heater resistor testing

TJay · March 01, 2021, 10:04 AM

I had the heater carp out on me last month, yup had to pick the coldest month, anyway took everything apart, checked and cleaned the switch (looks ok), took the motor out and bench tested it (ok) so now I'm at the relay and resistor as possible suspects. Can anyone tell know how to test the resistor and relay. Or is it possible to build a resistor because if I remember right the old ones were made from varying sizes of wire coils. New ones (if you can get them are $160 and the relay is 57 quid). I've got it wired direct from power with a fuse and rocker switch so I have heat but its not the greatest.
Thanks
Have a great day
Tom

Red90 · March 01, 2021, 11:17 AM

Measure the resistance across the resistors. Values are shown in the wiring schematic. They are just for the slower speeds. Full speed does not use the resistors. Sounds like you have a relay issue which should be easy to solve.

In order to make your own, you would need to understand how to build resistors. They are not just wires. You could use a heater resistor pack from another vehicle if you are comfortable working out the wiring.

ugly_90 · March 01, 2021, 12:53 PM

If someone had a cheap clamp-on DC Ammeter-DMM, they could do some measuring. ;)

Sounds like some too-expensive resistors and relays there. I wonder how the Britpart pricing would compare. Failing that, with enough effort, you should be able to identify the electronic component and order directly through a company like Digi-Key, www.digikey.ca

Their shipments arrive a day or two later into Edmonton, and I think they charge $8 or so for the FedEx from Winnipeg.

Matt H · March 02, 2021, 07:25 AM

I'll assume you checked the fuse?
In my experience it is very seldom a relay actually fails. It's not that they never do it's just unlikely. I would be 100% sure there is no extra resistance in the harness and connectors first before looking at a new relay.

The fan draws high amperage, especially at start up and the harness connector at the fan can become burnt. Have you checked for voltage and correct amperage at the harness connector to the motor?

If the resistor fails you will just have max fan speed available.

If you suspect the relay you can always bench test it. Same with the resistor. If it turns out either is bad I'd just get one from a wrecker.

My 2c.

TJay · March 09, 2021, 02:58 PM

Thanks everyone for the help, - Red90 I checked the resistance across the resistors with it out and got zero across the board also couldn't, rave doesn't give any values in the electrical troubleshooting manual.
Ugly-90 I took a look at digikey and I guess I'm not very electrical cause it was all greek to me.
Matt H yup first thing I always check - fuses, the connectors all looked good but I cleaned them and the pins anyway.
I plugged everything back in and still nothing at all not even high speed, however more testing and it appears all the circuits in the resistor are working , the relay has 12v at pin 87 and at fan speed 1 there is ground at pin 85. That is about all I can decipher from the manual though.
Anything else anyone can think of to check ?

Thanks Tom

Red90 · March 09, 2021, 03:21 PM

Is this a Disco 1? Do you have the Electrical Troubleshooting Manual. Are you able to follow the circuit diagram? I've attached it. It is important to understand that the switch is on the ground side of the circuit. Follow the circuit and isolate which component(s) is at fault.

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Trevor · March 09, 2021, 05:19 PM

Quote from: Red90 on March 09, 2021, 03:21 PM
Is this a Disco 1? Do you have the Electrical Troubleshooting Manual. Are you able to follow the circuit diagram? I've attached it. It is important to understand that the switch is on the ground side of the circuit. Follow the circuit and isolate which component(s) is at fault.

You cannot view this attachment.

A few pointers that usually helps me when sorting out these electrical diagrams:

...you sacrifice the chicken AFTER the first dial looking thingy on the diagram...and I have some extra eye-of-newt if you need any.

TJay · March 10, 2021, 12:33 AM

Hey Trevor, I sacrificed the chicken, prayed to the Lucas gods but DAMN I forgot the eye of newt, send me some asap lol
it's always good to have a laugh when ya need one thanks Trevor

Red90 · March 10, 2021, 06:00 AM

Start at the fan connector (C2069). Disconnect. With the ignition on and the fan switch on position 4, you should have 12V on pin 2 (white/red wire) and ground on pin 1 (black/orange wire). If you do, the fan is bad. If not, then follow the faulty side to the cause.

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TJay · March 10, 2021, 08:14 AM

Thanks Red90, Yup I have a '97 Disco 1 and as it sits right now I have hot wired it from the battery (fused) straight to the fan motor with a rocker switch so basically position 4 on the heater switch so the fan motor is good. Cleaned all the pins, connectors and ground, plugged everything in and still nothing not even position 4 on the fan switch which should be direct bypassing the resistor. If it is the fan switch I'll have to get the whole control unit for a measly $500 -600 US ouch the truck is hardly worth that except it's a 5 speed with no sunroofs, 508000 km's and runs great hence I really want to fix it.
My thought - is it possible to wire in a 12v rheostat bypassing the resistor and relay hence variable speed fan?

Red90 · March 10, 2021, 09:37 AM

You need to get in there and troubleshoot. Find the actual problem and then address it. As Matt mentions, it is most likely to be a connection somewhere. It is doubtful it is the switch or the resistor pack as they would not fail on all speeds. If figuring out the wiring diagram is an issue, see if you can find a friend to help that understands what it all means. The internet is not the optimal way to solve these kind of issues.

Connectors C2066, C2027 and C2022 are the most likely suspects. But please use a meter to trace the fault location. The ETM shows a picture of every connector. If you do not have it, let me know and I will show you where to get a copy.

You can't just install a rheostat as this is a very high current use. A suitable rheostat would be massive. What would normally be done in 2021 would be to use a PWM controller for speed control. These can be purchased for fairly low cost. You just need to make sure it is rated for the current of the fan at full speed.

ugly_90 · March 11, 2021, 07:21 AM

It sounds like you might need some help there, as you haven't done this before? It isn't inherently hard, and an experienced guy could redraw that factory schematic to something simpler by hand on the back of an envelope.

Other than ordering parts, there shouldn't be anything "land rover special" about it. The same rules and procedure for fixing a heater fan circuit in a Hyundai, Ford, GM, or any other make.

Test lamp, DMM should be all you need.