How to Replace / Repair the Heater Matrix on MX5 Miata Eunos Mk1 (NA) & Mk2 (NB)
In this repair article you will be shown how to replace a leaking (or blocked/inefficient) Heater Matrix on your Mk1 or Mk2 Mazda MX5 / Eunos / Miata, without the need to remove the dashboard!There are a couple of differences between the heater matrix fitted to the Mk1 (NA) and the Mk1 (NB). The Mk1 version used copper pipework that is attached to the matrix using two short lengths of rubber tubing, one end connects to the copper pipe, the other connects to the outlet of the heater matrix. The pipes are usually secured with spring clips. Try to avoid the temptation to replace these spring clips with jubilee clips as the former will expand with the heat and provide a constant securing pressure around the clip.
The second version as fitted to the Mk2 uses an O-ring type seal that inserts into the heater matrix and secured with two special screw clamps. Ensure the O-ring is in good condition and correctly located on the pipe before inserting, as shown in the picture below.
Step 1 - Drain the Coolant
Drain the Coolant from the cooling system. Collect in a suitable contain and re-use if still within its service life, otherwise use this opportunity to flush and replace with the correct mixture of water and antifreeze.Step 2 - Disconnect the Battery
To prevent short circuit, or accidental deployment of air bags, remove the Negative Battery Terminal and then press the horn to remove any residual power from the system.Step 3 - Remove Driver's Seat
We will be working up under the dashboard and so removing the Driver's Seat will give a bit more room to work comfortably. Slide the seat forward until the rear seat bolts are accessible and remove the two bolts with a 15 mm socket wrench. Slide the seat back and remove the two front bolts and carefully remove the seat from the car. Now collect all the bonus cash from under the seat!Step 4 - Remove Inspection Panel
Located immediately below the steering column is the Dashboard Inspection Panel that must be removed. Undo the two cross headed securing screws and pull the bottom of the panel outwards to release it from the two upper clips.
Step 5 - Remove Heater Matrix Cover
Looking through the inspection panel, up towards the center of the car is a square shaped cover with an electrical plug attached to its side. This is the Heater Matrix Cover and needs to be removed.
There is a Heater Control Rod that is in the way, as shown in the picture below. Set the heater control to Cold and then prize open the bottom white clip to remove the bottom of the control rod, which can now be moved to the side, out of our way.
Remove the two screws holding the cover in place and withdraw the cover enough to be able to reach behind it and release the wiring plug from the cover. The cover can now be removed, exposing the heater matrix and pipes, as shown below.
Step 6 - Remove Heater Matrix
Follow the pipes towards the top of the heater and remove the clamp holding the pipes to the heater module by removing the screw. This will give our tubes a bit of free play and aid in removal and refitting.
Loosen the two screw or spring clips that attach the pipes to the heater matrix and slide along the pipes to keep them out of our way. Pull the two pipes upwards to remove them from the matrix, taking care not to damage crush the fragile pipes. Also ensure that both O-rings are still located on the end of the pipes on later models. The heater matrix can now be withdraw sideways and removed from the heater module.
WARNING - There WILL be water! Ensure you have a suitable cloth located under the matrix to catch excess water once the pipes are removed. When removing the matrix, keep the holes in it as the highest part to avoid spillage as it will be full of water, don't pour it into the foot well or over yourself!
Step 7 - Inspection and Repair
The heater matrix itself can now be inspected, flushed, and replaced if necessary. As can be seen in the picture below, my own matrix is heavily corroded on one side and the most like location of the leak. Replacement in my case is via a used unit purchased from eBay.
Step 8 - Partial Reassembly
Place the heater matrix back into the slot in the heater module and securely reconnect the pipes using the spring clips on earlier versions, and the screw clamps on later versions, ensuring the O-rings are correctly located on the ends of the pipes before inserting into the matrix body. Replace the clamp that held the pipes to the heater unit using the fix screw.Step 9 - Test for Leaks
Refill the coolant system using the correct ratio of water and antifreeze. Reconnect the negative batter terminal and start the car, allowing it to reach operating temperature, checking for leaks at the matrix and pipe unions. Assuming no leaks are detected, turn off the engine and allow to cool. Disconnect the battery again.Step 10 - Final Reassembly
Now we know we are leak free, replace the heater matrix cover with the two screws, reattach the wiring block to the side of the cover, and reconnect the heater control rod. Ensure the control rod operates correctly to its full range by operating the temperature control from cold to hot on the heater controls.Replace the inspection panel by slotting the top into the retaining clips and then fastening the bottom with the two screws. Refit the driver's seat using the four bolts. Reconnect the Negative battery terminal. Finally, check the coolant level and top up where required. Total job time, around two hours.
Did you find this article useful? Have some tips on how to replace a heater matrix? Let us know in the comments below.