whalebird

well, I'm not sure. I used to have a spare overvolt relay laying around, but it's gone now. There would have to be some mods getting it into the harness. I do have the benz wireing diagrams and that would help. It is the same size as a DME relay, and most of the pins are the same regardless of carmaker, but I would want to sort it out first.

dshepp806

Yes,..I remember that thread (now). Will have a read.

Thanx, Bird!

Doyle

rusnak

well the possibility is interesting to me nonetheless.

I guess that's why Iceman has a voltmeter in his.

dshepp806

On his "car"? I, too, monitor the system voltage via the cig lighter connection via the radar/laser voltage readout. I check to quantify any voltage differences between the cig lighter point of measurement vs. the ACTUAL battery terminal.

Was able to sense an overvoltage condition years ago that went away,..it was a bit intermittent...saw it only 2-3 times before I had ALL of the HW changed (Alt/Vr found to be ALL original from 1989....[ahem]). Point is: damage to any DME would have already have "happened" should those voltages be significantly HIGH...I would have simply viewed the damaging voltage, although the detector hits a ceiling (I can't recall the ceiling..maybe 16 VDC??? Not sure) readout then renders an audible notice. I was lucky that the event horizon was only marginally "high" (I would only guess....) ,..and not of any values to actually damage the DME.....

New ALT,..NEW REG,....===salvation.

I wouldn't want to think of what could have happened should I have been indifferent, apathetic, or (outright) ignorant,..as these OV conditions are (probably) quite spikey....quite damaging........it may be "over" before you can even measure it (on a VOM)....

I see a new project on the horizon............should be one of those "accepted Porsche mods",...such as the instrument lighting,...or the headlamp current lift from the damned stalk switch.....

We'll see,...Bird,..we'll have to get on the DME protect circuit.....I must profess I'm prone to seek something (approach) that's active and not passive,..although either would work,..safer to work with the buss OV sensing (relay) outboard of the DME,..vs. modding the DME DC Buss circuit to protect itself.'

...as if I'm not busy enough ALREADY......................

Best,

Doyle

Carrera51

Thanks guys. I have a spare ignition switch so I will try swapping it in to see if that helps.

Uber 911

I had a similar problem with the oil gauge, jumping about giving weird readings, like 0 pressure!

=Bad earth.

Check behind the battery there's a cluster of cables strapped to one brass earth point.

scarceller

This thread is interesting: I have same exact issue, sometimes in my 84 the tach acts up, it pegs to 7K then seems to want to work backwards meaning RPMs go down as you rev engine. It does this only once in a while, most days it's fine.

I also have a DME re-boot issue once in a while as well. The DME simply decides to re-boot for some unknown reason. I know the DME relay is good since it's been replaced and I have tried several DME relays. I also suspected the ignition switch and this has been replaced with OEM switch but has not helped.

The over voltage scenario I had not thought of, but I will start monitoring system charge voltage when this condition occurs.

The other thought I had but have yet to investigate is the Speed Sensor on the flywheel could be acting up and maybe missing some pulses from time to time. I suspect this because the DME program code has code that checks to be sure it sees 10 pulses in a given time period and if it misses 10 or more the DME re-boots. I know this because I've dis-assembled the DME code from the 89 chip and I currently run the 89 code in my DME with an EPROM emulator that emulates the chip. The cool hing about this setup is I can see that the program code executes the re-boot (boot-up) section of code sometimes for no good reason. This causes the engine to die for about a split second and then simply catch again. When this happens the tach drops to 0RPM then starts up again on the re-catch.

I also know that the DME never losses power during this condition, meaning the relay that powers the DME stays active all the time. But the FuelPump relay within the DME Replay does get deactivated then reactivated on the re-catch. This is normal as during a DME boot up sequence the pump gets turned off then back on. I can actually hear and feel the FP relay within the DME relay get switched off then back on if I put my hand on the DME relay when the condition occurs. Also interesting is that this re-boot only happens at idle when the idle switch is closed and the IdleControlValve is active.

Just wondering if any others have seen something like this?

Auto_Werks 3.6

+1

Sounds like the same symptoms as my '83 when the voltage regulator went out! PS despite "expert" opinion, the 'internal' regulators can be replaced quite economically if you're doing the work yourself. Not sure I would pay several hours labor to replace only the regulator though... probably would do the whole shebang at that point.

scarceller

The 83 cars are not DME so I assume the tach signal comes off the CDI box? If your 83 tach acted strange when over voltage condition was occurring this seems to indicate that these vdo tachs don't like to see voltages above some level. Do you have any idea how high your voltage was when the tach wacked out? How high can the voltage go with a bad regulator?

I will be monitoring voltage levels right at the DME main power relay.

Thanks.

Auto_Werks 3.6

I can't say that I remember the exact number.... If you are recording voltage, you really should not be seeing below 12.5 (fairly low), or much above 14.5. 15v would make me nervous, 16v is too high, and 18v will most likely cause some trouble, or lots of trouble. The voltage should seem fairly constant, you should not see "spikes".

Auto_Werks 3.6

Just for reference, I have attached the voltage indicator from the 951. You can follow the markings on it for a guideline. You can see that the car is on but not running, so the gauge is indicating "battery voltage" only. If the alternator is working properly, the gauge would rise to around 13 or so after starting the engine.

scarceller

Ryan, thanks for the reference points.

Lorenfb

How high can the voltage go with a bad regulator?

Over 20 volts (20V - 25V)!

scarceller

Loren,

That certainly would be very bad for the DME!
Have you seen any other reasons why the DME would unexpectedly re-boot? My DME simply decides to re-boot sometimes, mostly at idle with idle switch closed. The DME relay is not the cause as the DME does not lose power. But the FuelPump side of the relay does turn off the FP during the re-boot cycle but this is normal during re-boot. I have a chip emulator in this DME and can clearly see that the DME steps through the processor initial bootstrap when this happens. It's as if the processor lost power for just a split second. It then boot back up very quickly and the engine just stumbles and re-catches. It's interesting that it only seems to happen at idle with idle switch closed, meaning the ICV valve is being serviced.

Any thoughts?

Thanks.

dshepp806

scarceller: Since I know you're in to the SW (have read all of your post, here and "there"), possibly hardware (I'm certain?), [[[and to certainly note that I live vicariously through many of you SW guys' post vs. "doing it"}}}, Can't help but draw from good tech experience/troubleshooting. I'd say break out that scope (multi-channel, right?) and scope those incoming DC busses to eliminate any power-related issues. While there, look at those pulses you speak of and determine if you've a reboot correlation between those two.

The other thing (and now I tread into Loren's domain) is to ID the CPU pins that offer the reset (they, no doubt exist)...

And (lastly),..:::: connectivity to the DME. I know the audio engineering side is coming out (garnering chuckes from a few, I'm sure),..but, nonetheless, the notion of resonances could prevail (sometimes) in connectiviy issues (,.somewhat of a long shot, but I've seen it before,...whereby various resonances will provide degrees of vibration, subsequently bearing intermittent connection integrities due to a faulty harness connector...of course, each and every damned pin on that DME harness connector (male and female) can be inspected/tested....not to head down the rabbit hole, just yet.

I;m betting Loren will chime with some reset data for this CPU and it's natures relating to reset.

For now, over-voltage conditions needs to be eliminated. I have grown to like my detectors' capabilities to monitor my voltage,..it stays there on the readout.....and will yield an audible when she exceeds 16VDC...I've seen her go there and return to normal a few times,..those days BEFORE repair [only weeks, actually]]. At least I KNEW the event HAPPENED.

GEEZE,..25VDC!!!!!!!!!!!!! Bet it's a nasty looking 25 volts, also. Never knew,...NOW one would be (only) relegated to what the tolerances are for the subjected (to) components (these OV conditions of THAT magnitude) within the DME. No doubt he's changed out the subjected components before in a refurb/repair process......

Keep at it SC!

Doyle


Again,..Loren could surely comment on his many findings of what "let loose" when the rails hit monstrous DC ceilings..I'm all ears, for sure.