Tachometer stopped working. Then briefly okay again. Now NG.
Car running okay.

 

probably a loose connection. the tach gets its timing signal right off pin 1 of the ignition coil, at least on bosch ignition systems, not sure about regina. what year 740 ?

 

It's a 1990.

 

hmmi I only have wiring diagrams on 1991+ 7/9's.

is this a bosch LH2.4/EZK116k car, or is it a Regina/Rex system? Rex ignition systems have a square coil that looks like a transformer, with an integrated power module, while Bosch systems have a conventional round ignition coil, and a seperate power module.

on the 1991 Bosch non-turbo (at least), the tachometer signal is a red/white wire that comes from ignition coil pin 1 and goes through pin 27 of the bulkhead connector at the left A post (this is a BIG connector with like 100 wires). on the inside of that bulkhead connector, this red-white wire goes to pin 6 of instrument panel connector "A", which is an 8 pin 'in line' connector on the upper side of the back of the instrument panel, behind the tach.

on the regina/rex systems, this red-white wire comes off one of the coil's 3-pin connectors, its the only red-white wire there. the rest of the wiring is the same.

if you take a 12V LED test light, with the black wire clipped to ground, and probe various places on this red-white wire circuit, you should see blinking when the engine is idling, these are the timing pulses to the coil and tachometer.

 

Wow you are very knowledgeable. Thank you so much.

FYI my system is Bosch. I'm going to follow up asap.
I'm very proud of myself. Replaced the in-tank fuel pump today. Quite a process. Took three hours all in. My main pump was working super hard when cruising. I could hear it.
I'm a bit nervous though. I tested the pump I removed and it works.

Could these two power issues be related??

Doug

 

my professional expertise is computer systems and electronics, software and hardware. back in middle school (we called it Jr HS) in the 1960s, I learned to read schematics and understand current flow. I happen to have the CDrom sets of greenbooks for the 1991+ 940s' (which encompass 91/92 740) as well as the 240 series.

if your main pump is working extra hard, then your in-tank pump is probably dead. the less gas you have in the tank, the harder the main pump has to work when the in-tank transfer pump isn't working, if you get below 1/3rd or 1/4th tank, it can really struggle. a new in tank pump (replace the little bit of rubber hose, and the pickup sock thats in there while you're at it) and your main pump should go back to being whisper quiet and happy.

 

Thank you again!

Too late to change the rubber hose (it appeared OK) but I did install a new sock. The old one was retrieved from inside the tank. It fell off. Maybe this was part of the issue.

It was obvious that the pump I removed is not original due to wire splice.

Question. Is the in-tank pump powered up all the time or only on demand? Like; when I turn the key should I get juice to the red wire?

Doug

 

both pumps are powered together, when you turn the key on, they should run about 1 second and stop until the engine is cranked over, then they both come on again, and stop about 1 second after the engine stops moving (as determined by pulses fro the crankshaft position indicator).

the tank pump is on its own fuse while the main pump is not