Oil Pressure and Temperature (Part 2)

In my last post about adding gauges, I suggested 6 possible alternatives. Today, I focus on the viability of option 5: fitting a Porsche combination gauge into the spare hole in the original bus dash pod. I do want to stress that I bought pretty much everything for this project before the end of January, before we knew what this CoVid-19 thing was about. The only exception is the oil temperature sensor. I had purchased the wrong one (English threads not metric) long ago so I needed to get one that would actually fit. On with the post...

Porsche Combo Gauge

blank held up by napkin-holder

First, we start with the question I've been dying to find the answer to: will a combination gauge from a Porsche fit well enough into the third hole on the bus dash to provide oil temp and pressure information? The short answer, I think, is "yes". I'll pause for a second to let that sink in. Yes, that gauge will fit. But, will it look good and do we need to do anything to the dash or the gauge or both to make it fit? Ay, there's the rub. In this case, quite literally... there's a rub. As in, the left-most edge of the combo gauge rubs against the large plastic column in which lies the speedometer. Could it get wedged in good enough? Sort of, but it would look pretty janky. So, could we shave off some of the outer edge of the combo-gauge to make it fit? Maybe. Alternatively, we cut a small section out of the plastic column, and have the combo gauge nestle into the small section. Either way, something needs to be damaged to further the investigation.

almost exactly 4" diameter

the original blank 3-5/8"

So, what's to do? Well, it turns out, my dash-pod, dash board or whatever you want to call the plastic thing that is held on with 4 screws, is cracked. In fact, there is a break running from the bottom edge of one of the climate control sliders all the way through the bottom of the dash-pod. And, the upper left corner is broken off. Cosmetically, it really should be replaced, but it does hold to the dash, and hold the gauges well enough. When I contrast it against the nearly flawless Porsche combo gauge, it seems rather obvious to me which one should be sacrificed to learn how to make it fit... or if it can fit: the cracked and flawed dash.

Before I start pulling out the dash, it would be wise to take a bunch of measurements. I took pictures of them so others can see how close everything is before I touch anything. This includes a test fitting in the cab of the bus. The pictures are strewn across this posting.

Pull the DashPod

combo gauge with glass
from blank on top of it

The dashpod is held on with 4 long Phillips screws. Remove them and stow them safely. Then, reach behind the dash and un-thread the speedometer cable from the speedometer. If you have the plastic tips on your climate control levers, remove them. Now, you can lift and tilt (top downwards) the dashpod so you can get to the wires behind. I can't stress this enough: do NOT just start pulling wires. Identify each one and label it with a toe-tag, a piece of folded over tape, whatever it takes so you can correctly put it back in at the other end of this exercise. Move slowly, be patient. The slower you go now, the more you will appreciate that you took the time at the other end. Once all of the wires have been individually labelled and removed from the back of the dashpod, you can remove it all the way by sliding it towards the center of the bus.

Measure, Check, Measure, Check then Cut
Even though this dashpod is cracked and will be replaced, I have no idea when I could get a replacement. Worldwide commerce is slowing to a crawl, my favorite parts places have been sending notices that they are shutting doors until further notice, etc. So, even though I expect to replace this dash, it could be months before I find a viable replacement. So, I will treat this exercise as if this is the only dashpod in earth.

test fit cockpit side

test fit from "above"

The picture on the right, here, shows where the interference points are. First, at the bottom of the photo you can see that the black ring around the Porsche combination gauge hits the side of the speedometer section before it can fully settle into place. This uneven look can be seen through the rear (front-is-front) of the dashpods in the picture on the left. It may not be obvious, but the oil pressure gauge is slightly less visible than the oil temperature gauge. The two idiot lights should be in the exact left-to-right center of the circle, but they are offset to the right. What cannot be easily seen from any picture is that the edge of the gauge hangs up on the little plastic tang between the threaded hole for the screw and the edge of the gauge hole. I started with those: I cut a shallow channel in-between with a hacksaw.

hole cut in speedo side

trimmed backing

Once I could get the gauge to settle in, I could mark where to cut into the speedometer slot. I covered the area with blue painter tape and marked up ideas with a sharpie. The hole ended up less than an inch from gauge top to gauge bottom and less than 1/2" in depth. It is oriented just below the edge of the outer gauge ring so the combination gauge can slot in. Ultimately, the gauge does not need much space, but it needed a little.

Last, the metal backing plate on the original dashpod needs a trim. It is not as obvious in the picture, but that slight overhang is enough to knock the combination gauge off-center. I marked the short (about an inch long) stretch that needed to be removed along the edge. With tin-snips, I cut that section out and then refined it with a file until the gauges could nestle together. Now, the backing plate threads back on without pressure and the combination gauge sets perfectly centered.

Gauge Wire Identification

labeling wires

If you are thinking of doing this, I strongly suggest that you verify the combination gauge you acquired still works before you grab a saw. I found this kind of fun. The gauge that I was shipped was removed from a late 70's Porsche and then the wires were cut. This is important because I would have had much more research to do had the seller pulled all the wires off. Now, there was a strange green over-spray on most of the wires, so my identification instead was based on what they were plugged into. Similar to the wires removed from the old dash on removal, I labeled these wires as well. There were 2 circuits of particular interest: brown and blue with green dashes. These were the ground and switched 12V respectively. These 2 circuits wired multiple things together, and in the case of the blue/green wire, it had 2 open ends. I think that is to prevent one failing gauge to eliminate the viability of the other. Once these were identified, the signal wires for the various gauges and idiot lights were easy to identify. I removed the gauge lights, after I demonstrated that the ground wires were also grounding those light sockets.

Validating the Gauge

testing oil temp gauge

Now for the fun part: testing the gauges. I grabbed an extra deep cycle battery I had in my garage, and set it on a grub-towel on my kitchen table. My wife loves me. To the positive side of the battery, I connected one blade of a 3-amp bladed fuse. To the negative side, I connected a 16ga blue wire and connected the other side to the ground wire from the gauge. I took a long 16ga yellow test wire which had a female disconnect on one end and bare wire on the other. The disconnect end fit properly onto the open blade on the fuse. I touched the wire to the 12V switched and the gauge clicked, sending the oil pressure gauge to the top. I figured this could be a bad thing, but, undeterred, I removed the wire from the fuse blade and went looking for my senders. I figure this behavior could be a way to signal the driver that there is no longer any signal coming from the oil pressure sender, since I did put the power on the oil-pressure switched 12V side.

combo-to-speedo "front"

I started with the oil temperature, after that odd reaction the oil pressure gauge had to power applied. To simulate installation, I ran another ground wire from the pre-existing blue one around the threads of the sensor. I then ran a jumper from the sensor signal to the temp gauge input. When I applied 12V to the switched circuit, the temperature needle moved off of the bottom post. It wasn't much, but that told me it was reading something. Considering that a success, I did the same thing with the oil pressure sensor. This time, instead of leaping to the top of the gauge, the needle lifted just a hair off the bottom post. Again, I took that to mean success.

I am concluding that the modern VDO sensors will activate the original VDO gauges. I need to conduct further tests, but this post has already gotten really long. I'll post on those tests next time. And, I'll post what the gauge looks like after all the trimming.

Thanks, as always, for following along. I must admit, if you are looking for a can't-leave-home project, this one has been a lot of fun. Please stay safe, and more next time-