740 Speedometer Sending Unit Question

  • Thread starter Thread starter Jamie
  • Start date Start date
J

Jamie

Howdy folks,
Yesterday I tried to find the cause of my speedometer issue. It has a
mind of its own. Usually only kicks in after driving down the highway
at speed for a short while.

I pulled the dash first and disconnected and reconnected everything. I
reinstalled it and no change. Next, I removed the pan hard rod from
the rear and unplugged the electrical connector from the sending unit
on the differential. The ends of both wires were exposed, so I cleaned
them and the connector with throttle body cleaner. From there I applied
liquid electrical tape, plus electrical tape over the bare wiring. The
wires did not appear to be broken.

I reconnected the unit and still it does not work. I am thinking my
next step is to remove the unit from the differential, but I do not
know what I will be looking for.

Any suggestions?

Thanks!
Jamie

1987 740 GLE
 
I didn't take apart the speedo or any components on the cluster, I just
unplugged the wires and reconnected them in case they were loose.

I wonder which end should I dig into the guts first - behind the dash,
or at the chunk? BTW- where exactly are the wires to the speedo on the
dash?

Can ya'll help me out from memory?

As I recall, from the drivers side going left to right are:

1) a multi pin connector on the lower left.

2) A yellow wire going high center with a plastic lock cover.

3) A brown/white wire going low, just right of center onto 1 of 2 pins.
I use the lower pin.

4) A multi-pin connector to the low right

5) A multi pin connector off to the high right.

6) There is also a 3 wire set (3 or 4 - can't remember), in a weird,
hard to disconnect plastic housing. It looks like you open the plastic
connector downward and back the wires out. I could not get this off and
did not want to break it. It was just to the lower right of the speedo
on the rear.

Can ya'll tell me what any or all of these wires do and which are for
my speedo?

Thanks
 
Thanks. I've read through all of these and while very informative, I
was hoping someone could give me a more descriptive answer on what I
should see when I remove the sending unit from the differential.

This gives a good detail of the wiring, but not much about the inner
workings at the 'chunk'. Haynes talks about using a feeler gauge to
determine the correct distance when installing a new unit.

I wasn't sure if there were any tell-tale signs of problems once you
remove the sending unit.
 
Thanks. I've read through all of these and while very informative, I
was hoping someone could give me a more descriptive answer on what I
should see when I remove the sending unit from the differential.

This gives a good detail of the wiring, but not much about the inner
workings at the 'chunk'. Haynes talks about using a feeler gauge to
determine the correct distance when installing a new unit.

I wasn't sure if there were any tell-tale signs of problems once you
remove the sending unit.
Click to expand...
There is a tone ring, or shutter wheel fixed to the ring gear carrier.
The pick up is a simple coil wrapped around a permanent magnetplaced in
a brass can and molded shut with plastic to form a seal and a plug
connector. The sensor is shimmed from below and held in place by a
threaded cap while it is sealed with an oil ring. The sensors only go
bad when the car is hit from behind driving the panhard rod into the
molded plastic and breaking it or the diff cover or both. Or if
something inside differential literally blows up. If you had to name one
piece on a Volvo that is absolutely bulletproof you have found it.

Now if you have ABS on the car the signal from the vehicle speed sensor
goes to a divider/multiplier circuit where the AC signal from the speed
sensor is split. One portion is converted to a square wave for use by
the ABS controller and the frequency adjusted for that use, IIRC the
tone ring for the hubs is 72 teeth and the ring on the differential is
48, the other half goes directly to the speedo head. The 4 pin plug that
connects to the speedo is the other end of the speed sensor wires, the
Gn/W, Bn wire twisted pair plus a R/Bl switched B+ and a SB-.

If you jack up the rear of the car enough that the rear wheels can spin
free you should be able to read a good AC signal from the sensor wires
with the cluster removed. There should be very little amplitude change
but some increase from wheels turning at idle, but a substantial
increase in frequency with each increasing change in rear wheel speed.
If the signal is there and reliable then the speedo needs to be repaired
or replaced.

Bob
 
Well, lots done and still no speedometer:

1) Removed and inspected the sending unit. Reinstalled - did not know
how to test it.

2) Cut off two-wire connector to sending unit, removed each wire-end
from the connector and rebuilt the wiring using a butt connector
installed in each hole with replacement 12 gauge wiring. I reinstalled
connector with new wires and re-wired wiring.

3) Removed the dash and inspected all connectors

4) Checked ground wires around left foot side of driver's floor. There
was an assembly that had dismounted from the chassis, so I remounted
it.

5) Checked fuses.

Everything else on the dash works - the odometer, tach, fuel -
everything but the speedo.

(sigh).
 
Jamie said:
Well, lots done and still no speedometer:

1) Removed and inspected the sending unit. Reinstalled - did not know
how to test it.
Click to expand...

Measure it with an ohm meter, unless it's open circuit then it's
probably fine. As someone else mentioned, all it is is a coil.
2) Cut off two-wire connector to sending unit, removed each wire-end
from the connector and rebuilt the wiring using a butt connector
installed in each hole with replacement 12 gauge wiring. I reinstalled
connector with new wires and re-wired wiring.

3) Removed the dash and inspected all connectors

4) Checked ground wires around left foot side of driver's floor. There
was an assembly that had dismounted from the chassis, so I remounted
it.

5) Checked fuses.

Everything else on the dash works - the odometer, tach, fuel -
everything but the speedo.

(sigh).
Click to expand...

And the odometer and cruise control don't work either I assume?
 
The odometer is fine, works well. The cruise works if/when the speedo
kicks in.

Everything works fine, only the speedo doesn't work.
 
What is giving me fits is the yellow wire is on pretty tight and I
didn't remove it out of fear of breaking it.

The only other set I cannot remove is I think the speedo set, a small
cluster of wires in a cube (not like the parallel snap-ons), that are
on the lower corner of the speedo. I can't see how these are clipped
in. I want to pull the cluster and inspect the back side, but I mainly
fear I will break this one cluster of wires.

Can anyone tell me how these are removed?

Thanks.

Jamie
 
OK, today the speedo decided it wanted to work. Let's see how long it
lasts...

I am beginning to think Volvo has a 24 hour rule. It seems like when I
adjust something, it doesn't take effect for 24 hours, then it works.
 
Jamie said:
The odometer is fine, works well. The cruise works if/when the speedo
kicks in.

Everything works fine, only the speedo doesn't work.
Click to expand...

Then the problem is obviously in the speedometer itself, the same sensor
and wiring drives both. I had to fix one which had some cracked solder
joints on the circuit board inside.
 
Thanks James.
Can you tell me exactly how to remove what I think are the speedo
cables from the cluster. It's the only group of wires like that. The
multi-pin clips just unclip, and singles wires just unclip, but this
one little group of 3-4 wires on the bottom right goes into a cube
shaped white-plastic housing. I can't tell if the whole housing just
unclips or I have to open the housing to release the wires or what.

This is preventing me from removing the cluster entirely. I really wish
I had a diagram that told me what every wire on the back of that
cluster went to. In an easy to read format.

Thanks.
Jamie
 
Arnold said:
Hi Jamie, take a look at this link. I believe the wires you are talking
about do not go through the wire harness, I think they are jumpers soldered
between 2 places on the cluster itself.

Take a look at this link.

http://www.brickboard.com/FAQ/700-900/ElectricalInstruments.htm#740InstrumentClusterRemoval
Click to expand...


That's for a newer car with a different cluster, I don't recall there
being anything unusual in the wiring of 700 series clusters though, I've
had a number of them apart before but not recently.
 
Thanks Arnold. I had seen this page before, but I read it more
carefully this time and see the section about the capacitor. I just
hope I can remove the speedo wires without breaking them.

Thanks again,
Jamie
 
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