Volvo 740 hall sensor hell - plug replacement?

ToomanyVolvos

One of the first 3-4 Volvo's that's started off the TooManyVolvos was a much loved 87 740T, acquired with about 100K and now up to 138 after 13 years of ownership. Never ever missed a beat.... Then just a day or two back stopped randomly 3 times on a 10KLM trip. Replaced the Fuel Pump Relay just because I keep a new one in the glovebox. Next drive same thing, only prob was the was a Sydney to Little Hartley trip and stopped every few KLMS. The Tachometer would read Zero during failures and attempts to restart: based on previous experience a clear hall sensor failure.

So the 87 among the above mess got a Brand New genuine distributor I had in stock and ran nicely for a minute and same sequence, not fixed. NOT happy Jan, a brand new distributor wasted.

I'd read about the plug disintegration in the distributor but that plug looked fine. After much agony I investigated the wires coming from the harness;

Looks like my likely cause of issues? (Seeking opinions on this). I still don't understand how this could all of a sudden cause 50 odd engine stoppages in 100KLMS and not have ever caused an issue once in 13 years of ownership, could it be something else?

The replacement of the receptacle plug in the distributor is widely discussed and available. I've not seen this plug available or discussed, anyone had this issue and a solution?

VolvoHordz

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 worse as soon as your disturb it and more insulation falls off. Id suggest just cutting it back to a point where you find good wiring and wack a new plug/wiring salvaged from somewhere else. Alternatively the plugs are likely to be universal enough to buy generic new to be crimped onto new wiring.

ToomanyVolvos

Once seen I thought this had to be it...? But I've done over 30K in this car and never stopped once, then she demanded this issue be fixed yesterday. This was likely in this condition since I owned it so it's hard to understand why one particular calendar day she said enough is enough and threw continuous failures at me.

Vee_Que

Failing engine mount could accelerate wear.

I've replaced the plug on my 740t, efi hardware sell the plug, do not solder them and the dizzy side can only be replaced with the sensor, you could hard wire it.

ToomanyVolvos

No need to mess with the dizzy side, the entire unit is brand new: sensor, plugs dizzy etc.

Just need a fresh plug with wires fitted to take back to good wiring. Is this what EFI hardware are able to provide? My plug is actually in perfect condition, it's only the wires coming from it that are just fragments of what was once..... Maybe a good auto electrician can be given the plug and refit wiring from it to suit.

SA940

Wow, I'm blown away by where you live.

carnut222

SA940;c-145787 wroteWow, I'm blown away by where you live.

I’ll second that - would like to see some more pics and info on your house @ToomanyVolvos as we’re interested in architecture too (maybe a common theme there - Volvos - safe as houses. Volvos - boxy but good? LOL!)

ToomanyVolvos

Will do, I'll do a bit of thread divergence with some photos of random Volvo's across the property when I'm there next.

There's a straw bale house which is cool (photos to come). Sadly I'm in Sydney most of the time and boss is already going nuts about the dozen cars there so I have to place them in various other places.

carnut222

ToomanyVolvos;c-145829 wroteWill do, I'll do a bit of thread divergence with some photos of random Volvo's across the property when I'm there next.

There's a straw bale house which is cool (photos to come). Sadly I'm in Sydney most of the time and boss is already going nuts about the dozen cars there so I have to place them in various other places.

Great. We live in a straw bale house...what a coincidence!

Vee_Que

You can get new pins from efi hardware.

https://www.efihardware.com/products/349/connector-distributor-3-pin-plug you will find it easier to just fit the new plug too.

ToomanyVolvos

@Vee_Que thanks for the link!

I rewired the plug (using the existing wires coming out of the plug) up to the point of good wiring and ran ok for 30 mins and then failed again. Engine stopped about 30 times on the way to work and made me very late.

Last night I swapped in another "541" ECU and still got the same issue (strangely she idles at 950 with the replacement ECU, not 700ish).

Inspected my rewire work and looks ok, so either it's bad rewire work I've done or something else. Any hints on what else could cause intermittent engine stoppages with Zero tachometer readings during stoppages?

Thinking I could use a new plug to distributor and new wiring all the way back to the ECU if I knew what wires to splice into. Or is it the LH2.2 EZK unit that should be swapped out for rpm sensor issues?

Couple more photos from Volvo heaven, incl shot of the boneyard.

jamesinc

ToomanyVolvos;d-9495 wroteI still don't understand how this could all of a sudden cause 50 odd engine stoppages in 100KLMS and not have ever caused an issue once in 13 years of ownership

Well initially it was working, but then it failed

ToomanyVolvos

The joy of mid 80's wiring. The wires into the radio suppression relay are rooted. Does the hall sensor circuit go via this relay? Something @Major Ledfoot would know for sure.

jamesinc

ramrod

Injectors are turned on via that relay

ramrod

While you're there, look inside the plug where those wires terminate, they often slip down when the plastic sleeves crumble away and you no longer have a connection

ToomanyVolvos

I've experienced issues with this connection on another car causing a no start. On this occasion I've got a definite loss of tachometer readings at each time of failure, will a fault on these connections / the relay cause a loss of tachometer readings simultaneously?

Tried to separate the wires and she's still stopping randomly every few mins, looks like the 142 is the work car today.

Vee_Que

I cut back to good wires near the strut tower, and put a length of shielded wiring in from there to the sensor.

ToomanyVolvos

Vee_Que;c-147317 wroteI cut back to good wires near the strut tower, and put a length of shielded wiring in from there to the sensor.

That's what I did after replacing the Distributor didn't fix it, that didn't fix it either. Next I'll swap out the EZK unit, not sure what else to do.

ToomanyVolvos

Ok, so my FAV 740T has been on the street pretty much dead for 3 months now. Need knowledge on how to get to the bottom of this.

So plug replaced, fresh wiring right up to where the wires enter the larger section of the harness near the firewall. Ran without fault for 10 mins, then intermittent stop followed by completely dead shortly after.

Anyone know how to test the output from the hall sensor wiring with a voltmeter?

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?

Next step is to start paying an auto electrician but the two local ones to me are no good - lack of quality work and sense of entitlement to get overpaid.