Hot/cold start issue - 10.8V on black cable to solenoid
Discussion
Had the hot start issue for years but very rare. Has now become a cold start issue but every time it happens I haven't had the car somewhere where I could investigate, until recently....
Had the car on the ramps whilst removing/replacing the steering rack for an overhaul and it wouldn't start so got underneath with the voltmeter.
Had a good 12V on the red thick supply cable to the starter motor but only 10.8V on the thinner black wire to the solenoid.
The volts on the black wire could be seen to switch between 0V and 10.8V as the key was turned / released so that rules out the starter switch and the relay not operating in the immobiliser.
It seems that this voltage is about the limit of what will operate the solenoid and kick in the starter motor. As later when it rose to around 10.9V, the solenoid operated correctly.
So the problem appears to be volt drop in the supply to the solenoid, rather than the supply to the starter itself.
I tried tracing back the black wire by hand and it goes up over the solenoid and into a black flexi conduit but then I couldn't tell where it went from there.
The black cable is pretty weedy and is beginning to feel somewhat brittle si I want to replace it but I want to measure the volts back along this cable and find out where teh 1.2V is being dropped. If I can cure that, that should resolve the problem.
I was tempted to just whack in a fused relay under the car near the starter, fed from the larger red cable. It would be pretty easy to do and would work but it's just putting off the issue.
Does anyone know where it originates from? Presumably under the dash, maybe from the immobiliser ?
Had the car on the ramps whilst removing/replacing the steering rack for an overhaul and it wouldn't start so got underneath with the voltmeter.
Had a good 12V on the red thick supply cable to the starter motor but only 10.8V on the thinner black wire to the solenoid.
The volts on the black wire could be seen to switch between 0V and 10.8V as the key was turned / released so that rules out the starter switch and the relay not operating in the immobiliser.
It seems that this voltage is about the limit of what will operate the solenoid and kick in the starter motor. As later when it rose to around 10.9V, the solenoid operated correctly.
So the problem appears to be volt drop in the supply to the solenoid, rather than the supply to the starter itself.
I tried tracing back the black wire by hand and it goes up over the solenoid and into a black flexi conduit but then I couldn't tell where it went from there.
The black cable is pretty weedy and is beginning to feel somewhat brittle si I want to replace it but I want to measure the volts back along this cable and find out where teh 1.2V is being dropped. If I can cure that, that should resolve the problem.
I was tempted to just whack in a fused relay under the car near the starter, fed from the larger red cable. It would be pretty easy to do and would work but it's just putting off the issue.
Does anyone know where it originates from? Presumably under the dash, maybe from the immobiliser ?
Are you saying that the 10.8v your seeing is under load or just loose? because if that is unloaded I bet you thats dropping below 8v with the solenoid load I would be checking the terminal block just inside the car around the radio cavity area, your also quite right that just wont do 
Sardonicus said:
Are you saying that the 10.8v your seeing is under load or just loose? because if that is unloaded I bet you thats dropping below 8v with the solenoid load I would be checking the terminal block just inside the car around the radio cavity area, your also quite right that just wont do
Hi Sardonicus,It is off-load, i.e. lead disconnected, so you're right, it will undoubtedly drop lower on load.
So if I remove the radio there is a terminal block in there that this black wire will terminate at ?
David Beer said:
how old is the car? After around 96 you can pick up the solenoid wire in the passengers footwell. Your idea of a relay is good, but I would site the relay rear of the alternator.
It's a 1997. When you say rear of the alternator, you mean up in the engine bay to give some weather protection?The black start wire (which as you say could do with being a thicker gauge) goes to a connector behind the dash and from there to the immobiliser. The connector is 7 way and brown with white cable retaining clips.
The most likely source of the volt drop will be the contacts in the immobiliser.
Steve
The most likely source of the volt drop will be the contacts in the immobiliser.
Steve
Steve_D said:
The black start wire (which as you say could do with being a thicker gauge) goes to a connector behind the dash and from there to the immobiliser. The connector is 7 way and brown with white cable retaining clips.
The most likely source of the volt drop will be the contacts in the immobiliser.
Steve
Hi Steve,The most likely source of the volt drop will be the contacts in the immobiliser.
Steve
Thanks for that. Is the connector under the main dash top or behind the radio panel please ?
Would be good to temporarily attach a few wires frmo various points around the circuit so that when it next fails to start I can trace the volt drop.
Interesting on the immobiliser relay. I had thought that the relay issue was that the relay didn't pull in properly but could be more like it does operate but the contacts are degraded.
The base problem is the starter location so close to the exhaust. This dries out the lubrication in the solenoid and pinion mechanism which in turn takes more current to operate.
TVR used the wrong circuit in the immobiliser which means it is using the lower amperage circuit for the start and the high for the fuel pump (which has its own relay anyway).
Net result is the contacts in the immobiliser burn. Putting in a relay kit may help but the contacts are now burnt so may still be unreliable and in particular as they will be drawing a lower current that may have issues getting through the burnt contacts.
You will most likely find the connector buried above the tunnel on the nearside but below the area you see when you take off the dash top. You may not be able to get to it with the lower dash in place. You may be able to reach it through the radio slot but I have not tried it myself. You may also be able to reach it by following the cable branch from the engine loom just after it comes through the front bulkhead.
Steve
TVR used the wrong circuit in the immobiliser which means it is using the lower amperage circuit for the start and the high for the fuel pump (which has its own relay anyway).
Net result is the contacts in the immobiliser burn. Putting in a relay kit may help but the contacts are now burnt so may still be unreliable and in particular as they will be drawing a lower current that may have issues getting through the burnt contacts.
You will most likely find the connector buried above the tunnel on the nearside but below the area you see when you take off the dash top. You may not be able to get to it with the lower dash in place. You may be able to reach it through the radio slot but I have not tried it myself. You may also be able to reach it by following the cable branch from the engine loom just after it comes through the front bulkhead.
Steve
Steve_D said:
The base problem is the starter location so close to the exhaust. This dries out the lubrication in the solenoid and pinion mechanism which in turn takes more current to operate.
TVR used the wrong circuit in the immobiliser which means it is using the lower amperage circuit for the start and the high for the fuel pump (which has its own relay anyway).
Net result is the contacts in the immobiliser burn. Putting in a relay kit may help but the contacts are now burnt so may still be unreliable and in particular as they will be drawing a lower current that may have issues getting through the burnt contacts.
You will most likely find the connector buried above the tunnel on the nearside but below the area you see when you take off the dash top. You may not be able to get to it with the lower dash in place. You may be able to reach it through the radio slot but I have not tried it myself. You may also be able to reach it by following the cable branch from the engine loom just after it comes through the front bulkhead.
Steve
Hi Steve,TVR used the wrong circuit in the immobiliser which means it is using the lower amperage circuit for the start and the high for the fuel pump (which has its own relay anyway).
Net result is the contacts in the immobiliser burn. Putting in a relay kit may help but the contacts are now burnt so may still be unreliable and in particular as they will be drawing a lower current that may have issues getting through the burnt contacts.
You will most likely find the connector buried above the tunnel on the nearside but below the area you see when you take off the dash top. You may not be able to get to it with the lower dash in place. You may be able to reach it through the radio slot but I have not tried it myself. You may also be able to reach it by following the cable branch from the engine loom just after it comes through the front bulkhead.
Steve
Thanks for all that. Sounds like dash off and poke around until you find it is the best approach.
Have you tried replacing the relay in the immobiliser itself, i.e. the one with degraded contacts ?
Just wondered if they are possible to open up. Would think you could unsolder and replace it if you can get into the unit without destroying it.
The immobilisers are sealed, not surprisingly. I doubt it's possible to gain access to change the relay without specialist equipment. There is a ribbon cable connector that is removeable, this is to the keytag socket on the steering wheel column and also goes to the alarm to synchronise the 2, i.e. when you unlock the car, the immobiliser is disabled, for 15s.
I've had the stereo out to locate my immobiliser following start failures. All of the wires into the immobiliser (9 on mine) are black and unmarked in one loom. One of these is the starter solenoid wire supply. I suspect it has a long current path! At the alarm end under the dash cover, the insulation on the loom is marked with the pin nos used.
More info here:
http://www.abacuscaralarms.co.uk/tvr-alarms.html
When mine is cold, the parallel voltage potential on turning the key to crank is about 11V. (Over 12V when disconnected) When very hot, like yours, it drops below 10V and this isn't enough. I attempted to put my multimeter in series to find out the current draw but it loads the circuit so much that I couldn't get it to work even when cold.
I've had the stereo out to locate my immobiliser following start failures. All of the wires into the immobiliser (9 on mine) are black and unmarked in one loom. One of these is the starter solenoid wire supply. I suspect it has a long current path! At the alarm end under the dash cover, the insulation on the loom is marked with the pin nos used.
More info here:
http://www.abacuscaralarms.co.uk/tvr-alarms.html
When mine is cold, the parallel voltage potential on turning the key to crank is about 11V. (Over 12V when disconnected) When very hot, like yours, it drops below 10V and this isn't enough. I attempted to put my multimeter in series to find out the current draw but it loads the circuit so much that I couldn't get it to work even when cold.
Edited by ianwayne on Monday 5th September 17:28
Just a thought, if you read anything less than the battery voltage, 12.6v I think for a healthy battery when the solenoid wire is disconnected you should be able to trace it, you can't have a volt drop without drawing current, so loosing 1.2v plus at any time when disconnected means that there is a circuit being made on this line with a resistance in it, the resistance that is causing the 1.2v drop will very likely be the same one that sometimes gives a bigger drop, multimeter leads with the needle probes to access into connectors or the ones that put a small pin hole in the insulation might be useful, standard probes are generally too fat, they don't cost much.
Paul
Paul
Forums | Chimaera | Top of Page | What's New | My Stuff