Hi Simon This truck sounds to me like its between 20 and 30 years old and is running on a mk seven sevcon controler
later models like the ES went onto mk10 cable form if it has a black box located on the R/H sie of the panel as you look at it then its a mk 7 you say there is 39 volts to the chassie with the battery disconnected this must be cleared then I sugest you check for earths on all motors and the system seems to be sensitive enough to detect this I know Because I had about sixty of these trucks to look after.
If my memory serves me correctly there are micro switches along the bottom of the direction contacts and shorting contactor these must be operating correctly disconnect the battery push each direction contactor in inturn and listen for them clicking sometimes they go out of adjustment or even go open circuit. when you do this ensure you push the wiper plate up flush with the directin coil bobbin plate. Remember safety first disconect the battery ensure the drive wheels are of the deck.
You are getting your Direction contacts in so you have proved your hand brake switch and foot control switches are ok. I think the problem is on the shorting contactor swiches I think you should check them all out


Titus

Hi Simon,
Is the controller in the front of the truck? and has the large thyristor got a square base because when you say that the thyristor has a purple wire it sound like the controller might be an old Sevcon MK7 controller. Do you know how old the truck is?
Regards Shane

Hi Simon
Feel free to call anytime after 9 am.

Regards

Hi sorry it is confusing as you say these trucks dont usually operate on this system il get your number will it be ok to ring you tomorrow. regards simon.

Hi Simon

I am a bit confused sorry the info i have for sevcon systems are the MC400 system, and the sc2000. The Yale ERC info I have run the General Electric and the Old cableform systems.

What does the system look like? You can get my mobile details off my website and send me a pic on the mobile, hopefully can identify the controller.

Hi there is no 15 pin connector, the thyristors readings are as you said. What i have noticed today is there is a reading of 39 volts through the chassis even when the battery is disconnected.

Hi Simon

If this is the Sevcon MC400 Series, which is the black logic card which sits on top of the controller.

Then for checking open circuit gate, the thyristors usually reads on resistance range checking between one side of the thyristor to the gate lead anything between 10 to 50 ohms, but does vary.

The pot voltage can be checked on the 15 pin connector to the logic card (connector B). On pin 2 you should get 5v rising to 10v as you press the accelerator.

On pin 4 around 5 to 6v and on pin 14 around 10 to 12v.

Also check you have battery voltage on pin 10, usually when start switch is activated.

HI NO I HAVENT CHECKED POT VOLTAGE WHAT SHOULD THIS BE? HOW DO YOU CHECK THYRISTORS? REGARDS SIMON

Hi Simon

By pushing the bypass contactor in you have have proven the motor circuit is ok. Usually if its a thyristor fault you may get a fail safe situation where the truck jumps and then drops the contactors out. If i recall correctly if one of the small thyristor gate leads goes open circuit it could throw a similar fault.

Have you checked pot circuit voltage etc?