The Strange Case...

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Spacenut
Posts: 1205
Joined: Thu Feb 20, 2014 9:43 pm
Location: Hampshire

The Strange Case...

Post by Spacenut »

…of the reversing oil pressure, a mystery in three acts by Agatha Spacenut.

Act 1 – An Implausible Occurrence.

As we enter our third month of lock-down, the Green Machine continues to provide good service, clocking around 12 miles per week, which is the round trip distance to get the weekly shopping. On the whole the weather has been fabulous, the roads almost completely deserted, car parks nearly empty (no free parking for East Hampshire, alas) which is wonderful. Only one small issue has blighted my driving experience, and that is the oil pressure reading, particularly the readings I have been getting in the last few weeks.

About a month into the lock-down I hopped into the car as usual and fired up the engine. Everything seemed fine, but not only was my engine warning triangle lit up (it monitors the water temperature and oil pressure and gives warning if either exceed their preset limits), but my oil pressure was reading 118 psi! I have had some issues in the past with the oil pressure sender so I was not immediately alarmed, but it was disconcerting driving around with the gauge reading flashing away and the warning light on.

Once the engine had warmed up, the pressure reduced to a slightly less scary 75 psi, and with the engine switched off altogether, the reading was 27 psi. Time for a new sender, methinks.

The sender in question was a VDO device, rated at 0-10 Bar, 10-184 ohms and with a 1/8” NPT thread which requires a brass adapter to fit the standard 14mm thread in the Alfa engine oil gallery. This is it...

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Note the single connector – the sender earths through the body and the engine block.

Act 2 – A Most Ingenious Disguise

So I ordered a replacement sender from an online Alfa Romeo parts supplier, and sure enough it arrived a week later and fitted directly onto the engine. I plugged in the wire and switched on the ignition. Hmm – the reading shoots up to 147 psi, with the engine off. Start the engine, and the reading drops to 60 psi, and drops further when I rev the engine. The darned thing is running backwards!

I swapped the new sender for the old one and the readings went back to (almost) normal. My oil pressure gauge was supplied by Dakota Digital in the USA. It was time for some further investigation!

VDO still list the sender, but while trawling the Interweb for similar experiences I discovered another curious cultural difference between Europe and the good ol’ US of A – it seems that as well as all the common differences (boot/trunk, bonnet/hood, wrench/spanner, tomayto/tomahto…), gauge senders in the US work on the principle of reducing resistance with increasing pressure, and European senders work the opposite way around – increasing pressure equals increasing resistance. This was further backed up by the VDO technical literature, which helpfully states that the normal resistance for this sender is either 10 or 184 ohms!

I could not find any differentiator to uniquely identify the American or European part – it appears to be entirely down to which side of the Pond the item is purchased. So rather than spending more money in Europe on identical parts which have the opposite characteristic to the one I needed, I went straight back to Dakota Digital, who fortunately still list a replacement sender for their Classic range of gauges (note that my gauge set was purchased when I was resident at Lockheed Martin, which was over 20 years ago now).

Act 3 – An Inspector (well, a courier actually) Calls

Just 7 days later a box arrived on my doorstep, courtesy of UPS, together with a fat bill from UK Customs which I had to pay there and then. This was interesting to say the least, given the current zero-contact restrictions imposed by the lock-down. Still, once the strange ritual of filling an envelope with cash and putting it on the door mat, then standing back and allowing the courier to step forward to pick it up was over, I finally had a sender that I knew would work!

Next problem – the replacement sender had two contacts, one for the gauge and one for a separate earth connection. This was a bit irritating as my wiring loom is designed for a sender that earths through the body, like most senders do. After all, to get the starter motor to work properly, the engine block has to be really well earthed – why wouldn’t you earth through the sender body?

The terminals were designed for crimped eyelet connectors, so I had to hunt around for some spare spade terminals raided off an old ignition coil. Also, as the brass threads appeared to be a US Imperial thread I managed to rustle up some 6BA Nylock nuts to clamp the terminals down with. The brass adapter was fitted and I used the copper washer that came with the Alfa sender for good measure. Then it was time to fit the new sender in place…

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The observant among you will have noticed that I have managed to retain the engine breather pipes and the flame trap with the K&N air filters. I just don’t like the idea of venting engine fumes directly to atmosphere, and besides, where would I put the catch tank???

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The green dot signifies the polarity of the sender unit. And before you ask, no, the gauge doesn’t read backwards if you reverse the polarity :D

Next I had to work out an earthing path to the engine or chassis frame that wouldn’t compromise service access. I settled on a mounting tab left over from my old fuel pump location…

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The 8mm mounting hole closest to the side of the engine bay was used to clamp a crimp eyelet, and the earth wire was routed through the rear chassis harness up the roll bar brace to the coil and then along the RH breather pipe to the sender unit.

So – now that it is all connected up, what’s the verdict? Well, with the ignition on, and the engine not running, this is the sight that greeted me – zero oil pressure and the warning triangle illuminated, just what I was hoping for!

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...And with the engine running at idle (and still cold), a much more sensible reading!

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A quick check around the sender shows no evidence of oil leaks from either the tapered 1/8” NPT joint or the 14mm adapter, so it looks like we are good to go, to the end of the lock-down, and beyond…

Overall, a costly and time-consuming exercise. Still, I console myself that Lamborghini must have had the same trouble with the early Countach, which was equipped with American Stewart-Warner instruments throughout...

Until next time, stay safe out there!

Lauren

Sud 145
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Joined: Mon Jan 27, 2014 11:08 pm
Location: West Sussex.

Re: The Strange Case...

Post by Sud 145 »

What a palaver but a result all the same.........does your awesome dash read the fuel in the tank.......or fuel you've used? :lol:

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Spacenut
Posts: 1205
Joined: Thu Feb 20, 2014 9:43 pm
Location: Hampshire

Re: The Strange Case...

Post by Spacenut »

Sud 145 wrote:
Mon May 25, 2020 6:54 pm
What a palaver but a result all the same.........does your awesome dash read the fuel in the tank.......or fuel you've used? :lol:
It reads the fuel remaining, in percent. Too difficult to calibrate for gallons or litres...

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LooLooSud33Spider
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Joined: Mon Dec 05, 2016 11:14 pm
Location: Staffordshire

Re: The Strange Case...

Post by LooLooSud33Spider »

Superb job Lauren and glad you’ve got to the bottom of the problem. This sort of thing is exactly why I’ve always preferred capillary gauges especially on race cars . I find they are much less likely to give false readings.
Of course you have to ensure a smooth path for the pipe to avoid it being cut or rubbing through but that’s All part of the preparation Process .
Alfasud Ti 1984
Alfa Spider 2.0 Ts 916
Alfa Spider 20v Turbo

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