glow plug control unit problem

[topscalcookie]

hello everyone

My car has suddenly packed in. The glow plug control unit has stopped working when the 6 pin outgoing plug to heaters is plugged in. yet when heaters are unplugged the coil pulls in the contactor for a the set amount of time and then disconnects and sends a light to the dash which is what its supposed to do.
when plugs are connected in the middle of the timeout, the coil just disconnects straight away.
new unit has been priced at the main stealers at £120 odd and i cannot find a 2nd hand unit locally.
its a lot of money to spend to find out its actually the plugs.
that said the 6 plugs seem to work when given a direct feed.
i am very puzzled please help

 

[Bolide]

Have you measured the resistance of each glowplug with a meter? It's easy enough to do via the plug

I'd hazard a guess that one is showing a dead short and so the relay is disabling the glowplugs. Either that or the relay is faulty

A glowplug is the world's simplest thing - it's a resistor. It's either the right resistance (IIRC about 0.9 Ohm) or it's not

Nick Froome
www.w124.co.uk

 

[topscalcookie]

cheers for reply
i did test the resistance between the 6 pin plug and the head and they all gave the same result of about 1.6 Ohm.
i thought it would carry on working with the other plugs and send a signal to the dash to say one was down

 

[Hibbo]

Have you got a healthy battery?

If your battery is a bit weak, the volts drop caused by the load of the glowplugs could maybe cause the relay to release.

 

[topscalcookie]

hello hibbo
the battery was quite healthy and normally goes flat straight away, but did'nt this time, which is how i knew the glow plugs were not working.
that and the dash light not coming on

 

[Hibbo]

Have you measured the current each plug draws when fed directly?

 

[topscalcookie]

not yet mate thats what i'll do next, but i've got to charge the battery back up.
do you not think this could be a control problem

 

[Hibbo]

I'm just trying to think of what would make the control box shut off.

There are current sensing bits in there, that's how it puts the light back on when a plug goes open circuit or high resistance. It MAY shut off if it detects too much current flowing. Although the plugs may read 1.6ohm, this may drop when they've got 12v across them. Not sure how (although Duraterms do have a control coil in them) but just an idea.

If you can get at them, try disconnecting the plugs one at a time and trying it again (don't forget to put some tape over the free lead! )

Where on the Wirral are you? Up the Rovers!

 

[topscalcookie]

thanks hibbo
all ideas are much appreciated, but if the plugs are coming out, the car is going to a better mechanic than me or at least someone with a better toolkit.

ps i'm on the woodchurch, 5 miles from the rovers

 

[Hibbo]

The Woody eh? I went to school there! (and was a season ticket holder at the Rovers, Halcyon days!)

At this point you don't need to take the plugs out, just disconnect them.

Changing them isn't difficult, all you need is a 12mm socket (and maybe a 12mm spanner) and an 8mm one for the nuts that hold the leads on. Easy!

 

[topscalcookie]

still in need of advice

hello everyone
please help
i got the new control unit from the stealer's (pocket still hurts) and the same problem is still happening.
i am wondering if the control feed to the coil (in the unit) from the ignition comes via the overvoltage relay?
if not, what are wiring details to this unit,s 5 pin plug

T
LA
15
31
50

one must be a feed to the coil. the 2nd maby a seperate earth,then a loop out to the temp probe and back and finally a signal to the dash but which is which???

please help as this old banger has been the best car i've ever had. and i dont half miss driving her

 

[Hibbo]

Get a haynesy mate. I know they're a bit hit and miss, but the wiring diagrams are pretty handy.

If you can't get the info you need, do some probing with your voltmeter; it shouldn't be too difficult to work out what's what. From memory there is 1) Switched 12v, 2) Line from lamp (switched to earth) 3) 12v from starter 4) earth 5) I'd guess some type of temp line.

 

[wireman]

Is the big fat power cable that supplies the 12v from the battery in good order?

If not the supply will fail to the GP control relay and give the fault you seem to have.

Check the voltage from chassis to the stud on the 12v input to the relay and fire up the glowplugs, if the volts drop appreciably then you have a faulty cable or most likely corrosion inside one of the crimped lugs on the cable.

PS
Old fashioned analogue meters show up transient changes better that those there new fangled digital things and the battery being flat won't stop it working as a volt or ammeter.

PPS
You can get a more accurate measure of the glowplug resistance by powering it via a big (headlamp bulb) measuring the current drawn and then measure the volts over the glowplug, divide the volts by the amps and theres your glowplug resistance (at whatever current was measured).
Being metalic the GP resistance is most likly to rise with temperature.

 

[Parrot of Doom]

I've got a control unit sat here doing nothing.

3 of the fuseable links have blown, but otherwise I believe its in good working order (it came out of mine).

The back of the PCB has a number 72091::LL

The back of the plastic case - PA66F30


If you want it, make me an offer. I can't say for certain if its compatible with your engine. If it has faults other than the blown links, you can of course have your money back


/edit: just noticed you have a new one. Oh well

 

[topscalcookie]

thanks for the reply's

got a haynesy but it's got no diagrams for this circuit

cannot get a voltage or current at the GP's as the control unit disconnects as soon as the GP's are connected

mad bizzy at work. but if i get chance this week i gonna test the five pin plug to see which is the feed via the ignition. then i'm gonna put a direct feed from the battery to that pin and hope i dont blow the pcb.

i'll let you know the outcome

 

[Hibbo]

the w124 Haynesy must be crap , my w123 one shows the exact pinouts and internal wiring of the GP relay.

All I can think of is that there is a short on the main harness between a GP feed wire and the wire from the starter to the relay, thus when 12v goes to the plugs it also goes to the pin on the relay that senses when the engine is started and so shuts them off. V easy to check with an ohmeter.

Although if it has afterglow they should stay on anyway.

PS. It's replies not reply's - what's the apostrophe for?

 

[topscalcookie]

nice one hibbo
you got me. no one can spell round our way
The haynesy for 124 is rubbish, but that sounds like like a plan. Hopefully i can have a look at that this week or weekend

 

[topscalcookie]

please help

just had glow plugs changed now but the problem has not gone away.
the relay clicks on and off straight away.

 

[wireman]

THe possibillity of a shorted plug would give the symptoms you describe but before you start taking it to bits make certain that you are getting 12v at the fat terminal lug on the relay when the plugs are active. I have often seen crimp connections that look fine but have an inner layer of corrosion that will have a fairly high resistance, the fault you describe suggests to me that there is a loss of sufficient current to hold the relay input voltage above that required for normal operation. Inside the relay there is an input contact for each plug which is powered via a very low value resistor from the battery input terminal. There are electronic comparators that sense the voltage dropped by the low value r's which signal that all is well or not well with the glow plug current draw to some electronic logic thingy that controls the relays operating coil and the dash board light. The logic bit also has an associated temperature sensor in the engine to determine how long to leave the lamp on at switch on, it does not switch off the plugs untill some 30 seconds or so pass or the starter is engaged.
If the comparator section signals low current in one or more plugs the logic turns the lamp back on after starting to indicate the presence of a faulty plug.
To check my cable crimp hypothisis use a voltmeter between earth/ground/frame (call it what you will but the engines metal work is just fine) and the threaded stud where the big fat power cable is connected.
Be sure to prod onto the stud and not the lug on the cable or you wont be measuring the volts at the relay just the volts at the cable (they should be the same but if its the cable at fault they wont be)
With the key off there should be 12v at the relay with the key on the 12v should remain ,it may sag by a volt or so due to the large current draw of the pre heater system but should definately not fall by more than the odd volt, if it does there is a high resistance connection between the battery and relay.
You should be able to hear the relay operate, listen closely to the relay while an assistant turns on the key it should click and click again if the key is turned off, the relay will also click on its own if you let it time out for 30s or so.
Poor crimp connections on battery earth cables (at the body end) was the most common fail to start fault on 1980's vauxhalls I ever found.

 

[topscalcookie]

here goes bear with me:???:

thanks wireman
i'm going to put a new feed direct from battery to prong on relay with new crimps and hope its that, but i think the problem is in the logic bit of the relay.
the light on dash is being a bit random now as its not lighting for the timeout but lighting up after the timeout. this is happening when the six pin plug is disconnected.
when six pin plug is connected no light on dash and the relay clicks in then instantly clicks out and no light after the timeout