pcs7556964

With the risk of posting a question that may already have been adressed, but I was unable to find when searching.

I have an early 1990 964, that has a mechanical pulse sender mounted on the side of the gearbox, on the inside there is a disc.

My issue is that have mounted a g50/20 six speed in my car and it does not provied any means of moving parts over as on the 91 gearbox.
The later gearbox have a different side housings that eliminates the hole for the sensor as these cars take the speed signal from the ABS.

Does anyone know the number of pulses that are needed if I am to create my own solution?

Can I convert the signal from the ABS sensor to match my current speedometer?

Thanks for your help,
Tobias

NineMeister

You can solve the problem with some wiring work and a factory control unit. As you rightly say, the later cars take their speedo input from the ABS ecu in the from of a front wheel speed signal, but because the frequency is incompatible with the speedo it is converted through a frequency control unit - a purple looking relay - which is located in the main fuse board. I suggest that you simply copy the later wiring and buy a new or used FCU, however be warned that they cost around $150 new.

pcs7556964

Thanks for the answer.

Do you know if the ABS box handles the signal and the pass it on to the Control Unit or if there a direct connection to the sensor.

One problem I currently have is that my ABS has been removed by previous owner to saver weight and improve control.
______________________
Tobias
964 C2 - 1990 ( with all comfort and assistance removed...)

Geoffrey

The ABS controller outputs a +5v square wave output equal to the number of pulse inputs from the ABS ring the Mag wheel sensor reads. The FCU Colin speaks of is in the central electrics box and then sends the output to the speedometer.

Another alternative is to drill and tap the outside drive flange with equal spaced triggers and relocate the 1990 sensor using a custom bracket.

NineMeister

As Geoffrey says, a modification of one of the drive flanges is probably the easiest solution if you no longer have the ABS system. The stock 964/993 "speedo" differential has a ring of magnets (I think it has 12 from memory??)which actuate the switch, so you would obviously need the same number on the drive flange to maintain correct speedo calibration.

pcs7556964

Thanks Colin and Geoffry for your valuable feedback.

What if I went for a real wild alternative;
Mount the ABS sensors in the front and mount the ABS control unit, after which I do the FCU upgrade that Colin suggested?

_________________
Tobias
964 C2 - 1990 ( with all comfort and assistance removed...)

Geoffrey

I thought there were 8 magents (from memory)???

pcs7556964

I have now studied the electrical wiring chart of the 1989 C4, 1990 C2 and the 1991 C2.

My conclusion is that the speedometer signal is taken from the LF wheel and the C2 has the FCU in R52.
One curios thing is this wiring chart from 1990 C2 which seems to provide two speed inputs, on from pulse sender ( gearbox) and one from the FCU.

Value your input,
Tobias

NineMeister

You're younger than me .....

Geoffrey

If you look at the note on the R52, it says it exists when you have option 249 (Tiptronic). The wiring diagram shows both sets of wiring.

pcs7556964

My mistake, missed the 249 on the 90 diagram and didn't realize it meant alternative.

See that on the 91 diagram the FCU is standard on the 964 C2, so I will try and get the signal through the ABS box, and install the left front sensor.

We have to much snow here in Stockholm to have any Porsche fun so I have to entertain myself with Porsche projects....

/Tobias

pcs7556964

Are the magnets visible on the plate inside the gearbox, I have a brand new one laying i the garage so I could verify the number.

/Tobias

pcs7556964

Having checked on the back of my speedometer 964 641 516 00 it states that K= 4052 impulse / km.

Will have to check my plate as well.

I am leaning towards remounting the ABS and going for a hydraulic 964 C4 type, that I could later upgrade the M/C on.

/Tobias

pcs7556964

Old post but I thought I would give back my findings to the fourm.

I have now installed complete 964 C4 pump and ABS, 993 TT Mastercylinder and booster. Reconected all the ABS wiring and new wires to the pump. I had some electrical issues related to other electrical components, which took some digging to get sorted out but I had good help.

Once all issues had been resolved I mounted the 964 618 220 00 converter in fusebox R52 and put some protecting tape on the connector at the gearbox .

Long way to get a speedometer working but if the ABS had been in place I guess it would have been a fairly easy process.

parkertseng

I am in the final process of converting 1990 C4 to 993 6 speed gear box.
If I understand it correctly,
I can purchase the FCU relay and install it on R52 and is there any additional re-wiring require?
Thanks