Testing Relays

[wayne robson]

Most will probably know this ,but to test a relay ,it is very easy by just using
regular continuity tester,from post 30 to post 87a ,you will know straight away
if it is faulty,i know it is easy to replace a relay,but sometimes it can be a fault
in another area.Also great testing bulbs and many other related items.

[famoroso]

Thx!

Is there a list for which relays should be red and which black? In my case, '88 Carrera cab and all currently installed relays are of the red variety.

[universeman]

Only FP relay should be red is what I understand, the rest black. Red has an extra safety circuit of some kind is what I understand

[sugarwood]

Wayne, is continuity from 30 to 87a doing a complete test of a relay?
I think it's more rigorous if you power 85 to 86 in order to close the switch, and then test 30 to 87?

[sugarwood]

I will add my DME relay notes here:

Originally Posted by sugarwood

Let's understand how the DME relay works and test it.

Following the diagram, power 12V is hardwired into 30 and switched into 86 via the ignition switch
Powering a closed 86->85 coil circuit with 12V closes the first switch.
When that happens, 30->87 is now a live circuit (DME and injectors are live)
This also allows 30->85b to close the 2nd switch
When that happens, 30->87b is now a live circuit (Fuel pump is live)



If you have electrical test leads (aligator clips, wires, and 9V battery), let's test our relay, since we know it works.

1. Connect 86 and 30 to (+) and 85 to (-) of your battery. This will click the first switch closed.
2. Now connect the (+) for a test light to 87, and ground the (-) side. Light should work. (Simulates power to DME)
3. Leaving the above connections in place, connect 85b to (-) of your battery. This will click the 2nd switch closed since 30->85b is a live circuit.
4. Now connect the (+) for a test light to 87b, and ground the (-) side. Light should work. (Simulates power to fuel pump)

[ossiblue]

Sugarwood is correct, all functions of a relay should be tested.

The purpose of a relay is to switch power, that's why it's installed in the circuit. Simply checking continuity between 30 and 87a only confirms the relay I will pass current between those two terminals in its unpowered state, as it should. It doesn't tell you if the relay will switch when powered, which is the whole reason why it's even there. You need to follow Sugarwoods plan of testing.

As far as the red relays, having them in circuits other than the fuel pump will cause no harm. They are more expensive, however, so if one needs replacement, and it's not for the FP, replace with black.

[sabeo.m]

Originally Posted by sugarwood

Wayne, is continuity from 30 to 87a doing a complete test of a relay?
I think it's more rigorous if you power 85 to 86 in order to close the switch, and then test 30 to 87?

Sugarwood- How do you power 85 to 86?

Thx!

[jbrown]

Originally Posted by sabeo.m

Sugarwood- How do you power 85 to 86?

Thx!

Give 12+ to 85 and - ( ground ) to 86 and that activates the relay which will connect 30 to 87. You can also give 12+ to 86 and ground to 85 and get the same result. Does not matter which one gets the 12+ or ground. It works either way. Once 12+ or ground is lost it will switch back to 87a and 30.

[sabeo.m]

Originally Posted by jbrown

Give 12+ to 85 and - ( ground ) to 86 and that activates the relay which will connect 30 to 87. You can also give 12+ to 86 and ground to 85 and get the same result. Does not matter which one gets the 12+ or ground. It works either way. Once 12+ or ground is lost it will switch back to 87a and 30.

Ahh Ok, makes sense- i was testing the same way the Op is, had no idea. Thanks!

[wayne robson]

Sorry sugarwood and others for late reply,not sure.Reason for posting this ,
i was adjusting my drivers window,as it has always hit my top targa rubber seal
when lifting up, while doing this up and down alot,after refitting door panel etc.
went to finally test alignment and no power windows not working, so fitted another
relay, great problem fixed. original faulty one is Wehrie, made in Germany part
no--911-615-109-01, so i tested that faulty one and the new one i purchased here
Porsche dealer $52.00 Ausy dollar,and that is what i found. Thanks for other
information you have posted.

[steely]

Originally Posted by jbrown

Give 12+ to 85 and - ( ground ) to 86 and that activates the relay which will connect 30 to 87. You can also give 12+ to 86 and ground to 85 and get the same result. Does not matter which one gets the 12+ or ground. It works either way. Once 12+ or ground is lost it will switch back to 87a and 30.

This isn't entirely true, 86 is the positive side, and should never be the negative side.

This type of relay has a snubbing (anti-chatter) diode and they only want to work in one direction (has polarity). If it works in the reverse, then the diode has been damaged (shorted). The relay itself may work, but it won't be as protective to the circuit it is in.

This feature can be tested with a continuity tester: with + on 86 and - on 85, you'd read low resistance (~<400 ohms), flip the leads and you should read a much higher number (thousands of ohms). If it reads much lower, the diode was shot/"punched through". my numbers aren't accurate, but the difference in the readings should be very large.

the schematic drawn above does not depict the diode, but the schematic on the relay picture above does.

the pin numbers follow the DIN standard, which identifies pin 86 as 'positive' as opposed to 'common' because of the presence of the relay (and this inherent polarity feature). for example, the coil pins 85b and 87 don't care about polarity b/c they don't have a diode.

[jbrown]

Originally Posted by steely

This isn't entirely true, 86 is the positive side, and should never be the negative side.

This type of relay has a snubbing (anti-chatter) diode and they only want to work in one direction (has polarity). If it works in the reverse, then the diode has been damaged (shorted). The relay itself may work, but it won't be as protective to the circuit it is in.

This feature can be tested with a continuity tester: with + on 86 and - on 85, you'd read low resistance (~<400 ohms), flip the leads and you should read a much higher number (thousands of ohms). If it reads much lower, the diode was shot/"punched through". my numbers aren't accurate, but the difference in the readings should be very large.

the schematic drawn above does not depict the diode, but the schematic on the relay picture above does.

the pin numbers follow the DIN standard, which identifies pin 86 as 'positive' as opposed to 'common' because of the presence of the relay (and this inherent polarity feature). for example, the coil pins 85b and 87 don't care about polarity b/c they don't have a diode.

This may be on factory porsche relays but on standard 5 pin SPDT. You can do it either way. I have been using relays for 30yrs and install about 200 a year and never an issue . I was going on the relay shown in sugerwoods reply

[ossiblue]

Originally Posted by steely

This isn't entirely true, 86 is the positive side, and should never be the negative side.

This type of relay has a snubbing (anti-chatter) diode and they only want to work in one direction (has polarity). If it works in the reverse, then the diode has been damaged (shorted). The relay itself may work, but it won't be as protective to the circuit it is in.

This feature can be tested with a continuity tester: with + on 86 and - on 85, you'd read low resistance (~<400 ohms), flip the leads and you should read a much higher number (thousands of ohms). If it reads much lower, the diode was shot/"punched through". my numbers aren't accurate, but the difference in the readings should be very large.

the schematic drawn above does not depict the diode, but the schematic on the relay picture above does.

the pin numbers follow the DIN standard, which identifies pin 86 as 'positive' as opposed to 'common' because of the presence of the relay (and this inherent polarity feature). for example, the coil pins 85b and 87 don't care about polarity b/c they don't have a diode.

Originally Posted by jbrown

This may be on factory porsche relays but on standard 5 pin SPDT. You can do it either way. I have been using relays for 30yrs and install about 200 a year and never an issue . I was going on the relay shown in sugerwoods reply

The diode mentioned is only in the red relays, not the black, that is what distinguishes the two colors. Polarity of the 85 and 86 pins does not matter in the black relays, but is definitely an important factor in red relays. In red relays, 86 must be positive and 85 must be negative.

[jbrown]

Originally Posted by ossiblue

The diode mentioned is only in the red relays, not the black, that is what distinguishes the two colors. Polarity of the 85 and 86 pins does not matter in the black relays, but is definitely an important factor in red relays. In red relays, 86 must be positive and 85 must be negative.

See that you learn something new everyday.

[Pazuzu]

Originally Posted by jbrown

This may be on factory porsche relays but on standard 5 pin SPDT. You can do it either way. I have been using relays for 30yrs and install about 200 a year and never an issue . I was going on the relay shown in sugerwoods reply

Actually, most standard Bosch style relays DO have a diode, and DO only work one way. It's drawn right there on the schematic on the side of the relay housing...

Edit: I take that back, somewhat. I remember all of the Bosch relays that I bought having a diode in the schematic, but an image search of "bosch automotive relay" shows a mix. Lots of other brands with Bosch style relays do have diodes shown, far more often.

[ossiblue]

Originally Posted by Pazuzu

Actually, most standard Bosch style relays DO have a diode, and DO only work one way. It's drawn right there on the schematic on the side of the relay housing...

Edit: I take that back, somewhat. I remember all of the Bosch relays that I bought having a diode in the schematic, but an image search of "bosch automotive relay" shows a mix. Lots of other brands with Bosch style relays do have diodes shown, far more often.

Just to clarify my post a bit, my reference to the diode and color of relay only applied to round relays as those were the type the OP was discussing.

The presence of a diode in other shaped Bosch relays is something about which I have no information so your post doesn't contradict mine in any way.

[steely]

Originally Posted by jbrown

This may be on factory porsche relays but on standard 5 pin SPDT. You can do it either way. I have been using relays for 30yrs and install about 200 a year and never an issue . I was going on the relay shown in sugerwoods reply

yes - standard form c relays may not have a diode. I don't disagree.
The DIN std designating pin 86 as positive is a convention in this case, but necessary in those relays equipped with a diode.

I agree with ossiblue too, but the red / black convention only applies to the round relays - but note the DME FP relay in post #5 isn't red and has a diode. on edit: oops, i missed L.J.'s last clarification

[uwanna]

As far as the round black and red relays are concerned, the red relay is the only one with a "back EMF" diode installed! Ergo, when you test a red relay with voltage you MUST respect polarity for pins 86 and 85 or you will burn out the diode. With a black relay, you can apply 12V and ground interchangeably between 86 and 85 as there is NO
diode. Check these pics of relays (by DB_cooper) with the guts revealed for proof!
Multi purpose relays

[Mervo]

Originally Posted by steely

This isn't entirely true, 86 is the positive side, and should never be the negative side.

This type of relay has a snubbing (anti-chatter) diode and they only want to work in one direction (has polarity). If it works in the reverse, then the diode has been damaged (shorted). The relay itself may work, but it won't be as protective to the circuit it is in.

This feature can be tested with a continuity tester: with + on 86 and - on 85, you'd read low resistance (~<400 ohms), flip the leads and you should read a much higher number (thousands of ohms). If it reads much lower, the diode was shot/"punched through". my numbers aren't accurate, but the difference in the readings should be very large.

the schematic drawn above does not depict the diode, but the schematic on the relay picture above does.

the pin numbers follow the DIN standard, which identifies pin 86 as 'positive' as opposed to 'common' because of the presence of the relay (and this inherent polarity feature). for example, the coil pins 85b and 87 don't care about polarity b/c they don't have a diode.

A revival on this. I have been checking the Red round relay on my 88 Carrera for the Cruise Control and it does not display these resistance characteristics at all - nor do the other relays. All are original. On the 86 - 85 I get 60 ohms - either direction. Other relays are the same except for the Blower relay that shows a lower rating around 12 mili Ohms. I can't imagine all the relays are blown as everything works (except the CC but that is probably something else).