Volvo 740 hall sensor hell - plug replacement?

ToomanyVolvos · 2018-09-15T09:53:26+00:00

That'd certainly do it. Age and heat is the wiring's enemy for Volvo's of that era. Insulation gradually breaks down and eventually the wires short. Made wor...

Major Ledfoot

ToomanyVolvos;151324 wroteAnyone know how to test the output from the hall sensor wiring with a voltmeter?

The sensor signal feeds the EZ-K at pin 24 via the blue wire and is probably best seen with an oscilloscope. Pin 10 is a ground and shield (black wire), and pin 4 is +12V (red wire).

I'm thinking if I've got correct output on this wiring I'd like to extend that through the firewall right up to the computer: anyone know whether this signal goes to the EZK or ECU?

For LH 2.2 wiring, see the Green Book TP30158/1 - pages 16 & 17 of the manual for the ECU wiring; and pages 32 & 33 of the manual for the EZ-117K wiring.

ToomanyVolvos

@bgpzfm142 just the man I was hoping would chime in with these GEM's, another bottle of red owed.

Can I just provide the sensor a new earth and 12V supply (to rule out bad circuitry) and just feed a new blue wire to the blue wire at the back of the EZK unit? (Or does the EZK need the full 12V loop?). Just for temporary fault elimination of course.

I guess the moral of the story is the 88/89 Turbos have the cool styling without the completely rooted wiring (although where I've cut back to seems perfect, much of the fender harnesses are toast).

Major Ledfoot

ToomanyVolvos;151469 wroteCan I just provide the sensor a new earth and 12V supply (to rule out bad circuitry) and just feed a new blue wire to the blue wire at the back of the EZK unit?

You could, however the blue wire (according to the diagram) is shielded, which suggests to me that the impulses from the dizzy being put on it are quite low level signals and subject to impulse interference.

I should correct myself and say the voltage at pin 4 may not be 12V, but that pin 4 voltage is clearly the positive power feed for the Hall sensor in the dizzy.

The distributor wiring might be okay now, so that former fault may not be the only wiring fault.

In your place, I'd be grabbing a test light plus a multimeter, and going through all the connections where a loss of continuity equates to engine shutdown. The Green Book diagrams are awesome for this, since they show all the terminal numbers, the connection points, and the locations of the plugs.

ToomanyVolvos

I'm 100% certain the engine shut down is associated with zero tachometer readings. Exactly the same as another 740 which new Dizzy/ hall sensor fixed. Is there anything else that could cause a zero tachometer reading and engine shutdown?

At this stage I'm feeling it could be:
- Small area of damaged wiring I haven't found
- Bad EZK unit
- Fault in new Dizzy / Hall sensor (least likely of course).

Lupica told me the output from the sensor is milliamps hence the earth shielding, not sure how to use the multimeter to test for tiny signals but I'll give it a go Christmas Eve. Of course where my new circuit joins I've not got no shielding, I've just rolled the shielding into a wire like join.

Of more stress now is whether my car's got bent in last night's weather, on the south coast ATM. Had two 740's exposed, a 240, two V50's, and a C70 only under a shade cloth ?

Major Ledfoot

You need spark and fuel for the thing to run. Do you have spark?

I grabbed one of these test lights gadgets from SCA, which connect in series to a spark plug, to go fault finding in Chloe the 145. Sometimes I did get spark; sometimes I didn't. After much teeth gnashing, it was found to be a damaged wire at the opto-coupler. But I preferred to use the test light than to open up the opto-coupler signal wire.

As for what could cause zero tacho and no start... whoa, that piece of string is very, very long, e.g. -

- faulty coil
- faulty connection to coil from power stage
- faulty power stage
- poor earth at power stage
- faulty connection from power stage to EZ-K
- poor earth at intake manifold
- poor earth at pin 3 at C connector on LH suspension tower
- poor earth at pin 20 of EZ-K

And so on.

Poor earths and damaged wiring generally cause more intermittent-type problems than failing components.

ToomanyVolvos

Holy shit Batman, I really thought based on prior experience that the dead tachometer / engine in tandem was a sure hall sensor / hall sensor circuit issue.

As for things like coil, obviously a dead engine but not imagined this would give a dead tachometer? I'll be looking at the other areas you've touched on.

My engine harness is updated and perfect, the fender harnesses are seriously ratty.

I haven't done the basic stuff like pulling a plug to check for spark because of the above is identical to my previous hall sensor issue.....in all likelihood I'll get no spark or fuel because the computer/s are not even getting the hall sensor signal?

@bgpzfm142 thanks again, I am super keen to have this brick rolling again so I can get some clean fuel in her and get her back under cover.

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