Mystery resistor from a Ferrari 308 - Page 1
Forum section: electronics / repair

author: borud
author_rank: Unknown
timestamp: August 06, 2015, 09:16:12 am
content: A friend of mine is fixing the heater on a Ferrari 308. There is a power resistor that has disintegrated and which will need to be replaced, but I am having trouble finding any information on how much juice the resistor is supposed to be able to handle. My friend sent me an image of the resistor, but sadly he didn't include a size reference so I have no idea how big the part actually is. However, there are still some markings on it, so on the off chance that any of you are able to make sense of it, I'm posting it here.It looks like a 1.5k resistor. But how many watts?(I'm stopping by the garage later today to have a look. I can take a picture with a size reference then)-Bjørn

author: Wh1sper
author_rank: Regular Contributor
timestamp: August 06, 2015, 09:19:02 am
content: 1.5 OhmsI guessand somewhat in the 7W - 11 Watts Area

author: firewalker
author_rank: Super Contributor
timestamp: August 06, 2015, 09:20:39 am
content: I guess 1.5 Ohm at about 10 watts.Alexander.

author: borud
author_rank: Unknown
timestamp: August 06, 2015, 09:23:18 am
content: Quote from: firewalker on August 06, 2015, 09:20:39 amWhat do you base the 10 watt guess on? The design? The markings?-Bjørn

author: firewalker
author_rank: Super Contributor
timestamp: August 06, 2015, 09:27:58 am
content: I guess that the width of the resistor is the standard width of those kind of resistors. I am holding a 1 Ohm 5 watts resistor now and it seems to be smaller than the half of the image posted.Alexander.

author: Deathwish
author_rank: Unknown
timestamp: August 06, 2015, 09:30:30 am
content: RYK11 gives it away as 11 watts

author: rickselectricalprojects
author_rank: Regular Contributor
timestamp: August 06, 2015, 09:33:40 am
content: It is also a 1.5 ohm 11w resistor. Got any pics of the 308? The 308 is a very Beautiful car(-;

author: McBryce
author_rank: Super Contributor
timestamp: August 06, 2015, 09:34:31 am
content: That looks like one of the resistors for controlling the fan speed (1.5ohm). Does the fan only work on one of the three positions at the moment? Can't remember what wattage they were, but you can work it out as you (12V supply / 30A fuse - I think). Unfortunately, if this is burnt, there's probably a short in one of the windings of the blower motor. Also, there was an updated, much more reliable fan released later, so if the car is stripped down for restoration anyway, he should consider looking for the newer as a replacement.McBryce.

author: tec5c
author_rank: Frequent Contributor
timestamp: August 06, 2015, 09:46:56 am
content: Quote from: Deathwish on August 06, 2015, 09:30:30 am+1

author: borud
author_rank: Unknown
timestamp: August 06, 2015, 09:53:37 am
content: Quote from: McBryce on August 06, 2015, 09:34:31 amI'm not sure if it is burnt or if it has just disintegrated due to "environmental factors". From what I gather the fan has three speeds so there is another resistor I might be able to have a look at. Ithinkit works at the other two(?) speeds, but I'll check. In fact I think I'll bring along my own multimeter to have a look. The measurement values my friend mentioned over the phone didn't make a whole lot of sense to me. Or him. For different reasons.The updated fan is a drop-in replacement? In order to figure out the specs of the resistor I stumbled across quite a few fan assemblies. My goodness Ferrari parts are expensive. You'd think they were made of unicorn tears.-Bjørn

author: McBryce
author_rank: Super Contributor
timestamp: August 06, 2015, 10:53:21 am
content: Quote from: borud on August 06, 2015, 09:53:37 amYes, that's why I gave it up as a hobby too. As far as I can remember, it's a drop-in replacement. It's made for that car, it was just a later revision. I was told it's much more efficient too, but I can't vouch for that, it was one of the last repairs I did before selling it on... and buying a 348 which burnt money even faster! (Gearbox failed after 12 months)McBryce.

author: borud
author_rank: Unknown
timestamp: August 06, 2015, 11:35:42 am
content: Quote from: McBryce on August 06, 2015, 10:53:21 amTalked to my friend on the phone and he could confirm that two of the other speeds work, and they have indeed upgraded the fan. (I was under the impression that there was an entire fan assembly of which the resistor was a part, but I probably didn't pay proper attention since I was only interested in the resistor).(I have an 1987 Alfa Romeo 75 Turbo Evoluzione. It is a homologation car so it looks like a teenagers idea of what an Alfa 75 should look like and it is terrifying to drive. I have taken some steps though. Like putting in a steering rack with a lower ratio and adding power steering since the car is awfully tail-happy. It involved a lot of flailing about to keep it pointing nose forward when the turbo figured it was time to do its bit).Okay, so now I'm off to see if I can find a suitable replacement on Farnell. Any recommendations?-Bjørn

author: McBryce
author_rank: Super Contributor
timestamp: August 06, 2015, 11:42:30 am
content: From memory it was around a 12W resistor I used (wanted to be on the safe side).McBryce.

author: amyk
author_rank: Super Contributor
timestamp: August 06, 2015, 02:22:22 pm
content: My first thought was, wouldn't PWM be better for adjusting the fan speed than dropping resistors? But then again, if this is part of a heater, maybe the resistors help...

author: KJDS
author_rank: Super Contributor
timestamp: August 06, 2015, 03:38:59 pm
content: Quote from: amyk on August 06, 2015, 02:22:22 pmModern cars certainly use FETs rather than resistors, but I don't know if they use the linear or on/off approach.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: August 06, 2015, 04:00:40 pm
content: If 2 ohms is acceptable you have a lot more options. Here are chassis-mount resistors:https://www.digikey.com/product-search/en?pv2=77&pv2=198&pv2=11&pv2=12&FV=fff40001%2Cfff80488%2C40059%2C400fa%2C40386%2C40387%2C40388%2C40389%2C4038a%2C4038b%2C4038c%2C4038d%2C4038e%2C403e4%2C40433%2C40436%2C40468%2C40871%2C408c3&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25If the failure was due to over-power then this might be a good alternative at 20 watts in through-hole:https://www.digikey.com/product-search/en?pv2=11&FV=fff40001%2Cfff80482%2C40059%2C400fa%2C40386%2C40387%2C40388%2C40389%2C4038a%2C4038b%2C4038c%2C4038d%2C4038e%2C403e4%2C40433%2C40436%2C40468%2C40871%2C40898%2C408b6%2C408c3%2C40e31%2C40e5f%2C40e75%2C40ee8%2C410bc%2C41782%2C417ae%2C417dd%2C41ace%2C8000a%2C8000b%2C8000c%2C80010%2C400001&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

author: c4757p
author_rank: Super Contributor
timestamp: August 06, 2015, 04:21:18 pm
content: Quote from: KJDS on August 06, 2015, 03:38:59 pmI can't really imagine why you'd use a FET in linear mode to control fan speed - you want a fixed resistance anyway, and all you're doing is replacing a resistor with a more expensive (yes, power FETs are technically cheaper than power resistors, but require more expensive thermal management) and less reliable substitute.

author: borud
author_rank: Unknown
timestamp: August 06, 2015, 05:39:27 pm
content: Quote from: amyk on August 06, 2015, 02:22:22 pmThat was my first thought as well.Then I pictured myself standing beside the smoking remains of the car with a "uh...sorry?..."-face. Sure, it's easy to do, but I'm not an electrical engineer so I might screw it up in precisely the way that produces viral videos. And then I'll be known as the "Ferrari burner" and kids will throw rocks at my house.If it was my car I'd probably go for that.-Bjørn

author: KE5FX
author_rank: Super Contributor
timestamp: August 07, 2015, 08:28:28 am
content: Wait'll you see the fuse box.Believe me, no actual electrical engineers were involved when the 308 was designed.

author: KJDS
author_rank: Super Contributor
timestamp: August 07, 2015, 08:36:01 am
content: Quote from: KE5FX on August 07, 2015, 08:28:28 amI was chatting to a friend that owns a Countach last weekend. It just never occurred to the Italian supercar designers of the 70's that electrics on a car had to work reliably.

author: McBryce
author_rank: Super Contributor
timestamp: August 07, 2015, 10:40:56 am
content: You've got to remember, the design work for this started around 1969. Using resistors for fan speed was normal practice back then. Almost all vehicles use PWM now.McBryce.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 07, 2015, 12:51:34 pm
content: Ford used resistors wound with Nichrome wire on a mica former in the 1988 Falcon.Unfortunately it was in the heater/ventilator airflow.Gum leaves find their way in to that too,so you get the smell of an incipient bushfire coming out of the vent.The resistors become open circuit very readily,& I was going to replace the circuit with a big fat BJT,but got lazy & bought another resistor.The later Falcons used solid state devices,so my idea wasn't so far from Ford's thinking.

author: McBryce
author_rank: Super Contributor
timestamp: August 07, 2015, 12:58:09 pm
content: On some Fords you'll also find a resistor hidden inside the wire harness going to both headlamps. This was because the specs for headlamp supply voltage doesn't work well with the newer alternator output voltages they used.McBryce.

author: mikerj
author_rank: Super Contributor
timestamp: August 07, 2015, 03:16:20 pm
content: Quote from: McBryce on August 07, 2015, 10:40:56 amResistors were used to control fans in cars less than 10 years ago, and probably still are in some cases.

author: PChi
author_rank: Frequent Contributor
timestamp: August 08, 2015, 03:49:26 pm
content: Resistor power rating is so dependant on how they measure it. Just choose one of the same or larger physical size.It'w worth checking that the fan motor spins freely.My Vauxhall Astra Mk1 used a series resistor to control the fan speed. One day with the fan on I smelt hot electronics and switched the fan off (though my nose isn't as sensitive as Dave's) before the resistor burnt up. It turns out the fan motor was hard to turn and the excess current taken was too much for the resistor. The motor was exposed to the dirt passing through the ventilation system so inevitably after some years the bearings started to seize up. After taking the motor apart giving it a good clean and lubricating the bearings it gave some more years of service until the car was stolen.

author: SeanB
author_rank: Super Contributor
timestamp: August 08, 2015, 04:21:20 pm
content: Trundle off to the VW dealer ( or preferably the aftermarket suppliers as they are cheaper) and get a fan resistor for a VW golf/polo/passat. They have 4 connections for spade connectors, and a thermal fuse that fails open circuit when it gets too hot. The resistor needs to be mounted in airflow, so will go in the AC ducting somewhere.I have replaced a few, as they both fail from a poor fan motor design, and from condensed water from the AC system. I buy the aftermarket one and do a transplant of the resistor block as the old one no longer is available except as OEM ( and at a price out of proportion to the size) but the connections are the same ( aside from 1 extra spade lug that is easy to cut off) and it can be put into the other housing with no problems.If you drive a BMW you will be well aware that they also have a fan speed control using a MOSFET cooled by the air in the ducting, which is rather notorious for self destructing and leaving you with either no fan or uncontrollable max speed.

author: Nerull
author_rank: Frequent Contributor
timestamp: August 10, 2015, 12:00:37 am
content: Ferraris have a reputation for burning all on their own, so I wouldn't worry too much about that.

author: borud
author_rank: Unknown
timestamp: August 11, 2015, 09:25:21 pm
content: Quote from: Nerull on August 10, 2015, 12:00:37 amI'm probably a pariah among purists since I have a pretty modified old car. (Turbo moved forward so as not to fry the brake reservoir, aftermarket engine management system + new ignition system + new injector system and custom made intake, much bigger intercooler, upgraded brakes etc).The status displays inside the car never worked right though. Brake and the wiper fluid light comes on (the sensor isn't even connected so I have no idea why it suddenly gets the idea I have no wiper fluid).Brakehard, and the master alarm flashes like you're on the Apollo 13.In fact, Alfa Romeo had so little confidence in the electronics the dashboard has a reset button. And that button isworn. If you drive those cars on track you pay attention when the dashboard doesn't go apeshit. Because that usually means the car is going to stop soon(Not many cars from 1987 that get 45-50mpg if you drive them carefully though)-B