Red Relay or Brown Relay or Black Relay?

autobonrun · autobonrun's Garage

When I purchased my 79SC, the fuel pump relay was red. All the others were black (#21131). Years ago, the fuel pump relay went out and in an emergency, I swapped one of the black relays in and it worked fine.

Physically, is there a difference in the red and black relay? Either works in my car. Also, the last relay I purchased came in tan in color (#21164). Now I'm really confused. If all these relays work interchangeably, why all the different model numbers and colors? Are the contact ratings different or something?

Jim Sims · 05-14-2002, 09:43 AM

I believe the red relays contain a diode in the circuit that permits a "free wheeling" decay of the current to reduce contact arcing or something like that. I've seen black, red and white relays but never a tan one. Jim

304065 · 05-14-2002, 11:22 AM

The red relay has an integral "Quench" diode wired in parallel. When the current flowing through terminals 85 and 86 (the relay coil) breaks down, there is generated a "back EMF" much like that in the ignition coil when the points open. This can be hundreds of volts, and can cause breakdown of the circuit over time. So the red relay includes a diode to prevent this.

For more info, do a search on "quench diode"

Early_S_Man · 05-14-2002, 01:17 PM

The red relays have a 'quench' diode accross the coil terminals 85 and 86 for supression of the inductive kick that would normally be induced on the +12 Volt bus when the relay was switched off. The presence of that diode means that the #85 terminal must ALWAYS be the ground side of the red relay' coil, but other than that precaution, the red and black relays may be used interchangably for troubleshooting purposes, and permanent replacements of black relays by red relays is not a bad idea in cars with alarm systems or other sensitive electronic components.

I assume that the 'white' relays that have been mentioned a few times in the past week are really the old aluminum-cased 901.615.109.03 that has the pinout (and uses the same socket) of the red and black relays.

I haven't seen the tan or light gray relays mentioned in other supplier's slick catalogs that may be substituted for the black relays, yet!

Jim Sims · 05-14-2002, 01:24 PM

No, the white one I saw had a white plastic housing (not aluminum) and I seem to recall it was made in Brazil. It worked in the fuel pump socket (as would a black relay) but I don't remember a schematic or diode marking on it. Jim

Early_S_Man · 05-14-2002, 02:46 PM

Well, that just adds a bit more fuel to the mysteries of the relays ... fire.

The presence of a Wehrle 21xxx number on the tan relay mentioned above seems like a good piece of news. The spare Red Chinese relays without the Wehrle number on them that I have with the OEM Porsche mark on them don't give me a 'good' feeling of confidence AT ALL!

johnd · 05-14-2002, 03:09 PM

My 84 911 has a black relay next to the DME box under the seat. Everyone says to carry a spare, so do I get one just like it, or will the red one fit? thanks

Early_S_Man · 05-14-2002, 03:51 PM

No, John, the DME is a special purpose relay.

autobonrun · autobonrun's Garage

Quote:

Originally posted by Early_S_Man
The red relays have a 'quench' diode accross the coil terminals 85 and 86 for supression of the inductive kick that would normally be induced on the +12 Volt bus when the relay was switched off. The presence of that diode means that the #85 terminal must ALWAYS be the ground side of the red relay' coil, but other than that precaution, the red and black relays may be used interchangably for troubleshooting purposes, and permanent replacements of black relays by red relays is not a bad idea in cars with alarm systems or other sensitive electronic components.

I do have an alarm system and a fairly expensive audio system. From the posts it seems it's Ok to replace the black relay with red ones to limit the inductive kick but as a minimum, I need to remove the black relay that is now in the spot for the fuel pump and put a red one in. John also points out that there can be circuit damage without the diode in place. I had not considered this issue.

FYI, the black ones I have contain a Porsche number 911.615.109.01 while the tan one does not contain a Porsche number but does have a Wehrle number. Also, on the black relay, there are two symbols ( I assume they are German equivalents of our UL or FM certification) while on the tan relay, there is only one. My tech curiosity got the best of me, so I fired off an email to Wehrle Engineering. If I get an answer that is relevant, I'll post it later.

jkeyzer · jkeyzer's Garage

Those red relays aren't switched on/off by an automotive computer are they? If so, substituting a black one might run the risk of damaging sensitive driver circuitry (high voltage kickback as mentioned above). This is the main reason for using those reverse connected diodes when relays meet digital circuits.

I didn't think the 79SC had a computer so this might be a non-issue.

Jim Sims · 05-14-2002, 06:58 PM

You don't have to have a computer for this feature (quench diode) to be helpful; it also reduces contact damage in the ignition switch when the car is turned off. Cheers, Jim

jkeyzer · jkeyzer's Garage

Good point. I didn't realize that relay went to the ignition switch. Those "modern" SCs and their bells and whistles.

Early_S_Man · 07:29 PM

The factory alarm, which I believe is installed in all SC's, contains a CMOS IC of the RCA CD4011BE type, and it can be blown when a Voltage regulator fails and the system Voltage reaches the 18 to 20 Volt level. Since a blown alarm system controller can keep an SC from running, and is a pain to get to and disable (assuming you are familiar with the procedure to do so with a jumper wire accross the two #61 terminals of tyhe connector) ... the perils of high Voltage spikes on the +12 Volt bus is NOT something to scoff at!

Just one of several good reasons to have a Voltmeter in your console!!!

winglesss · 09-08-2023, 09:34 AM

Quote:

Originally Posted by 304065

The red relay has an integral "Quench" diode wired in parallel. When the current flowing through terminals 85 and 86 (the relay coil) breaks down, there is generated a "back EMF" much like that in the ignition coil when the points open. This can be hundreds of volts, and can cause breakdown of the circuit over time. So the red relay includes a diode to prevent this.

For more info, do a search on "quench diode"

Thanks to you and the others on the differences between the red and the black relays.

That information is VERY helpful.

Unfortunately it is not possible to measure good/bad on that internal diode health on the external red relay terminals using my multimeter diode tester because of the permanent connection to the internal coil. I'm going to assume the diodes in my relays are still good.

autobonrun · autobonrun's Garage

Wow. I completely forgot that I had asked this question. This is from 20 years ago. Loved the tech discussions back then.