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i have been trying to setup a solid state relay to control my thermo fan on a pwm signal from my haltech elite ecu, my issue is that it will work initially then go into a closed state and stay like that and can not be turned on or off by ecu, it is like the circuit that controls the fan stays on, i have had 3 of the 100amp solid state relays do this so i have just gone back to an old style 60amp relay. what could be casing this as it was only on a 100hz frequency

G'day Ben. Can you chuck up some details of the Relays you were using, and maybe your Haltech config file too? I'll have a look and see if I can come up with anything.

This is the relay im using. Im no longer using pwm control so dont have it on hand. Also was setup as per manufacturer specs

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i run that relay on my elite 1000 wired as per haltech solid state power switching wiring guide and works great.


as per screen shot with 100hz, i PWM it at 10% when below temp then linear to 100% over a 10 degree window.

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Have you got an external pull-up resistor fitted?

Yes i have.

Hmmm, I can't see a reason it shouldn't be working. Have you got access to an oscilloscope at all? It would be good to get a scope picture of the relay triggering signal (ecu output channel) and what the relay is actually doing (relay output).

The trigger side works fine. Its the relay that isnt working it goes to solid contact so it runs continuously

Yeah, we need to see the shape of the signal the ECU is generating though, that way we can determine if its the input, or the output side of the relay which is at fault.

Fan will stay on even when i disconnected both wires on the trigger side

The relays you're using dont say 'AC' up the top, instead of 'DC' do they? I've not had a problem using relays means for switching AC circuits in automotive DC applications before, but its been bought to my attention that it can infact make a difference, and have the relay 'jam' on.

100amp dc


Do you have a freewheel or suppressor diode across the load to prevent counter EMF spikes? That can kill SSR if not addressed.

I have had it wired like this

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Is the relay meant to be mounted and have heat conductive compound between it and the mounting surface.

I have seen OEM solid state relays used for things like headlights fail because the heat compound breaks down, then they overheat.

Literally lasted 5mins if that before it failed. Its even mounted to a heat sink

if it was on a fan than you need a free wheel diode.

the fan spinning down generates EMF and cooks the SSR.

does anyone have photos and details of what diodes are best suited to this application?

Like said above, add a freewheel or TVS diode across the load and use thermal conductivity paste between the SSR and the heat sink. You can use the same paste used on computer heat sinks.

I am having the same exact issue on my Haltech Elite 2000, this seems to be an ECU defect, I have seen many other complaints online.

Different relays may have different internal designs that will affect the way they should be wired. That wiringschematic may be ideal for the HALTECH relay, but for the one you're using it says "Note: Inductive loads must have diode suppression (Refer data sheet wiring diagram)", and there's no diode shown there.

Under "specificaions" it also states maximum reverse voltage 32VDC, it's unlikely, but possible for such high voltage spikes, and this may lead to issues.

Ah, small but possibly crucial difference between your HALTECH wiring diagram and this one, which incorporates the diode - https://www.hpacademy.com/assets/forum/attachments/56e65b78cc/HT-030202_IS.pdf - and specifically states it's required for inductive loads, like fans!

So I guess I should try an actual Haltech relay and see how it goes.

With the diode.

What I suspect has been happening is that the electronic 'switch' was broken down by the voltage spikes and is now permanently a closed circuit. The diode bleeds off the voltage potential from the fan's spinning, preventing it reaching a high enough voltage to break down the relay.

Hi Gord, Thanks, I have attached the diagram and also the Haltech diagram, I have tried it both ways with no luck, I will also share a onedrive link if you could please review it.


Attached Files

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