Instrument problems
 

I have read a few posts about skitzy instruments and thought I'd share my
experiences. My 1988 560Sl had a problem with the enigine temp, oil
pressure, and fuel gauges (or is it guages?). At times, the engine temp
would peg high, the oil pressure would drop a bar (from 3 to 2), and the
fuel gauge would jump to "full"... I could thump dash cluster lightly, and
all would return to normal most of the times (or I could wiggle cluster by
grasping bezel and shaking it). When it was messing up, the needles would
dance or pulsate in time with the left signal... About the same time, I
noticed my radio would operate without the key being turned to acc, or on,
etc (always hot, it seemed). I read all about checking grounds, and
removing the instrument cluster to check contacts/wiring harness, and was
not looking forward to removing the steering wheel (complete with air bag)
to trouble-shoot. I finally got a good manual and began researching the
wiring... I found that my radio actually drew power from two fuses, #1 and
#12. It seems, my #1 fuse was blown, even though it appeared to be good
upon visual inspection (while still in the fuse box). I removed it and
checked it with an ohm meter and discovered it was infact blown, and, upon
closer inspection, could see the element was separated slightly... I
replaced the #1 fuse and found the radio began truning off after about 15sec
after key was switched off. Also, my gauges quit dancin! I even tried to
make the gauges malfunction as before, but to no avail. A couple of other
things began working which I had not checked into prior to changing the
fuse. My cigar lighter and glove box began working (but, according to the
wiring diagram, are only hot when key is switched to acc, on, or
starting...), as well asthe little light which illuminates the ash tray. I
do not know why this fuse affected the instruments as it did, but am glad,
at least for now, that I don't have to remove the steering wheel, etc and
trouble-shoot the instrument cluster.

Hope this helps someone... CHECK those fuses... somehow, current can run
around in strange pathways.

Awww.. rats! Thought my gauges were fixed, but I was fooled. Last night, turned on headlights and POW! engine temp and fuel jump to full scale, oil pressure drops to 1 bar... Tap on the dash and good as new. Guess I'm gonna have to revisit the steering wheel removal thread.

I'm reviving this thread because I had the identical problem and found the solution.
The instruments and most of the lights in the cluster are all grounded from a common point. The ground comes in on terminal 1 of the three pin connector on the back of the speedometer. From there is goes along circuit board traces to a large metal plate on the top of the speedometer section where each side panel picks up the ground through a mechanical connection (hex screws through the two panels sandwich them together - not the best design but it works). The contact plate is on the top center and I highlighted it on the attached photo with a yellow "T".

If you are having problems with irratic gauge readings, especially when you turn on the turn signals or instrument lighting it is almost surely caused by a bad path for ground. In my case it was a trace on the circuit board on the back of the speedo section that had opened up. On the photo I circled the open trace in yellow at the top left corner of the board. I also circled the ground input (pin #1) too, for informational purposes.

I simply soldered in a jumper wire across this break and all was perfect again after that!

I am sure that a loose screws at the junctions can cause the intermittent jumpy gauges we've seen over the years on these cars too so cleaning them and tightly screwing them down will eliminate that too. Like I said, it is a poor design but can be fixed easily.

So, if you have this problem, grab an ohm meter and trace the ground from the point it originates at pin 1 to the plate that distributes it to the side sections and clean up those connections well too so there isn't any resistance (which is what probably caused the trace to pop on mine).

On the bright side I have gotten very good at removing the steering wheel and airbag - I can do the entire job and get the instrument cluster out in about 15 minutes.

I also can remove a wheel, airbag or not, and cluster in 15 minutes at this point. About grounding, etc, the soldering mentioned should conclusively end this problem.

While I was working on my 380SL, I found another interesting problem; the round 12-pin connector is keyed. However, the key can, and does, break off (and I was careful), allowing this connector to go in offset. This can lead to very strange problems or possible damage to the leftmost gauges upon reassembly. If the plastic tab in the middle of this connector is broken off, be VERY careful in realigning the connector - align by the missing pin and associated wire on the connector. The flat part at the top is NOT correct in that it is NOT parallel to the instrument cluster, so you can't use that as a guide.

Yeah, I could see how plugging that connector it one pin off could cause all sorts of problems!!

Fortunately mine was intact...the only broken part I encountered was the two-pin plug pulled out of the housing on the tach connection but I simply glued it down so it would be secure.

Since you guys have obviously had the gauge cluster out, does this connector look familiar? As stated in an earlier post I found it dangling when the under dash cover was removed. (Picture attached) All gauges work, but tach jumpy with right turn signal. I figure this post is the answer to that problem.

There's a plug like that on the back of the speedo, but I would assume if your speedo works and the other instruments are otherwise ok that this isn't that plug but either for something else or it could be unused. Sometimes they used the same wire harness for all cars and options were pre-wired in if they were to be equipped then they were used and if not they were just tied up behind the dash...does everything else on your car function properly?

Everything works properly except the jumpy tach when right turn signal on. I am going to address the grounding of the cluster as discussed in this thread. BTW, I have seen a post somewhere that recommends running an independent grounding wire from the cluster to a good chassis ground.

Based upon my interpretation of the schematics the tach isn't supposed to be grounded in the same loop as the turn signals. In fact it has no independant ground at all, the two wires to the tach consist of a 12V+ and a wire from the engine speed sensor. I would look for an extra grounding that maybe someone added to try and fix a problem in the past that may be causing this jumping.

I should have also mentioned that the right turn signal indicator doesn't blink, but the turn signals work fine. I think you may be right in that someone has been in there messing around. I need to get the cluster out.

Yeah, pull the cluster out and see what's been done. If the signal doesn't blink it could be a burned out bulb somewhere or some bad wiring work in the cluster so since you are seeing the tach trouble I would start there assuming all of the outside bulbs are working.

Thanks, NhDoc. Following your lead, I jumpered a short on my fuel/temp printed circuit and it solved the prob of jumpy gauges on my 300tdt.