TR cold-start injectors - no power...?

Hello to All: My 87 TR isn't getting any voltage to the cold start injectors when the engine is cranking over. The fuses are good, and I swapped around some of the relays. But the problem persists. I looked for the obvious loose connections, but all appears good. I am not sure where to start looking next. I believe there are other devices that must function properly before the cold-start injectors spray, but not sure what. I thought about jumping 12 volts to them just to see if they work, but thought I might damage something. So I'm posting here in the hopes that some of you can guide me along in diagnosing this problem. Thank You for any help you offer.

 

That's an understatement! Sounds like you've already tried another ...113 relay in the "U" relay position, but another thing to try is reseating the round C12 and C13 connectors at the black triangular box (one of the lines from the U relay to the Thermo-time switch passes thru them as does the line going to the cold start injectors).

If you need to dig deeper, try these tests (note "engine cold" means the engine is at room temperature and you haven't done any starter motor cranking recently so that the thermo-time switch has had a chance to cool off):

1. Engine hot or cold -- unplug the U relay and measure the voltage from the 30 terminal position of the U relay socket to the battery (or chassis) ground -- it should always go to +12V when the starter motor is actually cranking (this confirms +12V power is available during starter cranking to run relay U and the cold start injectors).

2. Engine must be cold, key "off" -- unplug the "j" white connector and measure the DC resistance from the RV (red/green) wire terminal in the unplugged "j" connector (3rd position from the right) to the battery (or chassis) ground -- should be a low resistance (a few ohms maximum) which would confirm that the thermo-time switch is properly closed when the engine is cold.

3. Now with everything plugged back in and the engine cold -- measure the voltage between that same RV (red/green) wire terminal in the "j" connector and battery (or chassis) ground. It should be a low voltage during the first second or two of starter motor cranking and then go to +12V when the thermo-time switch opens (and the starter motor is still cranking).

4. Everything plugged in and the engine cold -- measure the voltage between the C (orange) wire in the "j" connector (6th position from left) and the battery (or chassis) ground. This is the line that (eventually) actually runs the cold start injectors so you should measure +12V for the first second or two of starter cranking and then a low voltage after the thermo-time switch opens (if things were working).

Try those tests and let us know what you find -- should narrow down the suspects.
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Is anyone else simply amazed at the detailed, and logically laid out answers provided by Steve? I mean, he gives you step by step diagnostics that even a fool like me could follow. Man, I just wish he would buy a 355!

 

Thanks for the kind words Dave -- although I've not had specific trouble in that same area, I've studied the Fig 3 schematic in the TR WD books enough to last a freakin' lifetime

One thing that makes it really tough on a TR to chase electrical problems is the limited number of areas where you can actually measure something when it's plugged-in because (without the special factory "interconnection box" like Hank's) the connectors are too well sealed -- at those white connectors and inside the triangular black box are about it for places to measure stuff. I'm sure your 355 has the same measurement difficulties, but I think the raw size and complexity of the electrical network around the Motronic is smaller than the KE Jet (so less probablity of a problem) and the Motronics are smarter (at least it can tell you what it thinks is wrong via the on-board diagnostics).

Haven't thought much about what's next lately...355?...hmmmm?...unlikely for now ...

 

Thank You Steve....kinda speachless at the moment after reading your post. I'm just sitting here shaking my head in amazement. I totally agree with what Dave says about you....!

I will try and follow your advice tonight. I'm very shaky when it comes to wiring, let alone Italian Automotive wiring! I recently replaced the temp sensor, thermo-time switch, and both O2s and was hoping that somehow this problem would go away...but oh well...I will post my findings..l

Thank You Again Steve !!!

 

No trouble Jeff. When I first read your post I thought to myself:

60% chance it's just a flaky connection problem somewhere,
20% chance it's a component failure, and
20% chance it's a problem in the PCBA on the +12V power side of things.

That's still my best guess, but those tests should confirm/deny which area needs the help.

PS Just to be clear, in test #2 the resistance measurement is made at the red-green wire terminal in the unplugged "j" connector half with the wires (not the unplugged "j" connector half on the PCBA).

 

Well it's getting interesting now. Steve...I was gonna do your tests, but first thought I'd verify that each relay was the correct type. My owner's manual states that the relays for both fuel pumps should be the "113" type. But whats plugged into the board is "101". When I attempted to plug a "113" into the pump the fuse blew immediately. The same thing for both pumps! Could the owner's manual be wrong? I'm afraid to go any further until I understand this situation.