repair manual for 1990 Testarossa

There really isn't one, overall, comprehensive, easy-to-understand manual for a TR (as Brian already indicated in your other thread asking this same exact question). The best you can do is these 5 items:

1. Owners Manual for your particular year & version TR

2. TR Work Shop Manual 330/84 -- but if you have a US or CH or CAT version (i.e., KE-Jet with Lambda) make sure it has pages D68-D105 included. The TR WSM CD available on EBay doesn't have this section -- i.e., pages D1-D67 only covers early euro TR using K-Jet without Lambda system. For the later euro TR, using KE-Jet without Lambda, they did a seperate booklet, but it's really just a subset of pages D68-D105 (and behaves like a KE-Jet with Lambda when the O2 sensors are unplugged) so either will suffice.

3. For electrically troubleshooting the TR Ignition and Injection Systems, F did a series of "Diagnosis Sheets" -- Diagnosis Sheet N. 1 covers all TR ignitions (MED 120B and MED 120B/D); Diagnosis Sheet N. 5 is for KE-Jet without Lambda; Diagnosis Sheet N. 6 is for KE-Jet with Lambda; K-Jet without Lambda doesn't have electrics involved so no Diagnosis Sheet.

4. TR Spare Parts Catalogue -- on-line copies are viewable at www.eurospares.co.uk or www.RicambiAmerica.com if you don't want to buy a hardcopy.

5. The Wiring Diagram Book for your particular year & version -- the wiring diagrams included in the TR WSM are only for the early euro TR (K-Jet without Lambda). For a US, or late euro, TR there are separate Wiring Diagram Books.

As you can see, the best technical documentation collection for you is dependent on what year & version TR you have, and, as Brian said, they really assume that you already are mechanically-trained/inclined and have good familiarity with other prior F models. Additionally, let me add that details for the coachwork (like taking apart the center console) are almost non-existant.

Since your TR is a 1990, it most likely will either be a US or late euro version. If you don't know the version, give the engine family number shown on the top of the engine block at cyl #6, and I'll decode for you (it will be something like F113A or F113A040 or F113A046 or F113B).

 

Your earlier thread is here:

http://ferrarichat.com/forum/showthread.php?t=224236

F113A040 is a US version TR -- and, for a later one like yours, you'd need the 509/88 wiring diagram (that looks like this):
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These guys:

http://www.ferraribooks.com/index.php?main_page=index&cPath=248_300

might be a source for you -- although on their website they only presently show having the 457/86 wiring diagram for the earlier US version, and they show having Diag Sh. N. 1 and N. 5, but not N. 6 -- but it wouldn't hurt to let them (or RicambiAmerica or T. Rutlands, etc.) know what you are looking for IMO (and, unless the price is unreasonable, an F dealer is just as good ). I've also used www.ferrariliterature.com for some stuff, and Maranello Concessionaires had some great prices on F Lit back when they were doing the F classic parts (but I don't know if that's changed).

PS I believe there are a couple of errors on Diag Sh. N. 6 -- since it's fairly small size-wise, send me your email address in a PM and I'll email you scans of Diag Sh N. 6 (with my corrections).