CoolCat fan and relay?

I bought a CoolCat fan and solid state switch for my old E-type about 10 years ago. I remember the J-L discussion in those days was that adding a relay should be considered almost mandatory to maintain acceptable switch life expectancy.

I was looking at the CoolCat website more recently, and I see the current draw on the CoolCat fan is only 0.3 amps more than the stock fan.

I realize that the use of a relay reduces the wear and tear on the thermal switch, but some of the original Otter switches have lasted 50 years with the original fan, so it doesn’t seem than an extra 0.3 amps would be too detrimental to the switch…especially since the CoolCat fan should be running less often.

Was the “need” for a relay related to the use of the solid state switch? Does anyone run their CoolCat fan with the original Otter switch, and no relay?

Perhaps you have a S1 (do S1’s have no fan relay?) as by S2 Jaguar was using a fan relay. The fact they they went to using a relay may suggest something.

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The early Series 1 cars used a relay but all of the current went through the ignition switch. Switches got hot and ignition voltage dropped when the fan kicked on. Contacts in the switch became iffy and the switch needed replacing on a regular basis. This makes more difference when using an electronic ignition as it is more voltage sensitive. Jaguar learned but it took years and not until the Series 2 cars did they add back in a relay and picked up main power from other than the ignition circuit.



Thanks for the response.

You mentioned the “early S1”, and that Jaguar “added back in a relay”. Does that mean the later S1 cars used no relay?

The early as in 1961 used a relay. It is pictured in the 3.8 shop manual. The later Series 1 cars did not use one but should have had one.

Coolcat warns against using the modern thermoswitch without a relay. I’d suspect that the modern-type switch is not meant to have a lot of current running through it. You can contact Coolcat for specifics (as they advise on their website.)

It was not terribly difficult to wire a relay up to my 67 S1 (which lacked one) using Coolcat’s “advanced” application diagram, even though I’m not very electrically minded. It’s not expensive either, just need a good quality Bosch type relay.

I splurged and got a pigtail harness to make it easier to wire up to the relay (and make relay replacement easier if it’s ever necessary) and bought fabric-like sheathed wire which looks a bit more vintage for power and ground. I also perhaps went a bit overboard and added an inline fuse to the power line as well (probably demonstrating my lack of electrical acumen, being ultra-cautious and perhaps spending unneeded money.)


There are supposedly some 3.8’s that didn’t have a fan relay, and no S1 4.2 had a relay. The old otter switches were pretty heavy duty and can work a long time with an 8 amp load. But nothing made this century is as robust, and that includes copies of the original switches. That’s why I always recommend adding a relay for applications where Jaguar forgot.