I'm sorry if the title is confusing - I'm not exactly sure how to pose this question/concern. I'm rewiring a 12 volt automotive engine control harness to be used stand-alone outside the original vehicle. Part of the process is to eliminate the rest of the original vehicle harness & unnecessary components of the relay & fuse box that are intertwined with the engine harness. The factory service manual gives me the following diagram (reduced to show only necessary fusing/relays for my application), but I'm concerned that it is oversimplified and may not show the necessary relay coil suppression to protect the ECM (engine control module). The "CPU" in the diagram is not the ECM I'm concerned about.

Power distribution module

I had planned to use TE V23232-D0001-X001 relays for the throttle control motor, ECM relay, fuel pump relay, & starter relay. The coil resistance is 46ohm, and the relays have no built-in suppression.

For the ignition relay I planned to use a TE V23132-A2001-X30 (used on multiple BMW models). I cannot find exact specifications for this OEM model, other than seeing it listed as 130A and with the schematic printed on the relay housing. It appears to have a resistor in parallel with the coil. The general version of this relay has a coil resistance of 37ohm, with a resistor for coil suppression. The coil rating has a footnote indicating the ratings are "with resistor". The coil does read 37ohm across the terminals. TE V23132-A2001-X30

My questions, as general as they are:

What coil suppression method(s) should I use to protect the ECM considering I don't know if it has any built-in protection? Switching speed isn't a concern, but I'd prefer to get decent contact life from the relays.

If the ECM already has protection built in, does adding another layer cause complications?

If the V23132-A2001-X30 relay has a resistor built in, is any other suppression necessary, and what method would be used if so? The FSM schematic as shown has a series diode & resistor in parallel with the ignition relay coil, but this is not built in to the components I'm using and would have to be added if necessary.

  • \$\begingroup\$ I posed the question to TE directly and got the following response: "For the ECU protection, a diode would be the best but it is the worst suppression for the relay for maintaining good switching life. Generally, a resistor is used for relay coil suppression. All of the Automotive OEMS use relays with resistor suppression and the resistor would be my recommendation. A 680 ohm, 1W resistor should work." \$\endgroup\$
    – objext
    Jun 25, 2017 at 3:38

1 Answer 1


Relay coil suppression is typically a diode placed across the coil with the cathode connected to the positive side and the anode connected to the negative/ground side. A diode from the 1N400X series would be satisfactory.

When the ECM removes power from the relay coil, the collapsing magnetic field of the coil produces a reverse voltage across its terminals. The diode then conducts, shorting out this reverse polarity transient and therefore protecting the ECM. If the ECM has built in protection, the extra diode will cause no harm.

  • \$\begingroup\$ Thanks, Glenn. I had spent a couple late evenings trying to research this subject, so I did familiarize myself a bit with the options. A lot of what I found suggested that a diode alone was somewhat harmful to contact life and other methods (like diode and zener diode) may be better, like this: ![zener][1] [1]:objext.com/expic04.gif \$\endgroup\$
    – objext
    Jun 17, 2017 at 0:45
  • \$\begingroup\$ The diode on the coil has no effect on contact life so deploy them freely. \$\endgroup\$
    – Glenn W9IQ
    Jun 17, 2017 at 0:47
  • \$\begingroup\$ That's contrary to what I've read (from TE & Panasonic no less), which is part of why I wanted to ask about this issue in general. Obviously there are different methods, each suited for different loads and/or use, but since this isn't my field of expertise I wanted to bounce the idea off you guys. \$\endgroup\$
    – objext
    Jun 17, 2017 at 17:53

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