Voltage regulator woes

Bosch Spannungsregler, Bauart RS/UA (Dreielement-Knickregler) voltage regulator, © Robert Bosch GmbH

Bosch Spannungsregler, Bauart RS/UA (Dreielement-Knickregler), © Robert Bosch GmbH

What does a voltage regulator do?

In one sentence, “It regulates the power from the generator”. If you want it a bit more complicated you can say “It’s a relay, in principle, that opens or closes different contacts (up to 250 times per second!) when a certain charging power is reached or exceeded” This it does to achieve two important things:

  1. With the engine stopped, the battery would be completely discharged after a short time via the field windings and the windings of the armature in the generator. Hence, the regulator decouples the generator when the engine is not running.
  2. The amount of electricity supplied by the generator would be correct at medium speed, but too little or too much energy would be available at low or high speed (during the latter overcharging the battery and causing water loss due to electrolysis, i.e. “boiling”).

Bosch generator, © Robert Bosch GmbH

Bosch generator, © Robert Bosch GmbH

In a 12V vehicle, the healthy operating voltage is around 13.8V, maximum of 14-14.2V. Therefore, the voltage on a regulator for a 12V system is 14V. In 6V systems, the "regulating value" is 7V.

For a deeper tour into the mysteries of the generator and the voltage regulator, google or see if you can find the following literature in your language.

  • Bosch's book around ignition systems and Oldtimer Markt's around car electricity
  • Bosch Generators and Voltage Regulators

One word about generators in classic cars

Theoretically a certain generator must be combined with a certain voltage regulator. Given what I’ve heard from people I’ve been talking to over the years and different internet sources I’ve read, the tolerances are so great that it is considered that the specifications must deviate quite a lot to create problems. This is also to some extent confirmed by the tests the Bosch Car Service workshop in Stockholm has done for me. What is however a must, is that they individually and together work as they should.

Another thing a Bosch guy told me is that the stamped part number on the generator doesn’t say anything, given that the generator is not New Old Stock. Over the years your generator has probably been rebuilt several times by different local workshops and they took what they had on their shelves when repairing/servicing it. The result was normally good in any case, and if it wasn’t would the workshop have told the client?! The question is however what the sum of all deviating parts has become now that you are the owner of the generator and has some sort of issue with your ignition system.

The only way to tell is to find a workshop with the old Bosch testing equipment, a mechanic who knows what he’s doing (and therefore he is probably really too old to work, but he's doing it because he thinks it's fun and he has nothing else to do) and finally the specifications for your original generator/voltage regulator combination.

You also have instructions in the Mercedes-Benz “Workshop manual 190 Part 2” on how to test. Since I have used my Bosch workshop, who seems to know what they are doing and are honest, I haven’t bothered to read those sections carefully enough to know if they cover all scenarios. Two important scenarios in my experience, which I think will be tricky to do without a test bench, are to be able to test the generator/voltage regulator, under controlled conditions, at different RPMs and expose them to a long-term test.

The Mercedes-Benz 219 W105 Ponton with Riddarfjärden and Stadshuset in Stockholm as background

The Mercedes-Benz 219 with Riddarfjärden and Stadshuset in Stockholm as background

The start of it all, I think...

Originally my MB 219 -57 Ponton had a Bosch LJ/GEG 160/12/2500 generator and a Bosch RS/UA 160/12/15 voltage regulator. According to the data sheets the following values applies to this combination:

  • Voltage regulation, idle 13,8-14,6V
  • Start current regulation, cold 19,5-22,5A
  • Start current regulation, warm 17,5-20,5A
  • Engage at 12,7-13,4V
  • Reverse current 2,5-6,5A

The aim of the restoration, when it begun, 30 years ago, was to re-use as much as possible, thereby not overdoing things and keeping the car as original as possible. As you can see in the article Electrical equipment, engine I had my generator gone through and I cleaned up my old voltage regulator, painted the cover painstakingly in semi-gloss black. I could not judge the inner parts but assumed it would work since it had done so before the restoration.

The two devices were actually tested together twice by mistake. First by a friend's friend who had a small workshop with the right test equipment and then by a Mercedes dealer who then still had the right Bosch equipment available. Both tests are said to have resulted in "Passed".

What I however had forgotten, and what the two testers hadn’t said, was that the generator was not the correct one. While my father still owned the car, but I used it, the original generator gave up, it was completely worn out. What I saw myself confirmed it and my dad was not surprised. The generator workshop, well known in Stockholm at that time, recommended me to take a slightly more powerful generator instead. They had a Fintail generator, a Bosch “LJ/GG 240/12/2400 AR8 Max 360W Max 30A, lying around. It would fit both the ignition system and my generator mounts. So I bought it and had no problems, only the positive experiences of brighter head lights.

The test drives go awry

Some years later the restoration started and too many years passed until I finally, 2019, could start driving it. And when I did, it didn’t go well; the engine run like a sack of potatoes and worse it got.

Testing the Mercedes 219’s combustion, or Air-Fuel-Ratio, with the LM-2 from Innovate Motorsports

Testing the Mercedes 219’s combustion, or Air-Fuel-Ratio, with the LM-2 from Innovate Motorsports

This is not the blog post where I will describe all my escapades, starting 2019, trying to get the engine running as I want. That will be a separate, even longer post. When I’m finally done with the testing…

However, during those I had huge problems, not only with bad engine behavior but also with interference which disturbed my radio as well as measuring equipment. In addition, I had sometimes had indications of "boiling” battery but then I, and several other people thought it was because I insisted on having the old type of “Tar top-battery”. I had other issues as well but here we focus on the ignition-related ones.

Old style tar top battery from Oldtimer Batterie ServiceOldtimer Batterie Service in Germany

The only battery that fits my battery tray, an old style tar top battery from Oldtimer Batterie ServiceOldtimer Batterie Service in Germany. You can read more about those experiences in the article Battery.

As you can see in the blog post Are my Ponton condenser/capacitor and ignition coil OK? my original coil died. So, I bought a new “Red coil” which unfortunately showed the same behavior and died. With the latter I had also noticed a burning hot pre-resistor/ballas resistor before it did so.

Now my suspicions were directed at the combination Ponton voltage regulator and Fintal generator. I went to my Bosch Car Service workshop in Stockholm and had them test the items separately and in combination. They did several test-runs in their test bench with satisfactory results and within the specifications, at least not too high values. My original regulator and the Fintail generator gave 10A and around 13V and the Fintail combo 15A and 14V (better). That was a bit confusing, what could then have caused my problems?!

In parallel with this, I started looking for a regulator better suited for the Fintail generator and from Bosch Classic information I got about which Fintail regulators would work with the generator (given that the components were not completely incorrectly replaced). The number of different spare parts numbers I have for voltage regulators, number changes and replacement products are immense. Finally, I found a good used Bosch voltage regulator, 0 190 309 002, by Bosch Classic called “14V 30A”. This is today (to be more precise, since the early 80ies) superseded by the electronic “F 026 T02 204”. But no, I didn’t want to decorate my car with that, shiny little box.

During my search for a new regulator, the Bosch workshop fine adjusted the old one (that’s an art in itself). When they were satisfied with the results they did a long-term test of it and the generator. This revealed that the original voltage regulator intermittently got stuck and delivered too high Voltage! If it could have caused my boiling battery burned out ignition coils and made the pre-resistor/ballast resistor burning hot, they could not answer but it seemed likely. It should also have resulted in occasional stronger low beam, affected the radio etc but I did not notice anything like that.

Mr mb219dotcom in action with his 219 Ponton

This is how my wife saw me during several years, except when she traveled abroad to avoid seeing me like this.

The Bosch workshop now got the used Fintail I had bought and tested it separately and as well as in combination with the Fintail generator during winter 2020. The tests gave perfect numbers, it cut at ~14V and ~16-17A. After installation, the car run better, albeit still not perfect, and the problems with the interference and “boiling Tar top-battery” went down considerably, albeit not completely.

If you are as picky as I am, you do not stop when it is "almost" perfect. The next step was to immerse myself in an article on the site "Mercedes-Benz Pontons (1953-1962)", Rebuilding Vintage Bosch Dynamo/Generator Regulators, an article I had looked at several times before.

Hmmm, shouldn't I try to convert my old, broken but original regulator and see what happens? Of course I should, the engine compartment would look original again and the performance and reliability increase! I contacted Electrodynamic Solutions around the product line DVR4, the version which is suitable for the Bosch generators.

They recommended me a DVR4N-12-22 based on that my Fintail generator produces 360W with a rated maximum 30A current limit. When it’s running it’s regulated to 14.3-14.4V so as to correctly charge the battery. So, for a maximum of 360W this would be 25 amps maximum current output. But to give a bit of room on a hot summer day with high engine temperature, risking overheating the generator, they wanted to go for lower rating, 22A. Bought it.

  • Something has melted in Bosch RS UA 160/12/15
  • Something has melted in Bosch voltage regulator
  • Completely worned out electrical components in Ponton voltage regulator

I took out the cutter, the Dremel and a soldering iron, disassembled, cleaned and soldered the electronic regulator in one evening. During the disassembly it got obvious that my old Bosch RS/UA 160/12/15 voltage regulator was done for. Look at the windings and the resistor (I hope it is such a one…) under the baseplate in the gallery above!. Also its contacts where worn, after so many years, closing up to 250 times per second…

I glued the DVR4-unit to the regulator lid with the thermally conductive glue Loctite EA 9497. I should have handed in the baseplate for plating, the insulating Bakelite sheet is not normally damaged by it, but I didn’t have the energy.

The DVR4 in the original Bosch RS/UA 160/12/15 voltage regulator housing

The electronic DVR4 in the original Bosch RS/UA 160/12/15 voltage regulator housing

  • The electronic voltage regulator soldered into place
  • DVR4 being glued with thermally conductive glue
  • Pressure to keep the glued DVR4 in place even during drives in rough terrain
  • The electronic voltage regulator being assembled in the original Bosch housing
  • Done, looks like the original Bosch RS/UA 160/12/15 voltage regulator


If you are interested in measuring stuff, I can tell you that there is a tiny, tiny leakage over the B+/51 connection, 0.04mA. It’s well below the thresholds and according to Electrodynamic Solutions it’s due to a “body diode of a power mosfet".


I don’t know if I dare to again write that the engine runs even better and that the problems with the interference and “boiling Tar top-battery” went down somewhat more, albeit not completely. But:

  • The engine feels more stable now and during my test runs with my AFR equipment I can, more or less solely, relate the results to what I’ve done with the carburetor. That was not the case before, there were always something odd going on. Not any longer.
  • Some interference is still there but I doubt you can get rid of it all with such an ignition system the Ponton has. Some of it I can also relate to how well, or not, the engine runs after my adjustments after the test runs (e.g do misfires result in interference).
  • The battery is another thing. Most of the “boiling”, or actually getting crystals on its top and especially around its two poles, turned out to be related to leakage. I suppose this is due to how it is built and sealed but probably also to that I had huge problems from the beginning, probably it boiled due to the failed original regulator. With Vaseline (old battery, old and odd, but "well known", way of curing things) along its joints on the top, especially around the pole most of the problems disappeared with the Fintail regulator. Now, with the DVR4 it’s completely gone.

Above all I'm happy to have the correct looking voltage regulator in the engine bay. Never felt comfortable with that chunky Fintail thing :-D 


Invest in an electronic voltage regulator, if from Bosch, Electrodynamic Solutions or any other supplier is up to you. Sooner or later, I will also invest in some type of electronic solution for the distributor. But I’m not there yet….

And always keep a spare at hand! I bought a New Old Stock voltage regulator from a guy in France who seemed to be sitting on a lot of them.

  • New Old Stock Bosch Voltage Regulator 0 190 309 028 UA 14V 20A
  • New Old Stock Bosch Voltage Regulator 0 190 309 028 UA 14V 20A - box
  • New Old Stock Bosch Voltage Regulator 0 190 309 028 UA 14V 20A - sideview
  • New Old Stock Bosch Voltage Regulator 0 190 309 028 UA 14V 20A - sideview
  • New Old Stock Bosch Voltage Regulator 0 190 309 028 UA 14V 20A - sideview
  • New Old Stock Bosch Voltage Regulator 0 190 309 028 UA 14V 20A - underside