Voltage Drop Tests for Dummies (Namely Me)

[Sparty047]

The issue is with a 2017 Kubota L2501 HST ignition system:

At the harness for the starter relay the red wire V reads = to Battery (ie. both 12.6 V) but the lead that is "switched on" by the main switch reads ~1.5V low (ie. 10.8V - not 12.6V expected) and the starter relay is NOT activated.

NOTE:
1) I can remove the starter relay from the harness and briefly jump from RED term (12.6V) to BLACK term (10.8V) with main switch on to start the tractor. Obviously I break the "jump" as soon as the diesel starts and the engine will run until I switch it off at the main switch.

2) The voltage at the B terminal on the starter reads = to battery voltage.

3) The continuity tests on all the safety relays checked okay. Bypassed the pair of PTO safety relays with short "jumps" just to rule out their being the cause of voltage drop at the starter harness.

4) Main switch was only $20 so bought one to rule out impedance problem in the switch.



BIG QUESTION IS HOW TO PERFORM A VOLTAGE DROP TEST TO "ISOLATE THE EXACT SOURCE OF THE VOLTAGE DROP???


I've looked at several videos posted. The ones I've seen usually measure from battery post to starter B terminal to verify no impedance issue with the cable run from the battery to the starter. Some refer to other "intermediate spots" that may require investigation.

CAN ANYONE PROVIDE A RESOURCE DETAILING AN ORDERLY APPROACH TO ISOLATE MY ISSUE?

As I understand it there is a wiring point somewhere in the route of the starter system that is offering very high impedance. This is causing the voltage to fall from the 12.6V battery current to 10.8V.

At what intermediate points would I check voltage with the key switch "on" to verify measured V ~ = to battery V ?
Wish to either start with the first accessible point (a) and proceed to (b) then (c) etc. or check the last point (m) where voltage reads 10.8 V and move to (l) then (k) etc until I find 12.6V.


Thanks to anyone who is able to shed light on this.

 

[Oil pan 4]

Probably got a bad wire connection point somewhere.

 

[BruceP]

You seem to understand that your problem is a voltage-drop between battery+ and starter (small wire)
Be aware, your root-cause is most-likely a "safety switch" somewhere in that circuit. (seat, PTO...etc)

I commend you for realizing that VOLTAGE DROP tests are the most efficient way to isolate the type of problem you are facing.

Although you had the right idea to check the resistance of the safety-switches... The OhmMeter only uses very small current so it may not 'see' a poor connection.

YOU MUST MEASURE VOLTAGE DROP WITH CURRENT FLOWING. When you are trying to crank engine, there is much larger current draw....hence there is a voltage-drop.

Lets start with some 'basics' regarding VOLTAGE DROP TESTING:

The assumption is that a straight wire should have 0.00 volts across it at all times. (this is ideal.... reality tell us that there is ALWAYS some voltage-drop across a wire as current goes up.)

A "circuit" consists of wires, connectors and switches.... but we still expect 0.00V across the entire length of it.

Given the above.... VOLTAGE DROP testing is a methodical way of applying successive-approximation to isolate where voltage is being dropped in a circuit. In other words, we keep chopping the circuit into smaller and smaller segments until we isolate the voltage-drop.

Done properly, it often takes less than 4 minutes to find a fault. (If you have schematic diagram) Without a schematic, I often take 30+ minutes to figure out the circuit.


Applying the above to your specific issue: (it is best to use meter with alligator clips on the leads)

1. Clip one meter-lead on battery+ (THIS LEAD DOES NOT MOVE DURING TESTING)
2. Other meter-lead goes on small connector of starter
   • expect under 0.05V(50mV) when cranking engine
   • I realize you are seeing ~1.5V
3. Move meter-lead from starter to where the power goes INTO the keyswitch
   • expect under 0.05V(50mV) when cranking engine
4. Move meter-lead to where power goes OUT of keyswitch
   • expect under 0.05V(50mV) when cranking engine
5. Move meter-lead to INPUT of first safety-switch in circuit
   • expect under 0.05V(50mV) when cranking engine
6. Move meter-lead to OUTPUT of first safety-switch in circuit
   • expect under 0.05V(50mV) when cranking engine

I hope you can see the pattern here. The 'trick' is to figure out where the circuit flows.

When you see more than 50mV... you are starting to isolate where the voltage is being dropped.

-------------------------------
BEFORE you begin voltage-drop tests, you can momentarily jumper from the BIG wire on the starter to the small wire on starter... the starter should instantly begin cranking engine. This will validate your starter actually works.

BEWARE: Make CERTAIN machine is in neutral and expect to see some harmless sparks when you do this.

 

[GreensvilleJay]

testing safety switches......
Can be done with a DMM that has a low ohms range ( < 10 ohms )
1) set to low ohms and SHORT the meter prods, should be ZERO, if not , write the reading down
2) now secure prods to switch and test, if not ZERO ,record the reading

3) the actual resistance of the switch is the 2nd reading minus the first.
say 1st reading is 0.2 ohms, 2nd reading is 1.3 ohms. 1.3-0.2=1.1 ohms.

4) if the relay requires 1 amp of current, that switch will drop the voltage by (1.1 x 1) ,so about 1.1 volts.
V=I x R.

If you have 3 switches in series and all read about the same..then you'll drop about 3 volts , so starter relay won't engage as it needs over 10 to work......

I don't know what the ACTUAL current a starter relay needs, that can be measured using an ammeter. Easy if you go from +12 at starter --->ammeter----starter relay control terminal. Be sure tractor's in neutral,though it shouldn't start as ign key is NOT on.....

 

[nbryan]

Step by step check turn on the key then check the voltage drop across EVERY wire and device that circuit flows through. The drop seems to occur, from your post, when the key is just turned to the run position, and not being further loaded by the glow plugs or starter cranking? Then there's a 1.8v total drop across one or more of the circuit components, INCLUDING the cables?

check voltage between battery post and cable clamps, + and -.
Check voltage between the battery cable clamp and the cable itself.
Then the cable, check V drop between the cable ends, then at the load end between the cable and its crimped-on connector, then between that cable connector metal and the switch or solenoid post it's attached to, then across the switch, then the switch's post to the output cable's terminal connector,
And onwards.
The sum total voltage of all the wires, connections, switches will add up to that 1.8V you're seeing.
The one device/wire at a time V drop check will locate WHERE the resistances (voltage drops) are.

 

[lugbolt]