Fun with Relays

Gary Gibbs · 05-07-06, 05:15 PM

Well boy did I just have fun tracing down a fuse blowing/wiring problem on CAV #15. Anyway I will try to boil this down as best I can, but suffice it to say that all CAV owners should be on the lookout for relay issues.

Where it began. Over the winter I upgraded fans to 1" larger SPAL fans that flow about twice the flow of the stock fans. Mainly because the car temp would creep up on the grid during very hot days (95 deg. +). I also moved the condenser to the front of the radiator and would need some additional help. When I first started the car after the upgrade I noticed a 15A fuse blew right away resulting in the fans stopping. In the "quick fix" way of correcting this problem I went up to a 25A fuse and all seemed fine.

About a week ago I noticed the Lucas fan switch acted backwards one day and then the next mechanically broke inside. I ordered a new switch and went to replace it. I found that the switch failed due to over current! Now by looking at the wire colors and going over the fuse increase in my head I was convinced that they did not use a relay for the fans and were using undersized wiring. I bought new relays and 12/14 gauge wire.

In my detailed tracing of the fan circuit from one end of the car to the other (including thermal switch) I eventually found that THEORETICALLY they are using a relay to turn on the fans! It is mounted in the front near the horns. It THEORETICALLY draws main power through another 30A fuse and the Lucas switch and thermal switch were used to turn on the relay. After lengthy and methodical check out the switch side of the circuit was drawing ~23A! Well that is a little overkill to turn on a relay I thought. Come to find out the relay was wired wrong. The fat wires running through the 30A fuse was being used to turn on the relay and the small wires, switch, and thermal switch was supplying the actual required current flow to the relay/fans. Very nice! Anyway after rewiring the relay and reinstalling all of the other wiring that I had disconnected for diagnostics the fans work fine with the 15A fuse. Here are a few summary points that owners should look at and consider:

1.) If when you turn on the main disconnect to the car with the key out of the ignition and you here a "click" of a relay turning on this is a sign of a problem. I always wondered why a relay was being turned on like this and now I know.

2.) If you do here a click or notice that the fan switch is drawing too much current then check the wiring on the relay.

3.) All of these 30A relays that look exactly alike are NOT!!!!!! We looked at the pins on the Bosch units which the car uses and they are definitely different the two other like configured name brands. So if you do replace a Bosch relay you better double check the pin configuration.

4.) I did notice that one of the relays near the fuse box was not a Bosch (only one in the car) and was pin configured like the other name brands. Hope this is supposed to be in the car. It is difficult to check the configuration of the wiring in the car to see if this relay is wired correctly.

Hope this long winded post helps.

JohnC · 05-08-06, 01:05 AM

Double post, sorry. I'm in the middle of the Sahara and our Internet connections frequently drop out.

JohnC · 05-08-06, 01:07 AM

I've also had cooling fan / relay / wiring problems in a non-GT40 project. This might be a good thread to mention them. I run a single 16" SPAL fan in my hotrod, which moves loads of air, but also consumes 22 amps running current. This fan was initially installed with a 30 amp fuse, 40 amp Bosch relay, and 12 gauge wiring. The relay was thermostatically triggered by my EMS.

The fan failed about a week after the install, and I found the fuseholder had actually melted around the fuse. Thinking the fuseholder/fuse had been defective, I replaced them both and thought all was well. A week later the same thing happened.

As I was scratching around trying to understand and diagnose the problem, I decided to put a peak & hold clampon ammeter on the fan and was shocked to see that it drew nearly 70 amperes for a second or so during starting, before settling down to a more normal 22 amps. I had simply neglected that most electric motors consume 3-10 times their normal running current during starting. Fans are notoriously difficult to start due to their wr2 inertia calculation, so this huge starting current was entirely normal.

To make a long winded post short, this fan circuit is now supplied by 8 gauge wiring, a 75 amp Bosch relay, and a 50 amp fuse. My EMS still does not like the instantaneous voltage dip caused by the initial 70 amp current draw, however I'm coming up with a PWM circuit to reduce the initial surge and run the fan at variable speed instead of switching it off/on so much, and I think the entire system will be much happier.

I'm a degreed EE and could kick myself for not realising sooner how fundamental a problem this was. Our 12v automotive electrical systems are at such low voltages, and most loads consume so much current that maximising contact area, and minimising voltage drop is an absolute must to avoid problems.

John

Paul W · 05-08-06, 04:44 AM

John,

I bought / fitted one of these fan controllers on my car and it works a treat - I just have a relay for the dash overide switch but the little 35 unit powers both of my fans.

Paul W · 05-08-06, 05:02 AM

Sorry forgot the link to the site :

http://www.dccontrol.com/

JohnC · 05-08-06, 08:48 AM

PDub, thanks for that. I'm out of PWM outputs on my EMS, so I was going to have to sacrifice something, or get a standalone box. I also wanted to go from a 16" fan to an 18" and I see that both the box and the fan are available from these people.

John

p.s. Gary, sorry for hijacking your thread!

Canuk40 · 05-08-06, 03:52 PM

Hi Gary,

Great post and simple logic. Looks like your car was fine before upgrading the fans and replacing the relays. It's a very good idea to use the relays to operate the circuit instead of the switch carrying all the load. Very good advice for any CAV guys going to larger fans and especially the caution about replacing relays - different pins, wow that's a biggy and it applies to all brands. I'll be watching for that one...thanks.

Cheers

Gary Gibbs · 05-09-06, 08:55 AM

Quote:

Originally Posted by Canuk40

Hi Gary,

Looks like your car was fine before upgrading the fans and replacing the relays.
Cheers

The car was functional before but I guess I would not call it fine. The factory having wired the original relay wrong was not good IMHO. Fortunately the Lucas switch and small (~18 gauge) wire on the "turn on side" of the circuit was sufficient to handle the stock fan current. The upgraded fans just permitted the wiring problem to be found. Now atleast the relay is being used properly.

JCoop · 08-21-06, 07:12 AM

I finished a relay mod (thanks to Bob C. and Veek) for my CAV ignition switch to alleviate a load problem that overheats the switch when the A/C, headlights, then the fans, go on during hot summer nights. It basically takes some load off the switch. However, when I did the mod, everything was good with A/C and headlights working; except the radiator fans wouldn't go at the designated temperature ( I have a programmable SPAL controller hooked up to 2 X 12" SPAL fans). Even with the override switch, I can only hear the fan relay clicking. I thought it was low voltage, I charged the battery, but still just clicking of the relay. Is that a symptom of bad ground? Or maybe a bad relay? Or is this a related problem to what Gary initially posted? The fans worked OK before the mod...

I DID have to settle for a Hella 20A relay instead of the recommended Bosch as my local source did not have the Bosch. Pin designation was the same.
Thanks IA

chuck smith · 08-21-06, 10:52 AM

JCOOP,I have noticed my ign switch also gets hot under those conditions. How did you go about rectifying the problem? thanks; chuck smith CAV mono14

JCoop · 08-21-06, 12:38 PM

Hi Chuck,

Basically from a diagram that Veek sent me, You'd be taking the hot lead from the ignition switch, routing it through a relay to feed power to the accessories that were hooked up to the "switched" terminal on the ignition switch. So, the load that was on the switch is now transferred to the relay circuit.