740 Speedo :(

[natmargetts]

Good evening all,

Typical 740 Speedo issue.

Very intermittent speedometer- usually doesn't work when car is cold but after a while of driving, will come back to life.

Speedbumps / potholes will either kill or revive it. A heavy smack on the dash sometimes brings it back to life but not always.

I've had it apart a few times now trying to find the problem. Could not see any cracked solder on the board that attaches to the speedo unit itself, but reflowed a couple of dodgy looking ones anyway. Also cleaned up the pins that go from the speedo to the main instrument cluster. Cannot see any blown caps either.

Do some people apply solder to the weird clip looking items on the PCB sheet of the cluster unit itself? (See picture with resistor on, its those kind of connectors that I'm on about).

Should I reflow ALL of the joints on the speedo board?

Kind regards,

Nat

[bob12]

Have you actually unscrewed the speedo pcb from the speedo unit and replaced the small drivetrain gear wheel below the stepping motor?

It's unlikely there's a fault in the blue plastic circuity on the white backboard or a the speedo pin connector. It's easy to misalign the pins when re-connecting the speedo to the white backboard.

Why is the dimmer in pieces .... that's likely a separate fault?

Looking again at the pic of the speedo I note that the pcb is shielded by the clear plasic casing. That suggests you haven't taken the speedo fully too pieces by removing the needle (with the utmost, utmost caution) and the dial face, to deal with the drive gear where your fault is likely to be found.

Bob

[natmargetts]

Hi Bob,

The plastic cog has been replaced in the past, but that only controls the odometer. My fault is specifically with the speedometer (no speed display).

I can safely rule out the coil windings for the needle, as when I get no speedo output, the odometer motor also stops. This would imply that something prior to these two motors is losing connection.

I have had it all apart in the past, but didn't want to risk that procedure again for a few demonstrative photos this time.

I also know that the pins weren't misaligned when putting it back together, as it still intermittently works as mentioned before- if they were misaligned then it would simply never work.

The dimmer was removed because I couldn't remember which screws needed to be taken out to separate the two gauge cluster halves, so simply removed them all in that area

This is why I'm in quite the predicament on what to chase next...

Kind regards,

Nat

[bob12]

Thanks for the update on what you have done Nat. Was the gear wheel part stripped?

I have experienced similar problems. I assume you are sure the signal from the diff is 100% all the time.

Reflowing the the speedo pcb and testing the caps and resistors values can be a fix but it can also be the chip on the pcb is playing up after all these years. I have even found that the tightness of the 2 screws on the stepping motor can be problematic. I regret to say have had to admit defeat on some speedos in the past!

Sorry this isn't much help!

Bob

[Laird Scooby]

It sounds like you're losing the signal from the diff sensor because both speedo and odo are stopping at the same time. Assuming the other instruments continue working, power is still getting to the speedo so that only leaves the signal. Does the car have ABS?

[natmargetts]

Morning both-

Bob:

Thanks for the update on what you have done Nat. Was the gear wheel part stripped?

Yes- one of the teeth had broken off- its the classic problem with resetting the odometer while moving I think- this was a blight of the previous owner though. I tried one of the £10 kits off ebay but the gear was too small (diameter, the teeth count was correct). Had to buy one from Germany for £25!!

I have even found that the tightness of the 2 screws on the stepping motor can be problematic

I wouldn't even know where to begin with this!!

Mr Scooby:

Assuming the other instruments continue working

Aye

Does the car have ABS?

Nay

power is still getting to the speedo so that only leaves the signal

Agreed. I have redone the connection to the diff sensor in the past and have checked it recently using an Arduino while driving the car so I know that it is fine.

The symptom of me smacking the dashboard which causes it to come back to life suggets that it is something in the cluster / board itself regarding the signal - but it is hard to isolate when the problem is intermittent- I've tried my best to check as many of the blue plastic traces as possible for continuity and all seem fine! Losing my mind!

[Laird Scooby]

Originally Posted by natmargetts

Morning both-

Bob:


Yes- one of the teeth had broken off- its the classic problem with resetting the odometer while moving I think- this was a blight of the previous owner though. I tried one of the £10 kits off ebay but the gear was too small (diameter, the teeth count was correct). Had to buy one from Germany for £25!!



I wouldn't even know where to begin with this!!

Mr Scooby:

Aye

Nay

Agreed. I have redone the connection to the diff sensor in the past and have checked it recently using an Arduino while driving the car so I know that it is fine.

The symptom of me smacking the dashboard which causes it to come back to life suggets that it is something in the cluster / board itself regarding the signal - but it is hard to isolate when the problem is intermittent- I've tried my best to check as many of the blue plastic traces as possible for continuity and all seem fine! Losing my mind!

Losing your mind? Welcome to full-fledged Volvo ownership!

Joking aside, where did you insert the Arduino i the circuit? Was it across S (signal) and earth (0V) on the speedo connectors?

Also try cleaning all the copper strips on the back of the cluster and the "fingers" in the multiplugs that contact them. Also use a thin screwdriver or similar behind the curve of the contacts in the plugs to "stretch" the curve out to increase the tension on the contacts and hence the pressure of the plug contacts against the strips on the PCB which usually solves a lot of problems.