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I've checked the FAQ & searched the archive. I found bare wires above the sensor connector so I taped them. Male and female contacts appear ok, they're both shiny & clean. After re-connecting the harness, the speedo will work ok for about 3 minutes, then quit. It does this on the lift as well as on the road (I had to shut it off for an hour to retest). Should these symptoms lead me to the head, wiring, or sensor as the next step? Will the sensor work for a while, then stop? Would having the wires bare and I assume shorting out, damage the sensor?
Thanks.
Marlin
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My 1993 940 had a similar intermittent problem. First I checked the resistance of the ABS sensor at the wiring connector near fuel inlet pipe under the drivers side panel in the station wagon cargo area (the connector was within a plastic box that pops off). Resistance was infinity (should be a few thousand ohms) so I replaced the sensor with a used unit. That solved the problem. While replacing the sensor (easiest with car on lift) I realized there might have been corrosion on the sensor and connector terminals so I cleaned them thoroughly.
The original sensor removed from the car measured about 10% higher resistance than the replacement, leading me to think that sometimes the original sensor was OK and sometimes it was not.
My recommendation is to run the car long enough for the problem to happen then immediately measure the sensor resistance as suggested above. If you measure resistance greater than 4000-5000 ohms then the problem is in the sensor or cabling. If you measure 4000-5000 ohms then the next step would be to measure sensor resistance at the speedometer head connector.
Auto recyclers should have rear axles with intact ABS sensors that can be removed - I would not pay $$$ for a new sensor. Check the resistance on any used sensor. 4000-5000 ohms is what I remember - a published specification may be somewhere on the web.
Speedo and Cruise control will not work unless the ABS sensor signal is present.
Mike
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Checked the resistance at the sensor on the rearend. Resistance was the same, whether the speedo was working or not. Looks like I need to take a look at a replacement instrument cluster.
Marlin
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Hi Marlin,
What ABS/Speedo sensor resistance value did you measure ? I reviewed my Oct 18, 2013 post "1994 940 Intermittent speedometer - OK above 45 mph". The resistance I measured was in the 1250-1350 ohm range, not 4000-5000 ohms as I incorrectly stated in the earlier messages to you.
If you are measuring correct sensor resistance at the speedometer connector behind the instrument cluster then the problem is likely to be the speedometer head itself or perhaps intermittent connections to the head.
My 1994 wiring diagram shows 4 wires on the speedometer connector: Power, ground, + sensor, - sensor. There is a fifth wire on a separate single pin connector that carries the speed signal from the speedometer head to the cruise control unit.
Mike
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The resistance was 1.25 K ohms, so that's in the right range. I have
another '93, so I plan to pull the cluster from it and try on the car that
doesn't work. I was measuring resistance at the sensor itself.
Marlin
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I've still got it on the lift. Can I just check the resistance of the 2
contacts on the sensor?
Marlin
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You can check at the sensor. If the resistance is OK there then check at the connector under the panel near the fuel inlet tube or pull out the instrument cluster far enough to check at the speedometer connector. Do you have a 1993 wiring diagram ?
I would suspect the ABS sensor, connector or associated wiring as a first step because of the severe under-car environment. Do you have any way to buy at low cost or borrow a used ABS sensor ? I have some friends at an independent Volvo shop and they let me try an ABS sensor from a spare rear axle.
Yazaki instrument panels have flex-circuit solder joint problems that commonly cause the fuel gauge to quit or become intermittent (see the FAQ). The speedometer uses a connector separate from the other instrument panel connectors and is less likely to have a problem.
Some failures that appear to be the ABS sensor might be due to corrosion in the connector or wiring. Replacing the ABS sensor could scrape off enough corrosion on the mating connector to restore a good connection. I was worried about fracturing the allen head bolt that holds the ABS sensor. First I cleaned the bolt with PB blaster and used a pick to clean out the allen recess. An allen wrench fully inserted into the bolt head turned the bolt with no problem. With the bolt removed the ABS sensor can be twisted and pulled out. There is an O-ring seal that can probably be reused.
1991 speedometers have a high failure rate but 1993 and newer are much better. Speedometers can fail, as in Spook's case. My experience is with a 1993 and 1994 940. Now that I think about it more, my 1994 had the rear axle ABS sensor failure, not the 1993. Both cars have about 200K miles.
The definitive test for an ABS sensor would be to check the output signal using an oscilloscope or AC voltmeter while the the sensor was connected to the speedometer and the rear wheels were turning. A resistance check is easier and may tell you where the problem is.
Mike
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Dear Mike91,
Hope you're well. I concur in your view, that the under-body environment can be harsh. That could certainly degrade the exposed wiring harness, for the ABS/speedo sensor, as Marlin Mangels reported.
At an extreme, the water-tight seal - where the wires enter the sensor head - could fail, as happens with the crank (RPM) sensor. I'd have thought such deterioration would be as obvious with the ABS/speedo sensor, as it is with the RPM sensor.
Further, if the ABS/speedo sensor in the differential fails intermittently, that should cause the ABS system warning light to come on and to stay-on after start-up. Marlin Mangels did not report any ABS system warning light.
Absent signs of failure in the water-tight seal, where the ABS/speedo sensor's wires enter the sensor head, it seems more likely that the speedo would have failed.
While accessing the speedo's printed circuit board is a tad time-consuming, an inspection takes a few seconds. Either there's some sign of failure - usually discoloration caused by over-heating or a leaky capacitor - or there's no discoloration.
Hope this helps.
Yours faithfully,
Spook
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I expected the ABS light to turn on in my 1994 940 but it never did. The bulb is not burned out - it lights when the key is turned before starting the car. Perhaps the ABS controller only checks for a missing signal under certain conditions.
The ABS/Speedo sensor is sealed with no exposed wires - the 2 electrical terminals are embedded inside the connector housing.
Marlin Mangels problem does seem to be the speedometer head, assuming the ABS sensor resistance is OK when measured at the speedometer connector. I would replace just the speedometer head if the rest of the instrument cluster was functioning properly. A used replacement cluster could have several problems including fractured flex-circuit solder joints and oxidized bulb connections.
The speedometer head in my 1991 740 failed in a dramatic fashion - smoke came out when one of the internal integrated circuits burned up.
Best Regards,
Mike
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Dear Marlin Mangels,
Hope you're well. Intermittent operation can result from signal loss - when the wires touch each other or nearby metal - or from a failure of an item on the speedometer's circuit board.
From the fact that the speedo works for three minutes, and then quits, I'd infer that a component heats-up and that heat causes the component to fail. This is more likely to occur on the speedo's circuit board, than in the sensor, mounted in the differential cover.
The sensor head - like that of the crank (RPM) sensor - is pretty simple. It detects the presence or absence of metal, as a "tone ring" - steel, with holes cut in it - spins, just below the sensor's tip.
I'd inspect the speedometer's printed circuit board, to see if there's any discoloration or other sign of damage. On my '93 940, the speedo went erratic and then ceased working. I found damage on the circuit board. A salvage yard unit - checked by an instrument shop and re-set to the proper mileage - solved the problem.
Your earlier report - of erratic operation of the cruise control - also inculpates the speedometer. For the cruise control to work, the cruise control module requires a signal, showing speed at or over 22 miles/hour. A failure of the sensor, that feeds a signal to the speedometer, should shut-down the cruise control. By contrast, you were unable to turn-off the cruise control. I wonder if that is rooted in a speedometer failure.
Hope this helps.
Yours faithfully,
Spook
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I swapped speedometers between the 2 '93's and the replacement worked ok, so I re-soldered (actually just re-heated the original solder) all the components I could find. I also loosened and re-tightened all the grounding screws. Replaced it in the original car and it worked for 5 minutes on the lift, haven't had it on the road yet but looks good so far. As an added bonus, the other '93's trip odo had been stuck for several weeks, but in the process of punching buttons (reset trip and reset service reminder) I freed that up. I also re-heated the solder on the pcb on that speedo, just to try and ward off evil spirits. With any luck at all, both are fixed. Thanks for the help.
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Dear Marlin Mangels,
Hope you're well. You're welcome!!! It is also a good idea - when doing speedo cluster maintenance - to clean the underside of each screw head. A layer of corrosion only a few atoms thick - and so invisible even under a microscope - can block the flow of electricity. The screw head undersides can be cleaned with fine steel wool or a pot scrubber. A few passes will suffice.
To reflow the solder, it is needful to touch the tip of the soldering iron (15-20 watts) for a second or two. The solder's color changes from dull gray to shiny silver. At that point, any micro-cracks - caused by years of heating/cooling cycles, as power flows start and stop - have been closed.
Hope this helps.
Yours faithfully,
Spook
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You can also buy a new packet of cadmium-plated screws from Volvo for not much. I use DeOxIt to remove corrosion on the ones I reuse.
--
See the 700/900 "FAQ" at the menu bar top screen left side.
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