I haven't posted in a long time. What prompted me to ask is discussion from another Ferrari owner (off this board) about relays and circuit load, especially with higher current devices like radiator fans. On my 308 GT4, the fans cycle on and off on a regular basis, obviously more often when the weather is hotter. My first question is over time, these repeated on-off mechanical cycles, should I be concerned with the longevity of the relay? When I bought my car 16 years ago(!), I replaced all of the relays except the flasher (side note later *). Out of all of them, the rad fan relays have been used the most. Now, I could easily replace with like, but could I install relays that have diodes bridged across the 85/86 terminals? I'm aware of the "flyback" electrical phenomenon (where you get large voltage/current spikes when the contacts open and close, much like distributor points) and I'm wondering if this would help increase longevity/durability? If it would benefit, what polarity should the diode be (I've seen relays listed that have diodes oriented either way). Second question; anyone use solid-state relays? There are ones made by Hella and E.T.A. that have the same footprint as a typical relay, but have no mechanical moving parts inside. The solid state relays tend to be four pin only (so: 30,85,86,87) so those would only work in a couple of applications. The Hella units have heat-sinks which tell me they get hot and need air flow around them to maintain safe operating temperature (and Ferrari relays are always buried deep within the car in small closed areas with little to no air flow). Or, am I overthinking this and should replace the relays with the like and keep driving for the next 16 years? Any advice and feedback appreciated. * I'm also thinking of upgrading the lighting to LED. I have an idea of what to use for the flasher (from Steve Jenkins' blog), but has anyone had experience with LED flasher units? I hope I can participate more often, I miss being on F'Chat...
Peter, the higher current relays do take a beating, and it will affect their life span as their contacts can weld together, or develop deep pitting, etc. The diodes help on the coil side, so they are installed with reverse polarity, ie the cathode end (with the stripe) should be facing the +12 incoming voltage. The diode won't allow current coming in to flow through it to earth, but the flyback voltage gets shunted to earth. MOV's are another way of shunting this current safely, but they have to be added externally. I've successfully used SSR's in some applications, but the heat sinking is VERY important for their longevity. I've normally used the little flat pack ones and had to mount them wherever I could in the past, which doesn't look all that tidy....... One serious mistake that Ferrari made in their electrical system was putting the high current circuits on the same "motherboard" as the rest. For sure the fan circuits and possibly the fuel pump and headlight circuits should have been located elsewhere. I don't know if they're grouped together for your 308, but in the later cars this grouping has caused problems with the copper traces delaminating, burning through, etc.
On regards to the LED flashers, I was told a resistor to load the circuit and use of original flasher was better than an LED flasher.
Yes This is because there is no industry standard for whether 85 is plus and 86 is minus -or- 85 is minus and 86 is plus. Consequently, it's up to the user to get the right one to ensure that the condition John noted is met: "the cathode end (with the stripe) should be facing the +12 incoming voltage". But the diode has nothing to do with the life of the relay -- it has to do with preventing arcing at the switch contacts of the thing that is turning the relay on and off (so might help its life - but you are "fixing" a problem that doesn't exist IMO).
Peter. Back when I ran a manufacturing plant, we ran high voltage and amperage relays for years on machines that cycled them 5-6000 times AN HOUR, 8-10 hours a day, 220 days a year. 16,000,000 cycles a year. Some were 100 AMP 408 Volt, some were lower amperage and voltage, all the way down to some 18VDC guys. We replaced them about once every 3-4 years. so, maybe 60,000,000 cycles before we replaced them? So, yes, you may be overthinking it. Oh, but when I had a 90's era 911, I carried 3 or 4 spares that covered the major items of operation, and actually had to use one once to fix faulty headlights. They're cheap insurance. well, relatively cheap. D
They now make "no load" flashers (EP34) that are superior to either. And back on subject, I have disassembled and inspected many 4/5 pin automotive relays to inspect. The best (by far) are AC Delco original. The Ferrari originals (Magnetti Marelli often) are actually pretty good, and some of the Taiwan ones are absolutely terrible. Image Unavailable, Please Login
According to DIN 72552 the coil should be fed with +12V to terminal 86 and grounded via terminal 85, however in practice it makes no difference which way around they are wired, unless you are using a relay with an integrated diode
Yes, totally aware of that problem which affects the group of models built long after my GT4 (TR, 328, etc). The fuses are separate from the relay board assembly and I'm in the process of updating the fuse boxes to modern units (not Birdman's, as I'm not a fan of that set-up). Well, I haven't in my 16 years of ownership. As I mentioned, I replaced them shortly after I purchased the car and haven't touched them since. The car did have a dead Stribel 87/87b relay (controlling external lights) when I bought it, but all of the original Bosch relays were working... For how long? I didn't know or trust... I figured after 16 years of use, they "may" be getting "tired". I haven't driven the car very much this year, but overall, I have owned the car longer and have put more mileage on it than any previous owner. Heard/read that too. How much resistance? Point well taken. Always appreciate your wise words! See my reply to Steve M. Thanks! Thanks for the ref. to the flasher and relay integrity. I always make sure to read the wiring diagram to see what they're using for "+" and "-" but as was pointed out by the others, the diode serves no real purpose to help what I'm concerned about. I should just replace if I have doubts (which more than likely there is no problem to begin with). Much appreciation to you all, thanks!
Relays don't get "tired" that quickly, they are really robust and cheap which is the reason they are still around and will be for a long time. It is also why you should not think about them much - they are cheap and very few times a failure will actually cause an issue. Contacts welding themselves together is usually the worst that can happen and a problem I have encountered a few times. You will usually notice it soon enough - e.q. fan blowing like mad even with ignition off. Quite a few 355/360 F1 pumps have died an early death because of it, too. For such an item like an expensive pump, I would probably have relay renewal on the long-term maintenance list. For a fan or light, it doesn't matter that much. Re-engineering for less contact burn is something I would not spend time on with my cars, and I hold a degree in EE... It really is an engineering choice of risk vs. cost. A word of caution here too, putting diodes etc in a circuit to rid the contacts of flyback, will mean you will either put a diode over the contacts (pushing the flyback into the car's electrical system) or introduce a diode parallel to the inductive load. While pushing the flyback upstream is bad, both solutions are to introduced with caution because a diode is a semiconductor - with the characteristic that a voltage, current or heat peak can cause it to short. If the diode wires you put in are too thin, you then have introduced a fire risk. So from an engineering standpoint, it's better to accept some contact wear. A solution could be a capacitance over the contacts or a parallel resistor over the inductive load - both dampen the voltage peak and therefore the arc across the opening contact.
Firstly: Don't "fix" what ain't broke. My usual process in replacing a worn part is: 1. sit down and design an "upgraded" (read "over-engineered") function, 2. then I realize the limitations of the "advanced" design, 3. then I replace the original part like for like. The Bosch relays do carry quite a bit of coil current, but it's typically the contacts that die. The high current coils don't have the huge inductance of electronics store relays, so while you'll often see "flyback" diodes on the high Henry coils, it's really a "frill" in automotive applications. And a car is a horrid environment for silicon parts. (Note the heat sink on the 328's fan transistor.) (I did a lot of reading on relays during step "2" on the 328's aircon thermostat: I designed, built, and tested a PC board OP-amp and thermistor aircon control (step 1), then discovered that the circuit couldn't supply the current to kick the Bosch relay. An interesting exercise in home-brew printed circuits, but, in the end, I just installed another capillary tube thermostat. The center console in a GTS can easily exceed the operating temp range of commercial electronics parts.)
The mention of diodes were that examples I had seen available had either resistors, or diodes (and those were available in either polarity) built into the relay. In my line of work, I've installed diodes between the contacts of heavy duty solenoids to prevent damage, so I figured there may be a benefit here... Looks like not. Thanks for the info. Merry Christmas. I suppose the heat build up is from the engine's coolant pipes running below. Thanks for your comments. Merry Christmas.
"In a GTS" -- from direct sunlight on the console. Even a tan interior absorbs solar energy. (When the thermostat died, I had a fuse across the contacts as a stopgap, but I needed to monitor the air temp to keep the evaporator from icing up. I had a portable indoor/outdoor digital stuffed by the console with the probe in the air vent. That let me see just how hot the console itself got.) I had to drape a cloth over the black steering wheel or use driving gloves. Not exactly a white xmas, here. Forecast high of 70F. Merry holidaze.
Thats correct but before relays with integrated diode across the coil were available, because it didnt matter, manufacturers did not pay any attention to it. This means if using a relay with a diode, its vital to check the polarity of the coil connections is correct. If they are not, the results can be catastrophic because the wiring harness will melt, as the relay coil is being replaced by pretty much a dead short of the forward-conducting diode. As has already been said, relays and high-current DC inductive loads dont really mix, because it causes a gradual migration of metal from one contact to the other, which results in a build-up and eventual sticking. So they do need to be replaced periodically.
