If you are checking for voltage on the positive post of the coil then there certainly is something wrong with what you see.
If you are checking for voltage on the negative side of the coil, you will see the voltage disappear when the ignition points are closed and reappear when the points open.
This is because the negative side of the coil becomes a grounded circuit while the points are closed.

Thanks for the offer, I will send you an email just in case. In the meantime I need to find out how this is wired. I am getting 12 volts at the coil with the engine running and 0 volts with it in the start position. Based on the comments above it sounds like I should have less than 12V when running and 12V when cranking (supplied from starter solenoid).

Running the wires backwards from the coil would be the place to start I guess.

Should it become needed I still have a schematic depicting how I added a Bosch 5 blade relay to bypass the ballast during cranking.
Since this forum has no means of posting images you can e-mail me (see profile).

Keep in mind also, that the accessory terminal on the key switch doesn't get power while in the crank mode so you don't want to hook the ignition to that one.

Thanks bbforks, we are reading the same sheet of music. :^)

The Clark wiring of the day was such that the Delco starter had a terminal on the solnoid that was energized only during cranking (not by the igntion switch). The wiring to this terminal was connected directly to the positive side of the coil, bypassing the ballast resistor.

I have had to dupicate this type of wiring schematic (with a relay) in numerous applications when the power to the igntion coil drops off due to high current load to the starter.

Changing to a external ballast resistor coil, adding the basllast resistor & relay with the related wiring has saved many a truck from needing a replacement wiring harness

I hope someone who is familiar with the Clark wiring of that era can offer up some info on whether or not a ballast resistor circuit (or a resistor wire) was configured into the ignition coil circuit to help with cranking/starting problems.
Here's why I say that:
For a long time in things automotive, ignition systems were fitted with a ballast resistor in the coil primary circuit. The ignition coil was built to be rated at a voltage level roughly 1/2 of the system voltage of the vehicle. During normal running operation, the ballast resistor did the task of reducing the voltage fed to the coil to a level the coil could tolerate.
But, there was a second wire that led to the same + terminal of the coil in addition to the wire from the ballast resistor. The second circuit was fed from a circuit that would only be energized when the starter was energized.
All of this was done to overcome the phenomenon of losing spark at the coil due to the tremendous current draw of the starter causing a corresponding drop in battery voltage.
The second circuit was usually fed from an ignition key switch having an independent output terminal that was energized at the same time as the starter control circuit, but NOT electrically connected to the starter control circuit.
General Motors accomplished the second circuit by feeding it from a dedicated post on the starter solenoid.
FoMoCo did it with a terminal on the starter relay.
Ballast resistors fell out of fashion and the job of lowering the running voltage to the coil was done by special resistor wires, instead of the ceramic body ballast using a iron wire resistor.
We had some Komatsu FG20-11 trucks back in the mid 90s that began to exhibit the same symptom you have regarding that they would not achieve an actual start up while cranking, only when you let go the key would they begin to run autonomously.
When I began chasing this problem I discovered that the second circuit (current direct to coil + during cranking) was just hanging loose in space at the ignition switch, and there was no empty terminal on the ignition switch to connect it to.
Queries to KFI revealed that the brass at the company I work for had specified the trucks to be built with a aftermarket ignition switch (that would match what we had on our shelves) instead of the Komatsu standard ignition switch for that model Nissan H-20 engine. The aftermarket switch did not feature a terminal to bypass the ballast resistor during cranking and so after some years of use, the ignition circuit in the truck would gradually build up resistance, and the current draw from an aging starter would increase until a point was reached where voltage during cranking would fall below the level at which the ignition coil could make spark.
The fault was not with Komatsu, it was because some bean counters at my company decided they knew more than KFI and Nissan engineers and substituted an incorrect switch for the correct one.
I was able to secure permission from our "powers that be" (the same ones responsible for the switch fiasco) to add a Bosch mini relay to the wiring near the ignition switch to feed the second circuit (ballast bypass) to the ignition coil when the starter control circuit was energized.
It is not as simple as just connecting the starter control circuit to the coil second circuit because if you did.....the starter would still be engaged after the engine starts due to being back fed from the coil primary circuit.
Sorry for the length of post but I thought maybe some background info about ballast resistor systems (or resistor wires) could possibly help in finding a cure.

Before getting into the wiring- check for power at the ignition stud of the ignition switch while cranking- I've seen this symptom many times being a faulty switch

Thanks for the thought. I checked tonight and the coil said Clark on it but just to make sure I put a volt meter on the coil. With the key in run there was 12 volts across the coil, with it in crank it went to zero. I hope to get a chance to chase wires this weekend.

Greg

What you describe about the key position and sometimes will begin running after you let go the key sort of points at either a very weak battery, or someone has put a resistor type ignition coil in place where a ballast resistor coil primary circuit exists.
I have seen the symptoms you describe on some old Nissan H-20 (stage 1) engines in Komatsu chassis.
Sometimes upon failure of an ignition coil people will replace it with whatever looks good or whatever is in stock, and that might not be what is correct for the particular truck.

I haven't had a chance to check the fluid level but will try to get to it this weekend. The closest dealer I have is about 100 miles away and their parts department does not answer emails from their website. I will have to call them next week to see if I can get a manual. Havent been able to find anyone online that sells them.

I need to get a wiring diagram for another problem it has. It will not start as long as the key is held in the start position. If I let it turn over a few times and release the key to the run position it will sometimes start. My guess is that the wiring is messed up that supplies power to the ignition in start.

Everything else seems to work great so if I can get the transmission and wiring cleaned up this should be a great machine! I do need to get the operators manual too.

Thanks for your assistance.
Greg

Are you sure the fluid is full? Dipstick is up front in the differential housing. Then is would check the inching valve. It has a brake line contected to it.

I finally went to pick it up and got a different story. They originally told me it would need to be loaded by fork lift but when I got there the guy that got it ready for me told me he was able to get it to work ok by slowly letting out on the pedal until it engaged. They were actually able to drive it up the ramps of my trailer although it was tricky because it was difficult to get it to engage everytime. I am wondering if the control valve is sticky/dirty of in need of overhaul. Is there a good service manual available that describes tests to the system?

Thanks,
Greg

mrfixit correct check the pressures cold be a bad trans pump sorry i dont have the locations for the pressure tests maybe edward t can shine some lite on this one or call 18002658433 ex 11 ask for fred he will send you the info call monday he is off fridays good luck

If it slips about the same in forward and reverse, the clutches may be ok. The pressure is just low. Is the fluid full? Do you know about the inching petal valve and how that dumps the pressure to the clutch packs when pushing on the left brake petal, causing them to slip? You should find the ports on the control valve and check the pressures before taking the trans out.

It is a C500-Y50. Late 70's model.

What model Clark is it?

Slips in both forward and reverse.

what is the problem with the trans