G50 Speedometer Sensor Problem

Woodyhfd · 10-07-2009, 08:41 AM

My speedometer ('87) was dead below 30 mph and then would spring to life and work properly. The odometer has always worked right.

I found this and replaced the $70 sensor:

http://forums.pelicanparts.com/uploads15/G50impulse1248984991.jpg

I still had problems. The speedometer would come to life at lower speed, but then would operate erratically with no real relation to actual speed.

I figure the problem must be within the speedometer so I sent it off to North Hollywood Speedometer, along with $147.

I reinstalled it and now the needle bounces below 20 and registers erratically above that, generally settling around 40 mph regardless of speed.

What do I look at next? Dirty or missing magnets possibly? How many magnets do the G50's use? I know that earlier transmissions use eight. Any tricks to getting them clean without disassembly?

Thank you.

Woodyhfd · 05:53 PM

UPDATE:

I was going to try swapping back to the original sender and try it with the refurb'd speedo. I had originally swapped it from underneath with the car on ramps. This time, I put it on jack stands and removed the driver's side rear wheel. Much better. I was able to remove the sender through the wheel well. After removing the bolt, I pulled it out and tried cleaning the magnets (eight). I used a small piece of dowel and a rag. I poked the rag through the sender hole and turned the brake rotor by hand. I decided to pop that sender back in and test it out.

I was a little surprised and very happy to find that the speedometer now worked much better. It was a still few MPH off from the GPS, but I expected that and, overall, I'm satisfied.

Next time, I'll clean the magnets first and maybe I can save two hundred bucks.

kidrock · 10-07-2009, 02:08 PM

thanks for this informative post, I wasn't aware of the magnets and their location, or their need for occasional cleaning.

flmont · 10-07-2009, 06:44 PM

I thought the magnets were on the inside of the tranny mounted to a spacer plate,..and the transducer was activated by the magnets ??? FM

twistoffat · 10-07-2009, 11:22 PM

I just got my Speedo back on the same car. The speed shown was slightly erratic and it wasn´t counting the miles. Its fine now but for what its worth my mechanic said the Tacho are like a good watch and the people who rejuvinate them need to know their stuff. Get down to Porsche and they should be able to pop a functioning replacement in quickly to see if it works. That way you can establish if the actual speedo is the problem or not. It only takes a second and should cost nothing. Mine had worn cogs that needed replacing.

FLYONWALL9 · FLYONWALL9's Garage

Quote:

Originally Posted by Woodyhfd

UPDATE:
I was a little surprised and very happy to find that the speedometer now worked much better. It was a still few MPH off from the GPS, but I expected that and, overall, I'm satisfied.

When I got my car (also uses a g50) I was following a buddy
home. I kept wondering why in the hell he was in such a hurry
to get home. My speedo was reading 90mph, on a beer/food
break I asked what the deal was. He said he had his cruise
set at 75. At 70mph my car would be very near 90, my ony
guess was that it was for a different model and the gear was
incorrect for the G50?

Cattler · Cattler's Garage

I have the same problem--jumpy below ~30-40 mph and then solid as a rock above. My speedo's pretty accurate (within 5 mph) once it settles down. I bought the sensor last year but haven't yet installed it.

How difficult was it? It just bolts onto the outside of the transmission? If it penetrates the transmission case, any concerns about fluid leakage when replacing it?

Sorry for the dumb questions.

Thanks,
David

Woodyhfd · 10-08-2009, 06:35 PM

David,

The only difficult part was snaking the wire up and over the transmission.

When I disconnected the old sensor behind the front seats, I tied a string to the end and pulled it up and over the transmission housing. Then I pulled the new sensor wire back over with the string. Just pull gently and help it along with your hands, as there are a lot of places where it can get snagged.

Use jack stands, not ramps, and remove the driver's side rear wheel (G50 cars). You'll be able to see where the sensor goes into the transmission from the side of the car.

Actually, I had a hard time pressing the rubber grommet back into the hole in the floor pan as well, but eventually it went in.

The second photo shows the hole with the sensor removed. The sensor wire DOES NOT go through the plastic clip as shown. I just put it through there to keep it out of the way when I cleaned the magnets.

When you install the new sensor, put a little oil on the o-ring, make sure the area around the hole is clean and make sure the sensor goes in straight. I used a new spring clip ($6) and I put some Loctite on the bolt. It's a small bolt so don't over tighten it. The the wire goes up and over the top of the transmission and back through the floor pan into the back seat area.

twistoffat · 10-09-2009, 04:21 AM

For what its worth when my Speedo went it was only a tooth on the Cog. They are actually soft and you could break them off with a finger nail. I still have the culprit part here.

[IMG][img=http://img522.imageshack.us/img522/1137/09102009169.th.jpg][/IMG]

rick-l · 10-09-2009, 06:55 AM

When you cleaned the magnets what came off? Any ferrous metal?

Woodyhfd · 10-09-2009, 10:53 AM

It was just a greasy, dirty smear.

Cattler · Cattler's Garage

Thanks again for posting this information--I replaced my sensor today. Some other things to keep in mind: the speedometer sensor wire likely will be held down by ~3 clips if it hasn't been replaced (2 of these clips are on top of the transmission); you don't really need to use a string on the old sensor wire to pull the new one along; jack the rear up and put it on stands.

David

rcaradimos · 11-29-2009, 07:35 AM

Quote:

Originally Posted by Woodyhfd

UPDATE:

I was going to try swapping back to the original sender and try it with the refurb'd speedo. I had originally swapped it from underneath with the car on ramps. This time, I put it on jack stands and removed the driver's side rear wheel. Much better. I was able to remove the sender through the wheel well. After removing the bolt, I pulled it out and tried cleaning the magnets (eight). I used a small piece of dowel and a rag. I poked the rag through the sender hole and turned the brake rotor by hand. I decided to pop that sender back in and test it out.

I was a little surprised and very happy to find that the speedometer now worked much better. It was a still few MPH off from the GPS, but I expected that and, overall, I'm satisfied.

Next time, I'll clean the magnets first and maybe I can save two hundred bucks.

Thanks for the tip, I checked mine out w/ a DVM and got 5 sometimes 6 readings, cleaned the sensor and magnets with some cue=tips. Rechecked and I got 8 readings.
Thanks again for the post,

rick-l · 11-29-2009, 07:58 AM

Quote:

Originally Posted by rcaradimos

Thanks for the tip, I checked mine out w/ a DVM and got 5 sometimes 6 readings, cleaned the sensor and magnets with some cue=tips. Rechecked and I got 8 readings.
Thanks again for the post,

Do you have an open differential and have both rear wheels jacked up? Non-ferrous dirt on the magnets should not make any difference.

I was thinking, but don't know, the magnets on the ring gear should go around half as fast as the wheel if the other wheel is stationary.

rcaradimos · 11-29-2009, 09:37 AM

Just the drivers side up, you can see the very top of the magnets as you turn the wheel... 8 per one full revolution. 189k on the clock so I'm sure the material was somewhat ferrous.

Roger Shultz · Roger Shultz's Garage

+2 on Jeff's post and procedure.

Based on Jeff's post above and followup emails, I followed what he did and it cured the problem. Like Jeff I cleaned the magnet area with a dowel rod wrapped in a paint thinner soaked Handy Wipe and the problem of an intermittent disappeared!

My symptoms seemed heat related. The speedometer operated normally for 10 to 15 minutes, then would start dropping to zero for 12 to 15 minutes, and then it would resume normal operation until stopped and cooled down. It repeated this cycle over the last 4 to 6 months with no exceptions!

I was surprised this cured the problem but it did and for now I'm a happy camper. If it occurs again I will replace the transmitter. Note: The transmitter on the 87 Carrera is on the drivers side, unlike earlier models. This change may occurred with the intro of the G-50 transmission.

THANKS JEFF!!!

Roger

88911coupe · 03-09-2011, 02:10 PM

I'm having a similar prob. Called a guy in Austin who really seems to know Porsche speedos and in my case I think I screwed up a diode (b31?) when I was taking the speedo in and out and accidentally may have gotten the wrongs wires to touch of the mess of speedo wires in the dash hole. More of an FYI to others to be very careful when pulling all those wires off. I'm going to send it off to be checked/fixed. I was never able to "test" my sensor (or it's dead) since I could not get any response doing the tests mentioned in another thread.. I'm pretty sure my sensor is okay since it seems unlikely that it would fail at the same moment I'm fiddling around with the wires behind the dash but the speedo guy says he'll test it first to see if it's working properly.

88911coupe · 04-04-2011, 07:28 PM

Can someone shed a little light on exactly what the magnets look like? I can rotate the wheel one revolution and only see what appear to be 4, not eight, dark spots? on the wheel like device just inside the sensor hole. Behind that when I spin the wheel are some large bolt heads or something. I tried to clean off the 4 things I could see, whatever they were, and got some dark gray dirt/grime off but this had no effect on the speedo. It was tested as fine by Overseas Speedo and I've put a new sensor from our host. The one thing that seems a bit odd is that the new sensor does not fit snugly in the hole, at first I thought it was missing the oring but it's on there...just not real tight. I tested the sensor with a VM and it seems to be working fine when I got a magnet near it.

KTL · 04-05-2011, 09:26 AM

Here's what the sensor ring looks like when it's installed on the differential

88911coupe · 04-05-2011, 11:05 AM

Thanks! When I look inside the hole for the sensor I see what looks similar to the outer edge of what's in your pic but when I rotate the wheel I only see 4 little spots that look like those magnets. Are there different versions of the sensor wheel possibly? I cleaned what I could see and didn't see much change that I could tell, very frustrating!