I guess it's worth the $50 to replace just in case something is at fault. I do have 2-3 other projects on the car to get done first though.

 

me being unaware of this topic until now, but keen to keep abreast..
have read a lot about the OVP now .
1) is it worth changing in a 1996?
2) where is it located in the 129?
3) ok to use the 9 pin?
4) part number?

thanks

 

I would not replace it just for the sake of replacing it. As things go it is a relatively simple device, especially the 5 and 7-pin ("one fuse") units. Even the 9-pin "dual fuse" device is far from being something one would expect to find in a rocket ship.

Both the 5 and 7-pin assemblies are, with the exception of the Zener diode, purely electro-mechanical devices--mostly identical but not pin-compatible.

5-pin OVP:

7-pin OVP:

The 9-pin unit discussed above adds the switching transistor and an additional fuse and relay to expand it's capacity. None of these are components "known" to fail due to age alone, as with any discrete silicon devices heat is their main enemy--and if they have survived 12+ years they are likely good for another 12+.

The Zener in the 7 and 9-pin devices (reports indicate the 9-pin unit can be used on a "7-pin" vehicle) can be tested as shown below:

This tests the forward voltage drop of the Zener if it is 0.4 to 1.5 V as indicated then the Zener is OK and overvoltage protection is intact.


Bottom line, IMHO, is that it's one of those "no need to fix if it ain't broke" things...

 

I changed mine simply because it looked original 22+yrs old and made up a shipping order. The early single fuse OVP as fitted to the early R129 is no longer available and as such is superseded by the later 9 pin 2 fuse item. It just means the unused pins locate into vacant holes in the connector.

I think the info cliffyK posted is well done and self explanatory. The bit I find particularly interesting is the last Paragraph

If a relay coil were to fail there would be no power to the control modules, and obvious failure, however should either relay's contacts wear and become "resistive" it could result in the modules being voltage starved to behave oddly.

This is also referred to as a cause of concern between the OVP and the fuel injection system in the Bosch FI book. Whether it is just the CIS injection systems that experience the rough running problems I don't know. But it would appear that a below par/resistive OVP with bad connections on the fuel relay side can cause some issues. There are DIY around that show how to re solder the bad joints if a new unit is not required. I feel having a spare functional OVP as a test part is good to have in the toolbox, I recall Albert being stranded but lucky enough to have an old serviceable item available

 

Your memory is excellent.
Yes I did, as an extra precaution.
I have permanently bolted an extra, right next, so it is on the fly connect, and play/go...

Man, am I ready. Lol.

One must not forget that the power to the OVP, is supplied by the ignition switch.
Voltage drop from the switch, will effect the coils, OVP, MAS,ECU,etc.

So don't blame the OVP.

The newer don't have an OVP, they have a module.

Senior moment.
I can't remember the name.....

Oh well, sorry about that.

Regards,
aam.

 

I understand 110% about the "rounding out an order", that's how the "free shipping for orders over $xx" works--been there and done that more than once.

"Bad" solder joints from age/vibration is a real issue. I repair/refurbish older electronic test equipment to finance my vices, degraded PCB solder joints are right up there on the list of common problems--and this is in equipment that spent most of it's life sitting on a bench!

My workshop

 

Good thread, I did a copy & paste of your posts to my info folder :thumbsup:

Looking at that workshop pic, seems you are up for some overtime. Think of all the new vices you could maintain if you had that lot sorted out

 

Gee quite a selection of electronic test equipment you have there - very nice.

I was under the impression that the later model SLs do not have an OVP.

Is this right or wrong and if so where is it situated?:thumbsup:

Oh by the way, your next project is a detailed description with schematics on how the soft top controller unit works.

 

:<snip>I was under the impression that the later model SLs do not have an OVP.

Is this right or wrong and if so where is it situated?:thumbsup:

I honestly don't know, the WIS and other sources I have are unclear about it. My suspicion is that they do not.

Microprocessor based controllers on every other vehicle I have "gotten into" have board level overvoltage protection built onto/into each module as do most modern electronic "thingys". At best I see it as an "overkill" first line of defense, though it may perhaps be sort of a kludge/OMG afterthought because the individual controllers were found to be vulnerable--IDK?

If it's there I would expect it to be in the "black box" with the backplane and most modules; unfortunately on my '98 the black box cover has two fasteners tucked in under the cowl vent that appear to be a real PITA to get at--in the absence of any strong motivation to pull that cover I have determined I will not. FWIW I have not been able to find it positively linked to the '98 MY on any of the online parts lists/vendor sites.

Oh by the way, your next project is a detailed description with schematics on how the soft top controller unit works.

Klaus at Top Hydraulics knows more about the RST than I will ever care to, AND is a wonderful resource here and in general--more than willing to any questions any one might have about the RST...

 

at Top Hydraulics[/URL] knows more about the RST than I will ever care to, AND is a wonderful resource here and in general--more than willing to any questions any one might have about the RST...

Cliffyk

I was only pulling your leg about the soft top controller - but yes I couldn't agree more about Klaus and his brilliant knowledge all things soft top and their workings:thumbsup:

Oh and guys thanks for the info on the later models not having an OVP - new technology ha ha

 

for my knowledge..where is the OVP in a 129 ( just to push the boat out any idea where it will be in a w140? i know thats a different forum but i got a SEL500 1992 yesterday and know that a lot is duplicated)
thanks

 

As a matter of interest cliffyk, what WIS did you use Star or Emanual?

 

It's the eManual version, which is a somewhat disappointing subset of the real thing (missing some "repair" documents, and having no electrical diagrams.

I also have another very incomplete R129 only system I picked up on eBay; it has some wiring diagrams, but again only a subset. Someday I will buy the real thing.

BTW/FWIW, the eManual parts system is also incomplete...

 

I belong to 3 forums here,r129,w124,w126,I own all 3 of these,the w124 forum talks about ovp problems the most, usually guys get electrical gremlins and running issues,replace and all is well, I replaced it on the black w124 I sold ,car had 230k, that car had a intermittent dieing problem,fixed it after replacing $300 worth of electrical and fuel parts, can't say if replacing ovp did anything? on the w126 developed running issues,replaced ovp,ran better,but still had issues ,out of curiosity took that ovp apart,found huge amounts of corrosion on the PC board,so felt better I replaced it,my r129? no issues whatsoever with the car ,so will wait on the OVP,this car has low mileage and lives in the garage!

 

In my case eManual...