MOVING THE PTU

The PTU (Power Transistor Unit)

It would seem, by the number of reports of problems with it, that the PTU is the "Achilles Heel" of this otherwise brilliant car.

What is it?

The PTU is a pack of six power transistors which behave as solid-state relays to switch 12v current to each coil pack under the control of signals from the ECU. It is the rectangular device bolted to the right hand cam belt cover. The wiring to the PTU runs across the front of the engine, then down the left hand side, where it joins the main ECU wiring loom. Several other harnesses are taken off the PTU harness bundle on the way back to the firewall.

What are the Problems?
The failure of the PTU is  reported increasingly  among Australian Z32 owners.
Problems with the PTU are not often reported in US cars, because Nissan USA acknowledged early on that there was a problem with the original (Mitsubishi) PTU and did a recall, replacing them with an uprated (Hitachi) unit. Any subsequent failures, if they have occurred, are not such an issue because the unit is much cheaper in the States. Collecting the experiences of various Australian owners the following points emerge:

• Failures can be intermittent - causing loss of ignition to one or more cylinders - or
• Failure can be complete and sudden, causing instant stalling - very dangerous in heavy traffic
• The component fails at 100,000-130,000 kms.
• Nissan did not apply its recall to Australian cars.
• The replacement cost is approx. $1100 ex Nissan Aus.
• The replacement which Nissan supplies is the Mitsubishi unit, NOT the upgraded unit.
• The replacement will fail again, often sooner than the original.
• Examination of the PTU reveals delamination of welded terminal contacts.
• Failure appears to be a combination of the effects of heat and vibration.

The placement of the unit on the engine is in itself a strange idea - The Z31 had its single PTU mounted on the left wheel arch beside the coil.

Is moving the PTU is such a good idea?

I'll play the Devil's Advocate for a moment and consider the case against moving it.

1. Nissan designed it that way for a reason (other than selling  PTU's!)
2. Fixing the PTU's position in relation to a vibrating wiring harness may eventually cause mechanical failure of the harness.
3. Will the PTU be in a "dead air" area where its own heat may not be dissipated effectively?
4. The cam belt case is plastic, which transmits engine heat and vibration less effectively than metal.
5. The air flow near the PTU is quite good - except that it's hot air from the radiator.
   

Having said this though, moving the PTU seems like a good option as it should reduce its exposure to heat and vibration. I decided to move mine because I have 135,000kms up on my Aus delivery 91 NA and so far no trouble. I put my good luck down to my car having spent all its life in Tasmania, where the ambient temperature is lower and there are very few traffic jams.

OK, so where to move it?

Despite the tight fit of the engine in its bay, there are several possible places to move to as there is plenty of cable length to play with.

• The Nose - behind the foglights/corner "lights". Access to here very poor. Possible splashes a problem?
• Beside the battery - Difficult to access. Need to separate a lot of the loom. Heat "pooling'?
• Behind the headlight near the relay box. - Good access, Easy loom conversion, but you need to make up a bracket to mount the PTU. Note: there may not be enough room in a TT to do this.
• I'm sure there are other options!

READ THIS BEFORE PROCEEDING!
If you do this conversion it is entirely at your own risk! I can't guarantee that it is safe for the long term health of the PTU or its wiring harness. If you have what you consider to be premature failure of the PTU or its harness please don't expect me to compensate you in any way as this is entirely experimental.

Behind-the-headlights seemed easiest and did not involve drilling holes in the car or doing anything not easily reversible. I also felt that I could more easily insulate the PTU from vibration if it was mounted on a bracket of its own, rather than on an engine bay component. The mounts used were the lower 6mm threaded bolt which secures the relay box, and the holes left after drilling out the pop rivets where the date-of-build plate was attached.

I made this bracket out of 40mm extruded aluminium section (left of picture) and an old PC drive mount bracket. A small L bracket was attached as an outrigger to the screw holding the relay box.
I drilled out the rivets holding the date-of-build plaque, and used these holes to secure the bracket. The final effect was to construct a tray suspended just over the left air intake trunk, beside the relay box. The essential features identified on the image are:

1. The holes which correspond to each end of the date-of-build plaque.
2. The hole for the lower relay box bolt.
3. I bolted an L-shaped bracket to the main bracket to reach the relay box bolt. This allows flexibility in placing the bracket.
4. The bolt holes for mounting the PTU.