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Speedo sensor differences???????????????

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Old Aug 25th, 2019, 22:39   #1
Laird Scooby
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Default Speedo sensor differences???????????????

Hi All,

in preparation to fit a replacement axle, i was looking at the speedo sensor on my 760 today. It seems to be the lesser-spotted variety held in place by a threaded collar.

However, the one on the replacement axle is the more modern in appearance style sensor held in place with a cap screw.

According to the Haynes Book of Fantasy, the plugs are the same for both sensors. Looking at the pics i was just about able to take with an old phone (that i can't upload from) it actually looks like it's hardwired.

Has anybody encountered the threaded-collar type before and if so, can they confirm the plugs are the same, different or what?

Thanks in advance!
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Old Aug 26th, 2019, 07:05   #2
mocambique-amazone
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Hy Dave.
There will be one problem:

Different type of teeth at the carrier.
Old style will be with 12 teeth
New style, and with ABS, 48 teeth.
You can't swap a axle with wrong teeth.
The cover will be to exchange. Zero problems to swap them. The old style of sensor will be very expensive now, IIRC 5 times of the new one.

Good luck, Kay
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Old Aug 26th, 2019, 09:29   #3
Laird Scooby
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Quote:
Originally Posted by mocambique-amazone View Post
Hy Dave.
There will be one problem:

Different type of teeth at the carrier.
Old style will be with 12 teeth
New style, and with ABS, 48 teeth.
You can't swap a axle with wrong teeth.
The cover will be to exchange. Zero problems to swap them. The old style of sensor will be very expensive now, IIRC 5 times of the new one.

Good luck, Kay
Thanks Kay, both axles have 48 tooth reluctor rings/tone rings and the donor car has ABS as does mine.

My main problem is the electrical connection to the sensor, the donor axle clearly has a socket to accept the same sort of plug as used elsewhere throught the Volvo, the 2-pin "D" shaped plug but i can't physically see how the connection is made on the one in the car.

Logic suggests that as Volvo often fitted whatever they had to hand, the loom would remain the same so the plug (in theory) to plug directly into either type of sensor should be on the car.
Then there's the possibility they changed sensor types at a given VIN number and all those before will have the older style with the threaded collar and possibly a different plug, those after will have the "D" shaped plug.

It has crossed my mind that the correct plug is further back in the loom and was tucked inside the spare wheel well, i'll look later today.

I really didn't want to have to change the covers, disturbing them and their gaskets is usually a recipe for oil leaks! Also i would then have to check the sensor to ring clearance and i don't think my feeler gauges are long enough to do that!
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Old Aug 26th, 2019, 10:02   #4
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Doing a bit of research online, i came across this photo :



For the purposes of crediting the original photographer and avoiding any copyright problems, here's the thread it was on :

http://forums.turbobricks.com/showthread.php?t=319179

It answers my question anyway, there's obviously a plug i can't see because of the position of it.

As soon as i can shift the chewed nut on the prop/diff flange i can get on with the rest of the axle swap. As long as i don't encounter problems with the handbrake it should be a simple spanner job. Everything else has been loosened (and tightened back up again!) so i just need to find a replacement M10 Nyloc for the chewed one and a bucketful each of energy and enthusiasm to do the job!

Thanks for your help and input Kay!
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Old Sep 15th, 2019, 21:45   #5
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Finally got to the bottom of this and have a (correctly) working speedo again!

Mine being a 1988 car, it turned out the diff had a 96 tooth reluctor ring fitted - thanks to Jim (tofufi) for the heads up on that possibility!

That meant with the replacement axle in place with the 48 tooth ring, the speedo was reading half what it should - 30mph @ 60mph true speed and so on.

I discovered through various means of research, Volvo fitted a converter (listed as an ABS Converter by Haynes!) intro the speedo part of the ABS line.

My first thought (after a failed attempt at using a bit of electronics to get round it) was i could simply bypass this converter to get the speedo to read correctly.

Tried that, it was awesome! At 30mph (real speed) the speedo read 120mph!

Back to the drawing board so to speak! PUlled the converter apart and found a binary ripple counter inside, 8 bit which also goes by the names of ring-a-round counter, divide by n counter and many others. Tracing the circuit back on the board, the output was taken from pin 9 of this chip which turned out to be Q3 or the 3rd binary bit, used to divide by 2^3 or 8 in decimal.

That means Volvo were still using the same type of speedo as they'd used for axles with 12-tooth reluctor rings and dividing the frequency by 8 so the 96 tooth ring would feed the speedo the same number of pulses as an unadulterated 12 tooth ring.

Looking at the datasheet for the chip, i found pin 11 was Q2 or the 2nd binary bit, aka divide by 2^2 or 4 in real terms.

The answer was simple, link pins 9 and 11 as pin 11 will give an output twice as often as pin 9 but the input resistance of pin 9 will be high enough to not do any damage or cause anomalies in the output.

Being an SMD chip and my paws not being as steady as they once were for various reasons, i decided my best course of action was simply to "blob" some solder onto pins 9, 10 and 11 - i wasn't bothered about pin 10 as it's not connected.

Here's the before and after shots :





It looks from that angle that i've got pin 12 as well but thankfully haven't. Refitted it and hey presto, a correctly working speedo again!

Now i just have to do similar to the ABS ECU as that registers a sensor fault above 20mph (real speed) as it's not seeing enough pulses. Stupidly the ABS sensor is a direct feed into the ABS ECU and as the previous ones would have been 12 teeth, later 48 teeth, i'm hoping there is a similar set-up in the ECU as both the "converter" and the ECU are Bosch items.

Although the ABS light comes on above 20mph, it is reset and stays off when the car is restarted so i can hopefully get it through the MoT without any problems. As for why my little circuit mentioned above didn't work, i found the reason for that inside the converter. There are input filters to protect the circuitry and "condition" the signal to something it could cope with easier. This was wiping out the output from the circuit i built around an LM2917 F-V converter set up as a frequency doubler.

All good fun but i wish VOlvo wouldn't try to be clever with electronics!
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