915 Transmission Gear Noise - help

[mdb400]

I have a 84 911 Carrera. I recently had my transmission rebuilt by a Porsche specialist. Why? Because 1st, 2nd, and 3rd gear synchronizers, dog teeth, and sleeves were weak and no longer doing there job. The shop replaced all synchros, dog teeth, operating sleeves, and most of the bearings (not all, only the bad ones).
The transmission was rebuilt and ready to be installed.

Now comes the bad part of this transmission rebuild adventure. My first drive in the newly rebuilt transmission came with a new noise. The noise was a clicking (metal) sound inside the transmission. The noise was very disturbing. Not a self-destructing sound, but more of a loud click, click, click, tap, tap, tap sound. Like something on gear rotating and hitting/tapping something on each rotation. The noise is most noticeable at lower speeds/rpms and on deceleration. I also had a problem with second gear being hard to engage and sometime grinding. Not good. All other gears shifted without issue. Although this second gear problem was a growing problem. Meaning in the few miles (50) of driving it was getting worst.

Contacted the Porsche specialist and at his request I drove the car to him (100 miles). He agreed something was wrong. Removed and rebuilt the transmission at no extra cost. He determined that the second gear synchro was bad and replaced it with a new one. After putting everything back together and driving it, he thought the problem was fixed.

Well...almost. The transmission shifted correctly in all gears, but after driving a few miles (fluid warming miles) the noise was started to sound again.

??? What is the noise? Does anyone have an idea? What is inside the transmission that could cause a clicking/tapping noise?

Below are some notes I took after the second rebuild.

The transmission is quiet when cold (fluid), and shifts correctly. As soon as the oil starts to warm, the noise starts. When the oil is up to temp the noise is very clear. Although noisy the transmission seems to be shifting okay, and second gear is much better.
When do I hear the noise?
• under hard acceleration the noise is not noticeable (any gear).
• under moderate acceleration the noise is barely noticeable, mostly at the beginning of acceleration/lower rpms.
• under soft/slow acceleration the noise is very noticeable, but gets quieter as the rpms rise. 3500ish and up is quieter
• decelerating = noisy. loudest at lower rpms (under 3000rpms in all gears).
• Coasting in second gear at about 25-30 mph is quiet with clutch pedal down, disengaged. With the clutch pedal is released (engaged with no gas) the gear/clicking noise is loud. Tick tick tick tick....
• As the gears are engaged the noise starts speeding up to the pace of the engine/gear speed. Doing the same thing at coasting speed in 3rd gear the gear clciking noise is slower, but at the same basic volume as it was in 2nd gear.
• Constant driving in 3rd gear at 30 mph the noise is noticeable and constant.
• The pace of the noise goes with the speed/pace of the car. Slow speed - slower clicking, faster speed - faster clicking.

I would really appreciate any thoughts on this topic. I'm trying to determine what could be causing this noise.

Thanks
Mike

I checked fluid level. It was good.

[sc_rufctr]

The gearbox guys should chime in...

But I suspect the noise is coming from the ring a pinion... "the noise is most noticeable at lower speeds/rpms and on deceleration"

I'm assuming a lot of things here. Like your mechanic checked the shift fork adjustments including the one for 5th/Reverse and all of this is within spec.

Sounds like an expensive rebuild. Some good news... I think you gearbox (1984) has the later smaller input shaft bearing so I don't think you'd have a spun pinion shaft bearing.

Hope it's a simple (cheap) fix.

[mdb400]

Thanks Peter for the reply.
I will note both of these possible causes. I'm interested in checking into the ring and pinion.

After the first rebuild my mechanic did have the thought that noise could be coming from the 5th/reverse shift adjustments. But after tearing it down he didn't say anything about shift adjustments anymore. His focus went to the second gear synchro problem, which I think he thought was the most likely reason for the noise and shifting issue.
Yes, this transmission has been a little expensive, but the most frustrating part is I'm not driving the car.

Thanks again.

"the gearbox guys"
Who are they?


Mike

[sc_rufctr]

Originally Posted by mdb400

"the gearbox guys"
Who are they?


Mike

Mike

Peter Zimmerman and Porsche Monkey are the "Gearbox guys"...

They should chime in soon.

[Peter Zimmermann]

Was anything done with the clutch while the car was apart for the trans work?

[Porsche_monkey]

I am a gear box guy. Sort of. But I am stumped. And I defer to Peter Z on all serious issues.

Did he remove/disassemble the the diff when he did the rebuild?

[Steve@Rennsport]

Mike,

If the frequency of the noise changes with the speed of the car, it can be:

1) Differential

2) Speedo sensor ring on differential

3) Axle, CV, brake rotor issue of some kind

Ring and pinions don't generally make those kinds of noises.

Its time to get it up on a lift (or on jackstands), pull the rear wheels, run the engine in gear, and see if its come from the transmission, drive train or brakes.

[Peter Zimmermann]

Originally Posted by Porsche_monkey

I am a gear box guy. Sort of. But I am stumped. And I defer to Peter Z on all serious issues.

Did he remove/disassemble the the diff when he did the rebuild?

Me too! That's why I asked about the clutch. I'm also thinking about CV joints...

After being apart a second time you would think the mechanic would have seen evidence of something touching/hitting - shiny spots, metal bits from contact...something. 5/R slider touching the idler... Did they damage a handbrake cable during R&I and its housing is hitting the outboard CV joint bolt heads on one side?

[mdb400]

I'm back. Thanks guys for the replies.

Clutch: I replaced the clutch with a new pressure plate, clutch disc, throw out bearing, and release fork (slightly bent).

I removed the engine/transmission for the first transmission rebuild. It was not my first time to remove a 911 engine. The second rebuild was by the Porsche specialist.

Interesting thought about the CV joints, but the noise speed is different in each gear when at the same mph. Driving at a constant 25mph and changing from 2nd to 3rd the clicking noise changes speed.

Differential? hmm. Interesting. What could have caused it to start making noise?

Speedo sensor ring on differential. Honestly I don't know much about this.

When I get enough possible causes I will contact my mechanic.

One other item that my mechanic and I thought we could try is replacing the synthetic gear oil with a standard gear oil. The transmission never had synthetic in it before the rebuild. The thought is since the gearbox is quiet when cold and noisy when up to temp. Using the standard transmission oil would be heavier than synthetic.

Thanks everyone.

Mike

[sc_rufctr]

Originally Posted by mdb400

One other item that my mechanic and I thought we could try is replacing the synthetic gear oil with a standard gear oil. The transmission never had synthetic in it before the rebuild. The thought is since the gearbox is quiet when cold and noisy when up to temp. Using the standard transmission oil would be heavier than synthetic.

Thanks everyone.

Mike

What brand "Synthetic" Gearbox oil are you using?

The default lots of people here are using has become Swepco 201 in normal climates. One side benefit is the colour of the oil. If there are any metallic particles in the oil they are very easy to see when you're changing the oil. It's dark blue...

However I don't think this could be causing the clicking sound. That has to be metal on metal contact.

At this stage I wouldn't drive it the way it is. Maybe you need to take it somewhere to get a second opinion. Has the side cover for the differential been opened? It is possible to rebuild/freshen a 915 without touching the side cover.

BTW: You should run mineral based oils in the 915.

[Joe Bob]

Does it match the speed of the car or the speed of the gear?

How are the CVs?

[mdb400]

I don't know the brand of synthetic gear oil used. I would guess the type of gear oil wouldn't make the noise go away, but I thought I would share the idea. I am interested in looking at the oil to see if there is any metallic particles.

I don't know if the side cover of the differential was opened during the rebuild. I will ask.

I looked at my invoice, I purchased a rear pinion bearing and rear main shaft bearing.

Mike

[otto in norway]

Hi there

I had a similar issue with my car. Turned out to be loose lug nuts for the spacers on both sides.(!)

link:
CLUnk noise when coasting...?

[mdb400]

matches the speed of the gear.

Mike

[mdb400]

I may need to clarify something.
With the car moving, let's say 30 mph, and the clutch disengaged the clicking noise is gone (silence). As soon as the clutch is engaged the noise starts.
So what I'm hearing is a gear (or something rotating in the transmission) clicking (tick, tick, tick, tick), and as soon as I disengage the clutch (push the pedal down, with the car still moving) the noise starts to slow down and get quieter until it's silent.

Question: Why is the transmission quiet (like a good transmission) when the oil is cold?

mike

[otto in norway]

OK.. -Sorry, I didn't get that.
But I can imagine a defective bearing on one of the shafts inside would give some similar noise. The cold oil can be enough to prevent it from vibrating, but when it's warm and thin, it will be flowing quick enough for the shaft to vibrate in the bearing.

Just a theory, of course, but if the sound follows the gear, not the car, it has to be something inside the box, that's for sure...

Good luck!

[al lkosmal]

If the idler gear (5th & reverse) is not spaced correctly (1mm) away from the slider gear (5th & reverse) it can make contact and click. If the sound goes away in 5th and reverse, that may be your problem.

[Peter Zimmermann]

Originally Posted by al lkosmal

If the idler gear (5th & reverse) is not spaced correctly (1mm) away from the slider gear (5th & reverse) it can make contact and click. If the sound goes away in 5th and reverse, that may be your problem.

That's what I was thinking in post #8 above...

Regardless if an "oil change" fixes the problem or not, you should NEVER use synthetic gear oil in a 915 trans. My preference is Kendall Three-Star 80W/90 DINO gear oil...

[Porsche_monkey]

I wonder if he 'adjusted' the shift arms?

[Mahler9th]

I like Steve's suggestion... but of course be very careful. Never use synthetic in a 915? Why not? A bunch of us in the Porsche Racing Club use Redline in our 915s... putting through more than 300 bhp...