Tony

I dont think it is. If i recall looking at mine when i replaced it, it's a sealed unit

Tampa 928s

I have a replacement tach to go in, at what location do I connect the 12-V and ground to test each bulb before installing.

underdog928

The easiest way to test the bulbs is to put 12v directly across each bulb. The circuit on the back on the tach is very complicated, using transistors and relays, and I'm not sure you can easily emulate the external circuity of the car's wiring with the tach out of the cluster. Power to each of the bulbs comes directly into the circuit board to the hot side of the bulbs through the connector. Tony (above ^^^) has labeled which connector pins go to which light. So all you need to do is ground the common side of the bulbs on the circuit board by finding the correct trace and you should be able to light each light individually. Hope that helps.

Tony

Well. I think we have a MAJOR breakthrough in this common 928 gremlin. I recently bought a used dash cluster from Mark to experiment with and to learn how to take apart in preparation for tackling my own.

I took it all apart and got to the circuit board behind the tachometer.


Using what Bob…"underdog928" posted about the broken circuit above i decided to check the used one i bought from Mark. Guess what!!! The circuit is broken in this one as well and i can see EXACTLY where.. I would guess if Bob were to show a picture of the other side of his board we may see the same thing! See pics below…What are the odds of getting a used one that perhaps had the exact same problem! (no PRND32 lights or back up lights) I took my continuity tester and placed it where the wire is fed to the circuit board and traced it all the way to the relay.
No Continuity!
I then kept moving the test lead back along the circuit pushing it through the protective "green" film over the solder "lines". By testing various points along the path i found exactly where the break occurs. You can see the areas below circled in red where I poked the tip of the tester into the solder.

The break appear to occur right between two resistors. (the orange arrow and the dark discolored area). Its very easy to pick out in the picture. The thin line of solder has to pass between two "anchor points" for a pair of resistors. Perhaps its the heat...the thinness of the line...the bend?? I dunno, but the circuit breaks right at the point shown in the pics below.

The true test will be when I pull my own cluster and see if I have the same problem. If so, Bobs jumper wire should be an extremely easy fix!

More to come for sure!

BOB!!!! you da man!

Randy V

Man, you still workin' on this?

Hang in there, T.

Tony

yup....

.well after the trouble shooting i had done up to this point i figured the issue had to be in the pod/cluster somewhere. So i waited. Now im going to replace all my bulbs and go LED etc...so i decided to attack it again.

You should see me at Christmas time with our light strings

underdog928

Glad to be able to help!

I just looked through the pictures I took on this fix and I don't see any of the actual break I found, but looking at your pictures that is exactly what I remember seeing. Apparently Porsche didn't use large enough traces on this circuit board to handle the load.

davek9

Thanks for posting this guys, I've been trying to get the back up lights working on "The Onion" (86.5) Auto.
My PRND32 lights in dash most of them work but very dim, thinking bad bulbs on the ones that don't.
However no backup lights.
This will really help,

Thanks again,

Dave

Tony

At this point it appears the circuit in my pod has the same issue. I cant get continuity between the same points.

Issue now is i cant get the F#$@%@g tach needle to come of its shaft to make the fix and repair the PRND32 bulbs...ive tried has hard as i care and its not moving...any more force an i think i will break somehting, which would be just typical.

Any tips on getttign the tach needle off?

I placed a layer of card over the face of the tach gauge to protect it...

ive pulled up directly as hard as i care..
Ive place a 45' plier under neath and gently pried..

Im now going to try and make a "puller for it"

I would hate to get this far and get stuck or destroy the tach needle and assembly...although i do have a spare.

Randy V

Use a dinner fork leveraged against a piece of cardboard or leather.

Tony

got the needle off and have soldered the jumper wire as Bob has shown. I also discovered that the other "power" feed on the circuit board looked brown and cracked also. Even though it tests ok for continuity i went ahead and jumpered that circuit is well. Both wires lead to the small green relay...one on one side...one on the other from the wires with RED w/black strip and a RED w/green stripe that is part of the wiring harness. I believe it is one of these two wires that send power back to the tranny switch...PRND32 lights and back up relay. I think they also power the dash "warning/status" lights as i all of mine tested ok with a 9v battery independently....yet they dont illuminate when i turn the key on. The only one i get is the "anitlock" light? Another mystery that perhaps all this will solve?

The proof will be when i manage to get it all back together and it works.



red circles = bad circuit
Yellow line = shows where circuit ends up on the relay
orange circle= show where both circuits originate RED/black...RED/green

2nd picture is the back of the board and the jumper wires. crued but it should work as Bob has proven!

Tony

i got it off..but that is a good idea!

as you can see below i managed to pull the metal assembly of with it also. The needle on the top is from my "experimental cluster" i was learning on. That one came of easier and left the "toothed gear on the shaft"

Tony

Some pics of what i did today...

#1 A small dremel drill is ideal for drilling out the new opening for the PRND32 lights after you remove them.

#2 I only noticed this when i viewed these on a bigger screen at higher resolution but on the "back lit" picture, the red circle shows where the break in the circuit is that is perhaps causing my issues. You can actually see a small gap in the solder trace.

#3 all the bulb type sin an 87S4 cluster. any one care to provide a LED cross reference for them? Some i can find others i cant.

Thanks.

dr bob

Tony--

Buy a cheap "solder sucker" to pull the solder out of those holes. The drill risks damage to plated-through holes if you don't do it perfectly. The little bulb-type sucker is fine. Just heat the solder at the hole with the soldering iron until it's melted, then use the sucker to pull the solder out of the hole. Or use a "solder wick", which is a bit of braided copper on a roll that draws the solder out by capillary action. Both are pretty cheap at Radio Shack.

Tony

Its been 11-12yrs since Ive owned this car....I now see the light!! FIXED!!

All bulbs work and are synced with the shift lever.

Now onto the next gremlin...Lamps check(only get ANTILOCK when turning on the key) and still a lack of reverse lights