Just bought the drivetrain/wiring donor for my 85 900T.....

[chester_copperpot]

So as my introduction stated I just bought my first Saab in mid-August, an '85 900T with really screwed up wiring. It has been sitting since then as I decided to just wait to find a decent later model donor car instead of trying to fix a problem I knew would be a constant battle.

A good friend of mine recently decided to give up on his 88 SPG so I picked it up on the cheap. It quit running on him due to an apparent lack of fuel pressure. I found that the fuel control unit (LH Unit) was not grounding the Fuel Pump Relay. After a few nights of examining wiring diagrams and scratching my head, I decided to check for spark as well. Low and behold, there was no spark. My attention quickly turned to this and I got some non-Saab wiring diagrams. (which are awesome BTW.) Using those diagrams it was much easier to see that the blue pulse signal wire from the Ignition Pulse Amplifier is the only wire (aside from power from Fuse #22) that the fuel system shares with the ignition system. It goes directly to the ECU and APC. I tested and found that the Ignition Control Unit was not providing power to the Hall Effect Sensor in the Distributor. One quick swap later and the car runs great! It helped that I have the 85T sitting next to the garage to harvest parts from.

So turns out the ECU won't ground the Fuel Pump Relay without a pulse signal from the Ignition Module. Not that it matters with no spark.

This is some pretty basic stuff that I should have been able to fix faster, but I thought some might benefit if I share the experience.

[SwedeSport]

If that SPG is in good shape, you would probably get more bang for your buck doing a light restoration on that.

You can still do a t5 swap on the 85 using one of Jordans wire harnesses.

My 85 had the same crumbling harness. Faced with having to replace the whole entire wiring, it seemed like a no brainer to just t5 it.

[chester_copperpot]

I really like the way the 85 looks and the 88SPG is pretty rusty. Pretty much all the wiring on the car that is not contained in the passenger compartment is screwed, including what comes out of the fuse block. That is why I am planning to just swap everything. Seems like I would end up with a better final product if I take the time to reseal and refresh everything as well while I have it out.

[Cark]

My 85 had the same crumbling harness. Faced with having to replace the whole entire wiring, it seemed like a no brainer to just t5 it.

My same story exactly! T5 rocks

[SwedeSport]

Jordan's harnes is really nice.

Pretty much just plug and play, at least on the engine management side.

You will still need to source a later chassis harness with fuse box. The stuff inside the car usually survives.

[Crazyswede]

You will need to choose a crank pickup method for the timing if you have not done so already. Not sure what the current options are.

[RobVonSAAB]

What is the benefit of t5? Cars get towed into my shop all the time that have t5. You need two di cassetttes all the time because they suddenly fail. My 9-5dad doesnt leave my housd without a spare in the trunk. But the good old distributors and coils rarely ever fail on all my c900 cars. Ill take the old setup anyday for a street car. Much more servicable, easier to diagnose, and cheaper to run and maintain. Just my two cents.

[Crazyswede]

What is the benefit of t5? Cars get towed into my shop all the time that have t5. You need two di cassetttes all the time because they suddenly fail. My 9-5dad doesnt leave my housd without a spare in the trunk. But the good old distributors and coils rarely ever fail on all my c900 cars. Ill take the old setup anyday for a street car. Much more servicable, easier to diagnose, and cheaper to run and maintain. Just my two cents.

The advantage is that it is a much newer engine and ignition management system that offers greater performance and tuning than any of the older systems. The system was hacked a few years ago and the hackers created a program called T5 suite...which I believe is free and allows you to map your computer with any of the fuel maps that saab created...also allows you to create your own tuned maps. The system controls both fuel delivery and ignition and without a doubt does it better than the LH systems on the c900. Arguably we could say that carbs were better than CIS, CIS better than LH, LH better than T5 etc. However the reality is that though each predecessor was simpler and could be tuned reasonably well they were less efficient than their following systems.

Sure T5 has its drawbacks but the reality is that those DI cassettes don't fail nonstop (maybe the remans). My red pack lasted 200,000 miles before it failed. The 3 main components to worry about on the later cars are the fuel pump, the crank position sensor, and the DI cassette. All things I don't worry much about until high mileage. Keeping a spare cassette in the car is pretty easy to do..I have one from a junkyard.

[Jordan]

There is nothing inherently wrong with LH/EZK/Hall Effect/APC systems, but they are far less sophisticated, harder to find new parts for (I'm sure used market will be saturated for a while) and if you had to go through the trouble of replacing a complete system (or 3 in the case of LH), T5 is simpler.

The DI cassettes have gone through several recalls and revisions since their initial release and have become much more reliable. Even aftermarket ones are proving to be reliable at a fraction of the cost of OE. They also tend to go bad by sputtering under load or going into limp mode, not just suddenly failing.

Specific benefits are:
- better fuel economy
- tune-ability for modifications
- better drive-ability (all around performance)
- Fewer components in engine bay
- Instant diagnostics
- One system instead of three systems operating independently
- better adaptability for aging components (fuel pump, injectors, sensors)

Also, 9-5 is Trionic 7, which is much more complex with the additional Air Mass meter, drive-by-wire etc.

Again, if things are all working on the stock systems, I'd say leave it. But if a major component goes bad that is NLA or it has to be re-wired due to failing wiring, it's a no-brainer.