Isolate Parallel Connection to Speed Sensor Cable?

[TrevorDennis]

I have a 2001 Mitsibishi Pajero 3500cc Petrol, and recently fitted a parking sensor that happened to have a digital vehicle speedometer. I can't say exactly what the make of this item is, because it was re-badged by SuperCheap Auto here in New Zealand, but THIS IS IT Unfortunately, T-ing off the speed sensor cable buggered up the speedometer and odometer in the vehicle cluster.

I have no way of measuring the input impedance of either cluster or parking sensor, (they both show an infinite DC resistance) but I took it in to the local Auto Electricians, and they tell me that it was indeed paralleling the connection to the parking sensor that broke the vehicle cluster. I'm actually not so sure about that, as when I had the cluster out looking for the right cable up from the vehicle speed sensor, I removed a subsection from the rear of the cluster (following advice from the UK Pajero Owners Club forum)and I suspect that might have caused the problem. It doesn't feel right either. I can see that it might have had an impact while connected, but my gut tells me that the vehicle speedometer would work OK again after disconnecting the parking sensor.

Incidentally, it has also messed up the odometer display like thus

I can get the cluster fixed OK, but I quite like having the big green LED speed display on my dash, and would like to have both, but I obviously can't risk breaking the repaired cluster again, so I'm wondering if there is a fool proof way I can make that parallel connection? An optical isolator sort of thing? If it is doable, I'd very much appreciate a circuit diagram and component list, or better still, if such a thing is available ready made, (preferably with a 12v feed).

TIA for any help.

Incidentally, I retired from Ford's Product Development site at Dunton in Essex in 2004 — most of my 37 years there was with a small design team producing specialist Test Instrumentation. My background is mechanical, but it was a small team, so I was able to a bit of everything. I moved to New Zealand soon after retiring.

 

[LionelHutz]

This site isn't for hobby work except there is a "Engineers With Hobbies" forum you could use.

That messed-up display isn't caused by the speed sensor but rather by messing up the connections between the driver and the display.

 

[IRstuff]

I agree; this is typical of the elastomer connector between the display and the board getting messed up. BTDT

TTFN
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[dgallup]

Too late now but why wouldn't you just use a GPS if you wanted a digital speed display? Maps come along free.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[TrevorDennis]

Thanks for the feedback. There is not a whole lot of point using a GPS here as there are not enough roads to get lost in outside of our cities, and I avoid those. I do have a dashcam with GPS speed display though, but the position of the parking sensor on top of the dash, plus its large, bright display, is nice and I have quickly grown to like it.

Incidentally, compared to the likes of the UK where the police ignore people doing 10mph over the limit on motorways, NZ police are beyond anal about speeding. Their cars have forward facing radar that can measure your speed as you approach them, and they have quotas to fill. You can drive for an hour without seeing a single car (that is not an exaggeration)and drive over a rise and there will be a cop car sitting in the hollow on the other side waiting to catch you. If you ever come here for a holiday, be warned.

The connection problem makes a lot of sense to me. I removed the white plastic cover in the top left of the image below. This was on advice from the Pajero Owners club, while looking for a connection to the speedometer sensor cable. The white plastic cover contains circuitry and sprung loaded fingers to connect to pads on the dash PCB, and it this area that I think has the fault.

Since starting this thread I have arranged for the cluster, and parking sensor display, to go to Auckland to the people who are apparently the best for dealing with this sort of issue. The parking sensor display is going for them to test driving both from the same signal.

Thanks again for your feedback, and my apologies for using the wrong forum.