740 Speedometer Sending Unit Question

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

this might help

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.

snipped-for-privacy@comcast.net wrote:

> 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 wrote:

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.

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.

James Sweet wrote:

Go to a U Pull It and yank out another cluster, if all else fails.

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

Jamie wrote:

Here are some photos:

-- the sending unit connector with bare wires

-- the sending unit connector with some liquid electrical tape

-- the sending unit connector cut off and rebuilt

-- an assembly near the driver's foot that has a snap on I guess for a ground

That about covers it so far...

Jamie wrote:

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.

Went out for lunch -- no speedo.

C'est la vie.

Jamie wrote:

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

James Sweet wrote:

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.

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

Arnold wrote: