubs hold the plastic holder and the innards stationary. If you were to cut them off, the gauge would not sit still, and could ground against the metal plate. I hadn't pieced this together when I cut them off in an attempt to get the new innards to mount. I would recommend instead that you expand the smaller set of holes so they are the same diameter as the larger set: so all 4 are the same size. Then, the plastic holder can go in in either orientation. If you hastily cut off the plastic nubs, like I did, you can solve by locating a pair on small plastic finishing tabs from one of those assemble-at-home pieces of furniture. With a little whittling, it can be formed to hold the innards in place. I suggest altering the mounting plate instead.

Re-Assembly

approximately 175*F

comparing top temp
modified on the right

However you solved for the attachment of the plastic bit to the metal plate, slide the new VDO innards onto the 2 posts. There are 2 ways the gauge innards can fit, but only one that actually works. So, when you do this, consider the orientation of the gauge so that the gauge sweep will be on the correct side. Press together firmly. Making sure the ground tab is on the bolt, thread on the 7mm nut and tighten with the socket. I considered the orientation of the new innards on the old plate and marked which side was for the signal (S) and with was for power (+) before I moved on.

If you intended to create your own gauge face, or paste on an aftermarket one, now is the time to do that. I would really like to have markings to show where important temperatures are, but I also like the simplicity of the stock gauge, so I left it alone.

To the gauge face side, I set the wax paper and then the old gauge face. I used the 2 slotted bolts from the new gauge to attach. Last, carefully press on the needle. Once it is all together, it fits back into the combination gauge housing just as easily as it was removed. If you are using the needle from the new gauge, you will need to insert a small washer between the mounting plate and the combo-gauge for each mounting bolt or the needle will hang-up on the contoured plastic near the rear (front is front) of the gauge housing. Using the 4 slotted bolts it was held on with before, mount the plate to the cluster threading each bolt through a small washer. Re-attach the wires.

Setting the Gauge Range

gauge re-installed into dashpod

As you can see from the pictures, I took the opportunity to test the new and old innards with various temperatures. First, I applied an open flame to the sensor to determine the top temperature. I then checked to see where the needles of the 2 different gauges sit when I put the sensor in boiling water. This took some doing since the "kitchen" table is 10 steps or so from kitchen-proper. I would boil some water in a small cup in the microwave and then dash over to test it. As a result, I think the picture on the above right is for around 175*F. If you consider how much gauge is left above the needle for the new innards, and that the top temp is 250*F, I will have a wide band for the temperatures we care about: 190*F to 230*F.

Well, that's an awful lot for this week. All that's left is installing it into the bus. I have to do some wire disconnects, but I don't expect it will be terribly dramatic. Now that I see the gauge in it's final form, I would absolutely recommend bus-pilots who still have their original air-cooled engine to think about installing one of these combo gauges. The original temp range is designed for the air-cooled hot oil, and it looks really good, in my humble opinion. Knowing that the new gauge innards can be swapped in eases my old concerns about long-term support as well. Just get a new VDO gauge (less than $40US today) when one of these fails, and do what I just did.

Thanks, as always for following along. Not sure what I'll work on, much less post on next. I guess I'll figure it out when I paw through my parts and incomplete projects. Stay safe and be well-