What device can be installed between the glow plug relay and the power leads to the glow plugs, to allow ANY voltage under 11.5 vdc, but allow no more than 11.5 vdc, regardless of how much higher the supply voltage exceeds 11.5 vdc?


The load is 8 glow plugs, which are rated at 11 vdc and 16.5a each. This multiplies out to 132 amps, but I said 270 amps "inrush" in the title, based on a factory note in the service manual. I had to communicate the highest amount of potential current involved, even though I haven't measured this amount of current in practice, so that it is clear that this isn't a small dc circuit board.

The initial power supply is two 12 vdc batteries, in parallel. Fully charged, at rest, available voltage is 12.6 vdc.

When the glow plugs are initially energized, the voltage drop measured at the batteries is typically 10.5 vdc. The glow plugs are rated at 11 vdc, so everything is great so far.


Once I start the vehicle, the high output alternator I installed is able to bring the system voltage up near instantaneously to the limit of the non adjustable voltage regulator, which is 14.2 vdc.

To meet emissions requirements, the glow plugs remain on for a minute or two after the engine is started. This wasn't a problem with the older weaker factory alternator, because the weaker alternator could not generate enough current at idle to recharge the batteries after starting and keep the glow plugs powered, so system voltage during the first couple of minutes while the glow plugs remained on never exceeded 11.9 vdc. Once the glow plugs were commanded off, the voltage then rose to 14.2 vdc.

But with the new high amp alternator, the voltage kicks up to 14.2 vdc immediately, even while the glow plugs remain on. Therefore, the glow plugs are being powered with at least 2.5 more volts than they were designed to operate at.


Glow plugs are self regulating, but are not designed to operate at 14 volts.

Glow plugs are self grounding to the cylinder head, so any arrangement of dividing resistors seems impractical, to the extent that I understand how dividing resistors must be arranged in order to work.

I have access to only two power leads... one lead for the left bank, and one lead for the right bank. Each bank has four glow plugs. Currently, both leads share the same output post on the glow plug relay. I was hoping that the mystery solution would amount to something that I can install between the output post of the relay and these two wire leads that feed the two glow plug banks.

In my uninformed imagination, I visualized a DC-DC buck transformer. Or a calibrated nichrome flat ribbon resistor that would somehow allow any voltage up to 11.5 volts, and then shed the excess voltage as dissipating heat.

No more than 14.2 volts is allowed past the voltage regulating device inside the alternator, and I want the voltage to be exactly that to properly charge the batteries. But I want to limit the voltage to just 11.5 volts on the circuit that feeds the glow plugs, even while the glow plugs consume as much as 130 amps DC, and according to the service manual, can apparently spike to 270 amps (I assume on inrush).

Any voltage limiting device I add has to be capable of handling this amount of current.

Likewise, any voltage limiting device must not be restrictive when the available supply voltage is from less than optimally charged batteries. So if the batteries are only 12.15 vdc at rest, and the glow plugs are energized pulling the system voltage down to say, 9.5 vdc, the limiting device I add must allow for this possibility and not add significant resistance when the power supply is low.

Self-Regulating Glow Plugs Defined

There are two coils, connected in series, inside each glow plug.

First inline and attached directly to the lead that provides power is the regulating coil, that is made of an alloy whose resistance has a large temperature coefficient, such that the resistance increases with temperature.

When cold, it lets through more current to the heating coil it is attached to, which is the second coil in series. The heating coil has a fixed resistance, and is agnostic to temperature changes.

As the heating coil heats up, reaching 1050°C within 2 to 5 seconds, it likewise heats up the regulating coil that is serving it power, which causes the regulating coil to increase resistance, thus limiting the current supplied to the heating coil.

However, once the engine is started, two problems are introduced with the self-regulating coil:

  1. With the engine belt driving a high end alternator that is able to produce 100 amps at idle, which therefore has no trouble elevating system voltage to proper battery charging levels, the voltage will increase to the point where the regulating coil will separate from the heating coil.

  2. Likewise, as the engine is running, the glow plug is also exposed to the high combustion temperatures within the cylinder, so it is rapidly being heated from the inside and the outside faster than the oil temperature can rise to command that the glow plugs be powered off.

These problems are identified by the glow plug manufacturer, the vehicle manufacturer, and, in my personal experience on my vehicle. I measured an open in one glow plug wire within 10K miles of installing the high amp alternator.

So there is clearly a limit to the “self-regulation” of the glow plug.

The glow plug manufacturer has since designed a different self-regulating coil that is designed for post engine start heating to reduce emissions. The new regulator coil can tolerate higher input voltage from a running alternator, as well as external heating from combustion.

But since my engine was designed 30 years ago, and was obsoleted not quite 20 years ago, there is no retrofit new style glow plug available that will fit my engine, so I do not have access to the new internal regulating coil design.

Hence, I am endeavoring to limit the voltage fed to the glow plugs, so that they can continue to perform their emission reducing function, even while in the presence of a higher amperage alternator that will reduce the environmental impact of having to replace the batteries repeatedly, which was the result of undercharging with the original weaker but glow plug friendly alternator.

  • \$\begingroup\$ It's a very well-written question. I have what I imagine to be a complete and full understanding of what you are trying to achieve. This is definitely a +1 to the question. Before I suggest anything specific to you, a couple of things pop to my mind. You say that the glow plugs are self-regulating. And this, I'm sure, is due to the fact that their resistance increases as they get hotter (more current.) So that's good. We can avoid worrying about the fact you only have two separate banks and that four glow plugs must share a single power source line. \$\endgroup\$
    – jonk
    Sep 6, 2020 at 23:40
  • \$\begingroup\$ It seems to me that you could simply devise a current-limiter. These will automatically reduce the voltage if there is an attempt to pull more current. I believe this should work okay as you can set the limit to what you feel works in your situation. Odds are, the resulting voltage will be exactly what you already know about and accept. It's just that this circuit will need to be able to dissipate 500 W for each bank. Up to a total of 1000 W for those several minutes you are talking about for emissions reasons. Does that sound right to you? (An additional feature might be a slight "foldback.") \$\endgroup\$
    – jonk
    Sep 6, 2020 at 23:42
  • \$\begingroup\$ Jonk, your question/comment on the self regulation aspect of the glow plugs inspired an opportunity for clarification in order to obtain the best answer. Per Stack Exchange directions, I added that information as an amendment to the original question, rather than in the comments section. The number of minutes post start will not exceed 2 minutes, unless the key is cycled again. \$\endgroup\$
    – Herbert
    Sep 7, 2020 at 3:01
  • \$\begingroup\$ If only others were as thoroughly well-informed about their subject and as willing to share with us what they've learned and bring us forward with them. It's a pleasure to read such a question. It gives as much or more to others than it asks from them. The simplest answer would be, of course, that you tool up in such a way that you can modify the existing old-style glow plugs using a newly designed (by you) regulating coil that is adapted to the physical structure. But an electronic circuit can be devised. It's going to be a pain, though, to consider all the elements that can go wrong. \$\endgroup\$
    – jonk
    Sep 7, 2020 at 3:31
  • \$\begingroup\$ +1 for a well-written question. Are you sure the glow plugs can't live with 14v as I find it incredible that something designed for a nominally 12v automotive system would not expect to see 14v or more in operation. And by that I mean I can't conceive of why anyone would be manufacturing 11v glow-plugs that would be damaged at 14v or what market there would be for them. \$\endgroup\$
    – John U
    Nov 18, 2020 at 14:09

2 Answers 2


Tough problem.

Given it's an old engine and should have lots of metal, I would try to just go for brute force and put a high current diode with a high Vf (forward voltage drop) in series with the glow plugs. That would drop the voltage to what you need.

Something like these two:

Vf of 2V to 3V at 100A (Silicon Carbide can run even when really hot--like 175C) https://genesicsemi.com/sic-schottky-mps/GC50MPS12-247/GC50MPS12-247.pdf

Vf of 2V at 200A but a nice metal screw to anchor for heat sinking https://www.vishay.com/docs/94050/vs-hfa135nh40pbf.pdf

You can tune the drop vs current depending upon your diode choice. You can also put multiple diodes in series to help with power dissipation.

That having been said, that diode needs to dissipate roughly 3V times 100A which is 300W of power for the time the glow plugs are on. Anchor it to something big and metal with good heat sink compound.

Good luck.

  • \$\begingroup\$ Thank you for your response Andrew! I've downloaded the information for both models of diodes that you linked, and really appreciate your taking the time to find and provide them. You are correct, the engine is about 1,100 lbs of cast iron block and cast iron heads. It would be nice to be able to dissipate that 300W into a visibly useful purpose, like an outside rear view mirror defrosting circuit, for example. But any solution that will forestall premature glow plug failure from over voltage is a useful purpose. \$\endgroup\$
    – Herbert
    Jan 2, 2021 at 13:02

The way to control alternator voltage is interrupting or cutting current for field coil. The field coil current should be cut then output voltage of alternator reach maximum allowed value. That system is already exist in your vehicle and need some alternation.

  • \$\begingroup\$ Thank you for your response! Unfortunately, the "system that already exists in the vehicle that needs some alternation" (assuming you mean "alteration") cannot be altered (to my knowledge) in terms of cutting current to the field coil. The vehicle's voltage regulator is an IC chip inside the alternator. There is no field terminal external to the alternator. It is internally regulated, and the IC chip wafer is fully encapsulated. Only the heat sink and brushes are visible, and this is with the alternator fully disassembled. The solution I seek is specific to glow plugs, not entire vehicle \$\endgroup\$
    – Herbert
    Jan 2, 2021 at 12:28
  • \$\begingroup\$ May be some couple wires for measuring system voltage going separately and you have access to it. Create circuit and connect it in series with existing. My thought it is easier to control low side current. And may be you need to create some temporary disconnect for alternator so the system gonna work only on batteries when glow plugs on. It is only few seconds. Some relay with coil feed from glow plugs circuit or even manualy \$\endgroup\$
    – user263983
    Jan 2, 2021 at 13:00
  • \$\begingroup\$ Your ideas are intriguing. You are also correct, in that there are indeed a "couple of wires" (exactly 2 wires) for measuring system voltage and detecting when the key is turned on for the vehicle to start. There is an ignition wire and a battery voltage sense wire that plug into the voltage regulator inside the alternator. \$\endgroup\$
    – Herbert
    Jan 2, 2021 at 13:07
  • \$\begingroup\$ Because vehicle electrical system minus is connected to chassis it could be one wire. Look at small gage wire from alternator. \$\endgroup\$
    – user263983
    Jan 2, 2021 at 13:12

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