New G50 Speed impulse sender over reading

[old man neri]

I have an 88 Carrera. As part of my work this winter I replaced the speed impulse sender because the speedometer was lagging slightly.

I installed part 911-606-910-01-M127. Today was my first drive. The speedometer now over reads by a rough ratio of 2 to 1.

Why?

[brighton911]

That is very strange. The sender is just a on/off switch reacting the the magnets in the trigger wheel within the transmission. The speedo circuitry converts the pulses from the sender to a speed on the dial. On my 86 (with a 915 transmission), the trigger wheel has 8 magnets. My speedo was reading low and after some sleuthing, I found some of the magnets in the trigger wheel were weak and the cause of the low reading. But reading high, I say the problem is with your speedo, unless your magnets have had offspring!

[old man neri]

I'm thinking it's the more obvious; the wrong part or bad new part. Luckily I still have my old sensor somewhere.

[Tippy]

That is really strange. It's just a passive system with a sensor (on diff cover) and magnets (magnet ring on ring gear). The speedo is doing the calculations for speed based off the pulse.

[GaryR]

I'm with Brighton911, sounds like a hardware fault. As that sensor costs 30% less than the Porsche-tax part maybe see if they will exchange it but look to see if there is any way the issue isn't installer related first...

[old man neri]

I've got a G50. I wonder if it uses a different impulse sender than the 915.

I don't know how I could have screwed up the install but I will check.

[brighton911]

If the G50 uses the flying magnets method, then a switch is a switch. And pretty hard to install it wrong. My finger is still pointing to your speedo being wonky. Of course post what you find, always a new lesson to be learned.

[KTL]

The G50 definitely has a different speedo sender than the 915. The G50 actually inserts through a hole in the transmission and is a completely different size & shape. No way they can be interchanged.

The 915 is a big "puck" that sits in a recess, not an actual hole, of the trans case. The magnetic pickup signal is somehow passed through the aluminum in the 915 application.

[dap930]

There is a reed switch molded into the puck. When a magnet passes by the reed switch, magnetic flux from the magnet acts on the reed to close the switch. The reed switch is actually small and delicate enough for this to happen.

[old man neri]

I took it on the high way. It was reading around 60mph....the same it was when I was cruising slowly in the neighbourhood. Slowing down on the off ramp it was going up and down. Basically it's useless.

So I put the old one back in. It's exactly like it use to be...lazy and lagging. I just want this damn thing fixed.

Could this be a bad speedo or is it still pointing to a bad sensor/sender?

[steely]

I think I read somewhere in the past (here) that the 911-606-910-01 is the correct model sender for the G50.
Consider using an ohm meter to measure the reed switch closure on each sender (jack car up and turn wheel). If they both show 8 open/close events per rev, then have your speedometer checked.

Frankly I am already leaning towards the speedo.