Goughary

I took out both accelerometers and cleaned them. Question is - if there is a build difference in the 964 v 993 accelerometers, and the 993 accelerometers dont need cleaning and dont seem to fail, has anyone tried them out back to back and noticed a difference in a C4?

Example, you are driving along and you get to a bumpy section of road, the 964 accelerometers lock the diffs and the pdas light is on. Does the 993 accelerometer do the same when there is no need to lock the diffs? I am making the assumption here that since the accelerometers change the braking characteristics when they light up the pdas, and the braking is much worse, does the 993 accelerometer design allow for better braking in some situations? Mine antilock is turning on way before it should in these conditions. I am imagining that the copper tab in the acceleromoter is just bouncing all over and coming up with force readings that are to the extremes when there is no need.

nvrlft

There is no difference. I put the 993 one "in front" of the 964 as it was far easier to screw back in.
Interesting that you have one of each as I did. This appears to be common.

Goughary

Nooooo, I don't have one of each. Mine are both the earlier version...and original to the car. (1991 C4)

Quesiton is for those of you that have swapped out both - Is there a performance difference particularly on dry bumpy roads under braking...

I'd love to test them both - but no way to do it given the cost of replacement units.

alexjc4

Sounds like either your accelerometers or wheel sensors aren't working correctly.

Are the copper pendulums completely free to move? It took some cleaning to get mine so they would move just by tilting the sensor.

From my understanding the abs first fires in response to lockup detected by the wheels sensors then, once firing, it modulates based on the g sensors and the wheel sensors combined input. So I suspect you have wheel sensor issues.

(Since cleaning my g-sensors my abs operates pretty nicely)

Goughary

yeah - i cleaned both and they work fine -I was tinking to check the wheel sensors next...Any link to a discussion on how to get to and clean/replace the wheel sensors?

alexjc4

the wheel sensors are pretty easy just take the wheel off and they are bolted into the top of the hub behind the disc with a little cap head bolt - not tight but reasonably likely to round out if its never been disturbed - the earth strap is also mounted to the bolt that holds the sensor in.

Once unbolted give the area a good blow so nothing falls into the hole and gently slide the sensor out, its about 2inches long and magnetic. Don't loose the little rubber O ring which will probably stay in the hole, when you put it back in fit the o ring to the sensor rather than leaving it in the hole to avoid it getting all bound up and dragged in to the hole.

I'd also check the continuity of the earth strap while you're there - just make sure there is a good connection between the terminal and the connector block, which is attached to the shock on the front / the swing arm at the back.

Goughary

Nice- thanks. I'm having my shop replace my shocks, struts and rear swaybar in a couple weeks, maybe I'll just add a check there onto the deal...

thanks for the help.

alexjc4

i'm not sure how often these abs wheel sensors fail, but on other cars you sometimes see it, the toothed wheel in the hub can break up, the earth can go bad etc also a bad wheel bearing can cause them to send bad signals.

Uwe_911

Alternative acceleration sensor?
The newer Version of the Porsche accelerometer is a Bosch part with the Bosch part number 0 265 005 108. They are available at the dealership for just under $2000. I did some research on the internet and believe that the Bosch part used in e.g. Mercedes SL (R129) ans S-Class (W140) are identical wrt the electronics (Bosch part No. 0 265 005 106). The difference seems to be the sensor case. Most probably only the hole patter for mounting. As the sensors are avaiable on ebay for about $70, it looks like a pretty good alternative.
Has anyone tried to use one of these sensors?
Uwe

apanossi

I thought someone was working on a solid state version of the lateral and longitudinal accelerometers. Does anyone know if there has been progress?

My pdas behavior is the same as yours with respect to bumps and such. I have also seen similar behavior described by others with C4's. To my understanding, it's the minute change in wheel speed over bumps that triggers the pdas through the wheel sensors, not the accelerometers in the cabin. I'm able to activate the pdas when turning in hard and at high speed over smooth surfaces. This is probably the result of the g sensors. Also, my car was lowered recently and pdas is easily activated as I ease into and out of driveways. This is most certainly the result of one wheel lifting off.

968norway

You'll find them at jdsporsche.com

Goughary

I was the first 964 to have the jds accelerometers, as far as I remember - he originally produced them for the 928...and when he posted on rennlist, I bought them immediately. They are remarkably better than oem. I've had them now for quite a few years.

That said, they function exactly how the oem accelerometers should perform, so unless you know what the conditions are that cause the oem to fault - you will not notice a difference, at all. The big difference here is how the pdas and abs perform under rough surface at limits, and then subsequently how the car behaves at limits.

So for this, when they are needed, big difference and I am happy with them.

All this said. Prior to buying these, unless you know your accelerometers are needing to be replaced, ie after a crash, if they have been dropped on the ground while out, or if after cleaning they still fail...you should be changing or cleaning abs relays, going through the pdas and abs system and looking for faults and issues, be sure your abs pump works properly, that the solenoid functions properly, that the bomb pressurizes properly and holds pressure, that your diff locks aren't seized (mine were seized), etc.
The accelerometers are a simple devise. These are awesome and way more consistent than oem, but real problems in the system are usually found elsewhere. With these you feel a difference in bumpy braking zones and bumpy corners. But for track you want to shut the system off anyway.

Goughary

Funny thing- I just saw I started this thread back when I was sorting my issues...

Six years later...love the new accelerometers- but - my issues were sticky calipers. Seized diff locks, bad abs relay, and also the limitations of the oem accelerometers.

So- I have the experience now of having sorted mine...but we also used these in the Alabama car from suspension madness- which was a fully rebuilt, sorted car, and using them back to back in a car with no issues...I can say I notice a difference - hence my previous comments - but then I also am sensitive to the conditions under which one would notice.

I don't know that if someone just tried one to the other, without really knowing what to look for, that they would notice a difference. You really need to replicate the conditions under which the oem suck. These perform better. Or rather, more consistently.

Uwe_911

Thanks, Goughary
That helps a lot.
I sorted out the ABS pump, the middle differential and the relais. My problem seem to be the accelerometers.
The anti lock is triggered be the wheel speed sensor. Once activated the anti lock system uses the longitudinal accelerometer to maximize the negative acceleration. Here the old Porsche / Bosch part with the copper pendulum gets blocked by debris. I cleaned the sensor. The debris comes from the permanent magnet in the hall sensor below the copper pendulum. My sensor builds up new debris after cleaning.
I already ordered a Bosch sensor 0 265 005 106 and will check, if the output is compatible with the original part. The sensor signal is very basic - a voltage output as a function of the g-force. Will report if it works.
The solid state accelerometers should be perfect. Will be my ultimate solution.

Uwe

Goughary

I've got a bunch of old accelerometers- 964 and 993, all should work perfectly, and a pair from a corvette which seem to be the same as ours, though I never tested the output. The Bosch accelerometers for the corvette cost me 20 dollars, lol.