964 Speedo Intermittent Failure

[Country 964]

Hi all,

I'm looking for a bit of info and/or pointers on a new issue I've encountered on my 1993 964 C2 Tiptronic. The speedo intermittently doesn't work.

 

A bit of background: a few weeks ago my alternator belt snapped and given the car's age, I thought I'd replace the pulleys/shims/bolts whilst I had things apart. Despite using a properly-set torque wrench and new (even genuine) bolts, one of them snapped in the alternator fan assembly and I had to semi remove the alternator to sort it out. Also at the time the belt broke, the ABS light came on but hasn't done so since. I can make it turn on by removing its relay (the fusebox one, R34), so am assuming the relay is OK and the monitoring of the system is as well.

 

Not long after, the speedo started playing up. By that I mean the speed reading = 0 km/h on both the analogue and digital displays and the odometer/trip meters don't tick over. Going over bumps can trigger this, suggesting to me there's a loose connection...somewhere.

 

My research has led me to look at the ABS sensors on the front wheels (cleaned, checked cables' contact), the ABS electronic control unit in the luggage bay (to which the cables eventually connect), the relays on top of the ABS hydraulic unit (they're there and seated), the pulse converter in the fuse box (relay 52, again, it's there, seated, no burn/smoke marks on the PCB inside it).

I've read that the front left wheel's ABS sensor is where the signal originates, but previously it came from a sensor near the [manual] transmission.

 

Any suggestions on where to from here? My take is that the speedo works OK, since it seems able to function properly and the computer also registers 0 km/h (unless one supplies the other with the info?) but the fact that it's intermittent suggests a loose connection. But where should I look now?

 

Cheers.

[tomo]

you might find something in these treads from R/L; http://rennlist.com/forums/esp_search.php?query=964+porsche+speedo+no+working&type=gcs

[Country 964]

Thanks, tomo. I think I've read a stack of them but I'm yet to find definitive information as to where the signal comes originates in a '93 C2 Tiptronic and whether the OBC talks to the speedo (and in which direction). Some post say it comes from the left wheel, some from the right....without specifying the exact model. Apparently, the '89 comes from the transmission. I've no fault codes registered on the motronic/tiptronic, either (someone mentioned there may be).

Tiptronic works 100% of the time. So, too, the gear display in the odometer and the lights work 100% of the time, so a loose connection on the back the speedo unit itself is down the list in terms of likelihood. Tacho works 100% of the time, as does the OBC in terms of display, so a loose connection on the back the tacho is less likely, I think.

Since my speedo doesn't have a trip odo, I've push the idea of gear failure down the list, too.

[Mike D'Silva]

When you say the Alternator belt snapped, and you replaced the pulleys/shims/bolts, which bolts are you referring to? 

How well does the belt tensioner sensor work?

When you turn the key on, do ALL the warning lights light up in the clock assembly?

Is there any chance, that one of your allen head bolts has come out of the rpm sensor pickup on the left side of the transmission? Or is it not plugged in correctly?

Are all the wires on the back of the alternator bolted down securely? I still don't understand which bolts you were changing or why...

 

 

[tomo]

Just reading a manual and it refers to this,"Triponic speedometer recieves its inputs via the transmission control unit. The speed input to the transmisson control unit is the same as the 1989/1990-pulse sender via the frequency control unit"  

[Mike D'Silva]

another unique 964 issue, is low battery voltage.

My 964 tipper had all sorts of warning lights, and wouldn't manually shift, if the battery was low on voltage.

Also, I found that the alternator did not start charging, until it had revved past 3000 rpm or therabouts.

2 things I did cured this:

1 - re-soldered all the pins on the back of the CLOCK... yes, the clock. This cured my "high rpm charging kick in". I also made sure that ALL the globes were lighting up; I heard that a failed globe might be a problem. Also, there was one globe, I was told would always drain the battery, and needed to be removed.. you'll have to google that one.

2 - I put a new battery in. Once it was completely drained, I think it was never the same, and would never hold a charge.

Feel free to call to discuss; I'm no expert, but I have been through a bit of stuff...

[Country 964]

When you say the Alternator belt snapped, and you replaced the pulleys/shims/bolts, which bolts are you referring to? 

How well does the belt tensioner sensor work?

When you turn the key on, do ALL the warning lights light up in the clock assembly?

Is there any chance, that one of your allen head bolts has come out of the rpm sensor pickup on the left side of the transmission? Or is it not plugged in correctly?

Are all the wires on the back of the alternator bolted down securely? I still don't understand which bolts you were changing or why...

 

 

Hi Mike 

When I said "bolts" I meant 'pan head screws' (apologies), specifically the ones circled in the attachment. They hold the fan belt pulley halves together/on and I don't think manual models have them (except turbos), as they may have lock nuts?

The belt tension sensor works fine - it was replaced 2 years ago and hasn't missed a beat.

When turning the key on, the lights in the clock assembly seem to function normally. By that I mean they all (except CAT(?) @ 11 and the one at 2.30 (next to the spoiler one) come on, then the airbag light goes out.

I've not heard of the rpm sensor pickup on the transmission - I'll investigate that, thank you for the pointer.

I didn't actually disconnect the alternator at all, just released its strap so I could maneuver it a little. I gave everything a wiggle before reseating it to make sure they were snug/fixed. I had the charger on it whilst it was out of action but I've not had to stick the charger on since.

I replaced the bolts and pulleys because I saw it as cheap preventative maintenance. The car is roughly 23 years old and when considering what all those parts together do (stop the engine from cooking and give it electric power and keep me cool!), for how long (100%), and under the conditions they function (hot), I thought a couple of hundred $ was worth it. I had read stories of pulleys breaking and since I'm in country NSW, getting the right part quickly can be a drama, so if financially feasible/not stupid I'll try to replace associated parts where possible.

I'm relatively new to this car stuff (probably obvious to most readers!), let alone Porsches, so I'm finding my way as I go with a bit of research and help from the crowd, so I appreciate you taking the time to respond.

Cheers.

 

Just reading a manual and it refers to this,"Triponic speedometer recieves its inputs via the transmission control unit. The speed input to the transmisson control unit is the same as the 1989/1990-pulse sender via the frequency control unit"  

Sounds like useful info - I don't suppose there's a handy illustration to go with it? Which manual is that, please?

Cheers.

another unique 964 issue, is low battery voltage.

My 964 tipper had all sorts of warning lights, and wouldn't manually shift, if the battery was low on voltage.

Also, I found that the alternator did not start charging, until it had revved past 3000 rpm or therabouts.

2 things I did cured this:

1 - re-soldered all the pins on the back of the CLOCK... yes, the clock. This cured my "high rpm charging kick in". I also made sure that ALL the globes were lighting up; I heard that a failed globe might be a problem. Also, there was one globe, I was told would always drain the battery, and needed to be removed.. you'll have to google that one.

2 - I put a new battery in. Once it was completely drained, I think it was never the same, and would never hold a charge.

Feel free to call to discuss; I'm no expert, but I have been through a bit of stuff...

Yes, the former custodian of this car (my brother) mentioned that if he'd had to jumpstart it, the tiptronic system was a bit of a pain and wouldn't shift (or something along those lines). 

Resoldering the clock sounds like a bit of drama, but if I start getting low battery issues I'll give that a crack. I've noticed in posts around the web that some dodgy soldering in the clock module can stuff up things like the airbag system etc., so maybe that too would be some preventative maintenance!

I've also fixed the one globe you mentioned. It's in the tacho and to fix it, all one needs to do is replace a single 75c transistor! Tore Bergville's got a great DIY article on it. For what it's worth, his remote central locking add-on ("T-Lock", $85 USD)) was the first thing I ever did to this vehicle and was well worth it. He also makes an OBD dongle that works with some free software (ex-Rennlist) that's handy when it comes to clearing faults, adaptations etc. Those two devices are the best $ I've spent thus far.

 

So I guess now it's time to stick it on the hoist and check the left side of the gearbox for that sensor pickup / cabling!

Thanks again.

[tomo]

Sorry Mate no photo's it was Adrian Streather "Enthusiasts Companion for the 964

[Country 964]

I've finally had some more time to research and troubleshoot this PITA, but it's still not yet fixed.

To summarise:

• '93 964 Tipper - the speedo works intermittently. This includes the OBC displaying 0 km/h and neither the odometer nor trip meter tick over whilst speedo = 0 km/h.
• Conflicting information as to where the data is generated - as @tomo has noted, Adrian Streather's book suggests it's from the transmission (pp 373). However, a technical bulletin (#9106) says "The road speed signal for the speedometer and several other control units is produced by the left front ABS wheel speed sensor. The reed contact in the transmission was eliminated." In relay position #52, a frequency conversion unit (964 618 220 51) converts 45 ABS pulses into 8 pulses for the speedo. The bulletin is applicable to 911 C2 with tippers from MY90 (plus a couple of others).
  
  Streather also notes in the troubleshooting section (pp 421) for 91-94 tippers that if:

1. Speed pointer is not moving;
2. Odometer is not functioning;
3. Rear spoiler warning lamp remains illuminated after driving off 
   
   then either relay R52 has failed (i.e., the FCU) or
   there's an internal electrical failure of the speedo.

In my case, if the speedo is working when I start the car and drive off, then the rear spoiler light does extinguish as it should. If that were the case most of the time, then Streather suggests it's a mechanical failure in the speedo.

Short of replacing the speedo, I'm running out of ideas on this one, but this is what I've tried:

• Recharging the battery, just in case it was something along the lines of what Mike suggested above;
• Cleaning the electrical contacts on the back of the speedo;
• Cleaning the pins on / reseating the FCU;
• Removing/cleaning the front left ABS sensor, reseating its plug. The cable from it through the grommet into the luggage bay and into the wiring loom seems undamaged;
• Replacing the FCU (old part 964 618 220 00, new part 964 618 220 51) - $330

The ABS light doesn't (ever) come on, so I don't think the wheel sensor is the issue. I know the ABS light works, too - pulling the ABS relay out tests that and the buzzer . 

On Rennlist, someone mentioned 7 years ago that "there apparently is a connection between the ABS sensor speed, and the Tiptronic speed sensor" - but without elaborating. Does that ring any bells for anyone? According to the '93 tipper wiring diagram, sheet 16, the transmission control unit does have an input for the wheel speed sensor (pin #12) and also connections to the speedo, but just for the gear selection lights.

Any suggestions on where to next? The fact that it's intermittent says to me a loose connection somewhere...but where? This is doing my head in  .

[Tit]

Maybe it's time to go to an auto electrician?

[tomo]

Yes Country, I think that i agree with Tit, Time for a Professional on the job before it sends you bonkers

[Country 964]

I think I've worked out that it's the speedo itself.

I ran a couple of wires from the pins of the frequency converter (R52) (output & gnd) into the cab and stuck a voltmeter on it. When the car is doing 0 km/h, the voltage is about 8.08V. As the vehicle speed increases, the voltage starts dropping in a linear fashion. At 75 km/h, the voltage was about 7.5V. 

Even when the speedo decided it had had enough and said I was stationary, the voltage readouts remained consistent in so far as they declined as speed increased. So without an oscilloscope handy, to me that means that there's no break in the feed to the FCU. Also, as the lights in the speedo stay on, that suggests that there's no problem with the GND connection which is shared between the light bulbs & the electronics.

I also spent a bit of time deciphering the electrical schematics and seeing if there was any truth to the idea that the transmission was involved on these later 964s, and this is what seems to be the case (from memory, I've not got them in front of me):

Front left wheel ABS sensor --> ABS control unit --> FCU (R52) --> Speedo --> other control units (OBC, spoiler, cab roof, cruise control). Interestingly, there also seems to be a connection from the transmission control unit to the ABS control unit and the front right wheel sensor. Tomo/Tit - you were right: reading the schematics did send me a bit bonkers.

So from here, I guess it's off to Howard Instruments, unless anyone has other suggestions?

[Country 964]

It turns out it was a 'faulty stepping motor' in the speedo, diagnosed and repaired by Howard Instruments, roughly $200 worth.