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1989 300TE
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Discussion Starter #1
I've had discussion in other posts about my '89 300TE and its OVP relay. After replacing this device and having my problem (apparently) go away, I got curious about what this thing really does. (Never seen one in any other brand of car!)

The main component in the OVP is a BYZ50K22, which the manufacturer calls a "silicon-protectifier with TVS (transient voltage suppression) characteristic". That's a mouthful for what is really a power Zener diode. This device sits connected to +12 and ground.

In normal operation, the Zener doesn't conduct at all. When system voltage reaches a nominal 19.8V, it reverse biases and begins to conduct. This action is called clamping, and it prevents overvoltages from being passed through to sensitive electronics. The reverse-biased diode has some inherent resistance, so it doesn't become a dead short across the power lines. Instead, it can absorb short-term overvoltages and then go back into its non-conducting state once the threat has passed.

There are fuses in the OVP also. If the overvoltage lasts too long or is too large (like if you hook the battery up wrong, or charge with a completely dead battery), one of the fuses may blow. There's also a relay which I suspect just connects and disconnects the load when the ignition comes on.

What makes the OVP fail over time? It's most likely not the Zener diode, since it does absolutely nothing except in extraordinary circumstances. It's also a very robust device and can carry 50 amps when forward biased. And, with an operating temperature limit of +215 degrees C, it's not going to be affected by high underhood temperatures. More likely a minor component goes, or the relay contacts get resistive and cause a voltage drop, or a solder joint fails.

Bob
 

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W124 Moderator
86 190E 2.3L 16V, 2 95 320TE's, 02 S500
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Nice, easy to understand write up Bob.

Thanks for posting it.

Jayare

Sent from my XT1080 using Tapatalk
 

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Premium Member
1993 300CE Cabriolet (mine) ; 1994 E320 Wagon (wife's) ; 1990 Benz 300E 2.6 (son's)
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the zener is in the coil circuits of the OVP per the drawing shown in the various wiring diagrams. There's three separate relays... on HFM-SFI cars, at least, one relay switches circuit 15 from fuse 1 and powers the ECU. One switches circuit 30 and powers the CC/ICS or EA/CC/ICS and MAF, and the third relay switches circuit 30 and powers the ABS. The fuse is on the ABS circuit.

When the zener triggers from an overvoltage surge, all three relays open.
 

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1989 300TE
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23 Posts
Discussion Starter #4
LCG, my '89 must use an earlier, less-sophisticated system. Per the circuit diagram on the OVP, there are two relays, with one switching circuit 30 to 87L and the other switching 30d to 87E. Circuit 15 is hot with ignition, and that turns on the relays. Each relay load is protected by its own 10 amp fuse. The power Zener is connected between 30d (hot at all times) and 31 (ground), so it protects the electronics even when the ignition isn't on.

The Zener clamp is old technology, but effective. A Zener can begin to conduct and protect much more quickly than a relay can de-energize, too - in microseconds versus milliseconds - thus the "TVS characteristic" the Zener manufacturer touts in their data sheet.

Bob
 

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1993 300CE Cabriolet (mine) ; 1994 E320 Wagon (wife's) ; 1990 Benz 300E 2.6 (son's)
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hmm, yer right. I was looking at the diagram for my 1993-1994 cars...

older diagram on page 43 here (labeled 118) http://new.freescruz.com/.priv/W124/w124CD1/Program/ETM/ETM.pdf#page=43
show it as your describe... circuit 15 closes the relay, which connects circuit 30 from X4/10 (power terminal near battery) to all the engine stuffs.

further, if that zener shunts, it will blow the fuse, which will immediately kill power to everything connected to it (the zener would not last long shorting power to ground.
 

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1989 300TE
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Discussion Starter #6
Just pondering what might make the OVP unit cause running problems, I remembered a mid-'80s Alfa Spider I bought a few years back from a garage. (I'm an Alfa guy first and foremost; a weakness, yes, but one I embrace!) Alfas of that era used Bosch L-Jetronic injection and engine management.

The garage had had the car for over two years and couldn't make it run for more than about five minutes. Mostly it just refused to start completely. They had thrown hundreds of dollars worth of parts at it, replacing things as far-flung as the alternator (!), and still no result. When I bought it, I immediately went online and found a very complete troubleshooting guide for the L-Jetronic system on that car.

According to the guide, the most likely culprit was low voltage. If the ECU sees less than 10 volts (which can happen with a discharged battery during cranking, for instance), the ECU simply won't fire the plugs. I checked fuse block voltage and discovered that the voltage supplied to the ignition fuse was about 2 volts less than battery voltage, even without the starter motor engaged. I jumpered battery voltage to the ECU supply, turned the key, and the car started instantly. Took me all of 30 minutes. The problem, it turned out, was a spade connector somebody had installed in the ignition lead from the switch. I reworked the connector and the problem was gone.

Different models of Bosch fuel injection likely shared design philosophy during this time. If the relay contacts in the OVP unit become resistive (which can happen over time and many operating cycles), or if a solder joint fails (more likely, IMHO*), the voltage supplied to the ECU could become marginal. That might explain rough running or hard starting on occasion. If somebody experiences these symptoms, maybe you could monitor output voltage from the OVP to see what's happening there.

(* I once had a small cottage industry working for a local Jaguar specialist. '80s and '90s XJ sedans brought to his shop had no intermediate AC fan speeds. The problem turned out to be bad solder joints on the TO-3 power transistors that supplied drive to the fan. Over time and many thermal cycles, the joints would fracture - not enough to be visible to the naked eye, but enough stop conducting current - and the fan would only operate on one speed. Such a failure could easily explain intermittent low voltage output from the OVP.)

Bob
 

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1993 300CE Cabriolet (mine) ; 1994 E320 Wagon (wife's) ; 1990 Benz 300E 2.6 (son's)
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way back when, my very first car was a ratty MG Midget. Previous owner had built the engine from parts, mechanically the car was in fine shape, body was seriously ugly. But it wouldn't reliably crank over, and he never figured it out, so you always had to park on a hill and bump start it. luckily, pushing a midget was easy peasy.

I quickly found out that the engine block was only grounded through the throttle cable, which got red hot when you tried to crank the car. added a fat ground braid, car started right up every time after that. Way WAY too many mechanics don't understand that demon Electricity.
 

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'95 E300 DIESEL, '91 600SEL, '92 600SEL
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...Way WAY too many mechanics don't understand that demon Electricity.
Neither do certain manufacturers, especially British and Italian products made during the 1970s where the employees spent more time on strike than on the assembly line.

Even my 2015 Italian motorcycle, required a complete teardown and rebuild when it was brand new because of sloppy assembly. Example, the main wiring harness was attached so badly that the waterproof connectors were exposed to the elements.

Reminded me of a early build DeLorean
 

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W124
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Just pondering what might make the OVP unit cause running problems, I remembered a mid-'80s Alfa Spider I bought a few years back from a garage. (I'm an Alfa guy first and foremost; a weakness, yes, but one I embrace!) Alfas of that era used Bosch L-Jetronic injection and engine management.

Bob
Need more cowbell...er, Alfas! Welcome!

Great opinions on the OVP; especially related to ECU voltage. Like the old Jags you spoke about, these older Benzes still benefit from close inspection of solder joints. I have solved more than a handful of problems over the years with just a soldering iron.
 

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89 190E2.6- 5-speed Manual, 95 E320 Sportsline-sold, 2001 E320 4matic Wagon-sold
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So it turns out the clamping device used in these OVP's are not zener diodes but rather avalanche diodes (in what is also called a TVS component) .
Do not want to get too technical here but there is a difference. Zener diodes are more "delicate" compared to avalanche diodes and they do not posses the multiplicative effect in clamping. In an avalanche diode for every electron that gets over the barrier after the clamping voltage is reached multiple electrons get knocked over so they are "quicker" to clamp.

Other than a burn-out event or a long term stress that can create opens or shorts, the only other likelyhood of long term damage is "leakage" beyond the specifications of the device.

I would like to simulate the circuit if I had the exact circuit diagram as something must be causing the output to be shunted from 13V to zero under this heavy "DC" leakage, intermittently or at elevated temp or other conditions.

This is the failure mode I have seen in my original OVP (11 years of age) and the JY OVP (18 years of age).

At this point this is just a curiosity as I will be replacing the OVP every 7 years from now on and avoid all the intermittent problems.

Anybody have an exact circuit diagram?
 

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1993 300CE Cabriolet (mine) ; 1994 E320 Wagon (wife's) ; 1990 Benz 300E 2.6 (son's)
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the way I read the diagram in the ETM of the M103 version of the OVP, if that diode shunts power to ground, that fuse is GOING to blow... so if they are failing with a good fuse, then the failure mode is more likely related to the relay getting funky.

http://new.freescruz.com/.priv/W124/w124CD1/Program/ETM/ETM.pdf#page=43 has the diagram without any 'black boxes'.
 

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89 190E2.6- 5-speed Manual, 95 E320 Sportsline-sold, 2001 E320 4matic Wagon-sold
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The unit I opened had a lot more components in it, I'm wondering if the diagram is simplified/symbolic in nature LCG.
I think I will reverse engineer the board and see what I come up with.

The failures that are not due to blown fuses or cold solder joints that I experienced must be due to the TVS leaking and falsely triggering that relay and disconnecting the output from the battery voltage. A TVS can not be shunting to ground as that would be catastrophic. It must be a leakage triggering the relay(s). I saw two or three coils/relays in the OVP I opened (mercedes brand)
 

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1993 300CE Cabriolet (mine) ; 1994 E320 Wagon (wife's) ; 1990 Benz 300E 2.6 (son's)
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the HFM-SFI M104's show a much more complex OVP, with 3 separate relays, and two e-blocks (boxes with a transistor symbol)
 

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W201 Moderator
89 190E2.6- 5-speed Manual, 95 E320 Sportsline-sold, 2001 E320 4matic Wagon-sold
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the HFM-SFI M104's show a much more complex OVP, with 3 separate relays, and two e-blocks (boxes with a transistor symbol)
Yep, I think most M103 replacement parts are also using the more complex OVP's with all that and 2 10A fuses.
When I get back home, I'll check into it. Do you happen to have the schematic diagram for the M104's LCG?
 

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1993 300CE Cabriolet (mine) ; 1994 E320 Wagon (wife's) ; 1990 Benz 300E 2.6 (son's)
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Yep, I think most M103 replacement parts are also using the more complex OVP's with all that and 2 10A fuses.
When I get back home, I'll check into it. Do you happen to have the schematic diagram for the M104's LCG?
only the block diagram shown in the ETM... it doesn't show what the eblock do, or what the passive components around the relays are, either.
 

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2003 G500, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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Attached is an excerpt from a write up about the OVP that I did years ago (but never finished) which was based on my dissection of various MB OVP relays. I hope it provides some useful insight into the inner workings of each type of OVP. Please forgive the less than ideal conversion to PDF and any typos, errors, etc. - I did this a long time ago and haven't really updated it since.
 

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W201 Moderator
89 190E2.6- 5-speed Manual, 95 E320 Sportsline-sold, 2001 E320 4matic Wagon-sold
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Attached is an excerpt from a write up about the OVP that I did years ago (but never finished) which was based on my dissection of various MB OVP relays. I hope it provides some useful insight into the inner workings of each type of OVP. Please forgive the less than ideal conversion to PDF and any typos, errors, etc. - I did this a long time ago and haven't really updated it since.
Very helpful Floobybust, thank you!
Mine were what are called #3 in your document. At this point my suspicion with the "aging" effect from my symptoms would be that "inverting" circuit or the relay itself that maybe intermittent and very infrequent.

I'll start poking around on the bench to see if I can replicate the behavior on either OVP.

Thanks again.
 

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1990 190e 2.6l Calif. auto 1987 Ford F150 4.9L Calif. man 4OD 4X4 2013 Suzuki C50 Crusier
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Yes on those cars if you lose your ground the choke cable will get red hot and fry.
 

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Attached is an excerpt from a write up about the OVP that I did years ago (but never finished) which was based on my dissection of various MB OVP relays. I hope it provides some useful insight into the inner workings of each type of OVP. Please forgive the less than ideal conversion to PDF and any typos, errors, etc. - I did this a long time ago and haven't really updated it since.
wow! this relay description helped me a lot. thanks
 
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