25 replies to this topic

[dale]

When you start the car the tacho seems to work ok.After about half hour of driving it creeps up higher and higher. When you stop at the lights and come back to idle the tacho goes higher up to as high as 6000 rpm and when you move off it goes down but probably to around 4000 rpm. Does anyone have any idea what my problem is. Car is geniune v8 SS with 308. Thanks in advance

[Potta]

 

Problem Solvered!

[Rockoz]

Wonder whether that is the symptom of the toroid falling of the tacho.

Pretty common for them to fall off.

Very fine wire and the glue holding them on has past its use by date.

[VDO]

Tacho most likely needs balancing.  Have you painted the pointer..?

 

Also with the 4-6-8 switch, I always solder the switch contact into the desired position when I finish refurbishing them.

[RallyRed]

Tacho most likely needs balancing.  Have you painted the pointer..?

 

Also with the 4-6-8 switch, I always solder the switch contact into the desired position when I finish refurbishing them.

yeah...maybe for a simple quick test...run the switch thru its positions about 20 times,,,may just be a dry contact?...not uncommon with little switches and small currents.

[dale]

Thanks Chris. I will be pulling the dash out later this year so I will get the tacho looked art then. I see you are in brisbane and call yourself VDO, do you refurbish gauges?

[TUF 308]

Able instruments Unit 6/10 Babdoyle St, Loganholme QLD 4129

Had a problem with my taco not liking new ignition system. They fixed it up for me. Great service and reasonable price

[arrimar]

I use Lionel Otto Instruments Salisbury, Brisbane.

[hanra]

Tacho most likely needs balancing. Have you painted the pointer..?

Also with the 4-6-8 switch, I always solder the switch contact into the desired position when I finish refurbishing them.

Hey mate did you have a pic of the PCB in the tacho? I’ve got to solder a jumper on a mates tacho and I’ve forgotten what the PCB looks like. Does the needle have to come off to do this?

It’s been a while.

[VDO]

Hey mate did you have a pic of the PCB in the tacho? I’ve got to solder a jumper on a mates tacho and I’ve forgotten what the PCB looks like. Does the needle have to come off to do this?

It’s been a while.

You mean the original PCB or the replacement PCB..?  The pointer on a LH LX tacho doesn't come off, it's fixed to the movement.

[hanra]

Ahhh yes. That’s right. Sorry.

Do you have a pic of the replacement PCB at all??? Or a circuit diagram?

Edited by hanra, 26 June 2022 - 02:19 PM.

[hanra]

This dosnt look like the original PCB. The cylinder select switch does not exist.

Attached Files

•  DFCE8AB9-8EE9-4764-ACA9-C1FC9ECDC15E.jpeg   194.53K   6 downloads

[VDO]

This dosnt look like the original PCB. The cylinder select switch does not exist.

No, that's a replacement board.  When installed they are calibrated for 6 or 8 cylinders.  The cylinder switch was nothing but problems.

[VDO]

Pics of an original PCB.  You can see the crappy way the switch is set up, no wonder it gave trouble.  289181203_5078891378885729_6481682327820120604_n.jpg   98.87K   5 downloads  289283081_1195227421241270_2973402437082914368_n.jpg   102.31K   6 downloads

[hanra]

Good one. Thanks for those images. I’ll run this one up on the sig gen and check for accuracy. The car has an MSD ignition system. And the tacho isn’t working properly. Was going to try a tach adaptor next.

[VDO]

Good one. Thanks for those images. I’ll run this one up on the sig gen and check for accuracy. The car has an MSD ignition system. And the tacho isn’t working properly. Was going to try a tach adaptor next.

MSD ignition systems will usually require a 3 wire tacho board conversion to pick up the low voltage tach pulse, unless it still uses a coil.

[hanra]

Ill double check with the owner. Do you manufacture/supply the replacement PCB's Chris? 

[VDO]

I can get them from the supplier, but installation will require calibration.

[hanra]

Yes, the owner runs the MSD box with an MSD Blaster coil. 

I can get them from the supplier, but installation will require calibration.

 

To easy, I should be able to do that with a sig gen and freq counter yeah? 

 

Im just thinking, if he still uses a coil, can the tacho signal still come from the Neg terminal on the coil instead of the tacho signal coming from the MSD box? 

[VDO]

Yes, the owner runs the MSD box with an MSD Blaster coil. 

 

To easy, I should be able to do that with a sig gen and freq counter yeah? 

 

Im just thinking, if he still uses a coil, can the tacho signal still come from the Neg terminal on the coil instead of the tacho signal coming from the MSD box? 

Yes, sig gen is what you'll need.  Do you know the settings for calibration..?

 

Mostly, if the coil is still being used, you can take the pulse from that, but in some cases a matched partner coil is designed to trigger with a lower voltage signal (5v) and won't run the tacho.  You need to check what trigger voltage the coil is designed for.  It needs to be 12v square wave.

[hanra]

Cheers. 

 

I was thinking it would be a case of: 

 

1000 rpm= 66.66Hz

2000rpm= 133.33Hz

3000rpm= 200Hz

4000rpm= 266.66Hz

5000rpm= 333.33Hz

6000rpm= 400Hz

7000rpm= 466.66Hz

8000rpm= 533.33Hz

[VDO]

Cheers. 

 

I was thinking it would be a case of: 

 

1000 rpm= 66.66Hz

2000rpm= 133.33Hz

3000rpm= 200Hz

4000rpm= 266.66Hz

5000rpm= 333.33Hz

6000rpm= 400Hz

7000rpm= 466.66Hz

8000rpm= 533.33Hz

Yes, correct, for 8 cylinders.  Due to the no-linearity of the scale, after 3000 rpm it tends to go a bit higher incrementally.

Edited by VDO, 27 June 2022 - 02:25 PM.

[hanra]

Yeah Im def finding that on the bench currently. It really over reads above 3000rppm... 

 

Do you use 12v P-P as your level? 

Edited by hanra, 27 June 2022 - 02:43 PM.

[hanra]

How are these replacement PCB's set up for cylinder selection? I cant get this thing to read very well at all.