Voltage Drop testing across starter circuit

Hello all,
Hopefully everyone is well!
I have been having slow cranking issues when my 1989 XJS is warm.
Cranks fine when engine is cold. I wanted to check out if anyone has been able to efficiently test along the whole crank starter circuit before condemning the starter.
I think the large wire from the RH post on the firewall down to the starter would be the hardest part without, at the minimum, disconnecting the RH exhaust down pipe so one can reach the starter solenoid. All the battery connections look fine, but I’m going to go through all the wiring, including the starter relay.
Here is an interesting video on basic starter voltage testing for anyone else that’s looking to troubleshoot with a multimeter :

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Good video, thanks for posting. It really outlines the procedure well. Agree that the wire from the bus bar to the starter is going to be your biggest PITA. No easy answer there, I’m afraid.

Thanks for the response BobPHx,
Dan Sullivan has a bunch of videos. I bought his Loadpro leads 2 years ago:


but haven’t found a situation on the jag that I needed it. The tool is somewhat controversial. Some mechanics love it and others think its a waste of money.

It’s certainly convenient, and the principle is sound. But many of us have done the same thing (load the voltage source) in other ways. Battery testers (those for lead-acid batteries, plus those for dry cells and watch batteries) work the same way–they apply an appropriate resistive load. The ubiquitous “test light” does the same thing. And old timers like me often choose low resistance analog meters to avoid the over-sensitivity of DVMs.

This thing offers the convenience of applying a load resistance with the push of a button, although only one value of resistance is evidently provided. So as to not blow fuses, it probably draws a couple amps at most. That won’t help in diagnosing starter or starter solenoid wiring, or even horns and headlights. But it would be about right for tail lights. The correct resistance needs to be roughly comparable to the resistance in the faulty circuit–not much more and not much less. And of course the voltage drop when loaded indicates excessive resistance in the circuit but gives no indication as to where it is, which is the OP’s concern.

You could make your own…include not only the button but a selector switch to choose among various resistor loads. You need to avoid holding the button down on any such device–it will get hot. Especially true for the lead-acid battery testers. IMHO.

My other expensive addiction besides my XJS is large RC airplanes. I load test the batteries (which operate the radio electronics, ignition and servos for control surfaces) before each flight to confirm remaining capacity.

The point being that a basic load tester is easy build, just gotta know Ohms Law V=IR and also P=IE
The device above results in a .5A draw, so it must be a 24ohm resistor. (12/.5). It must be a 6w resistor, since 6=.5*12.

Heat is OK as long as your resistor is rated for the power (watts) you are consuming.

The box I built has two buttons and a parallel circuit inside with two 5w resistors. One button gives me .5A load, the other 1A, and both together 1.5A. Simple to operate with one hand. Never thought about using it on the car. Might have to give it a try.

Remind me to tell you of my giant scale Ugly Stick (wingspan was 78 inches), powered with a Saito 5-cylinder radial.

It was going to go on my giant scale GeeBee Senior Sportster… then 911 happened.

Love the saito 4 strokes, have owned a couple. Beautiful pieces of engineering. That engine in the GeeBee would have been money in more ways than one!

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Hello everyone, I just watched a utube video where the car tech tested the starter solenoid cable at the front of the car so he could do a voltage test on it. He said with no power at that circuit, one would read through the wire, the brushes, etc of the starter and then to ground (since starter motor is grounded to the engine block). I thinking same thing can be done with the large positive cable at the top of XJS rear right in the engine bay.
One would need to know a reference ohm reading of known good xjs starter circuit.
Here is the video:https://www.youtube.com/watch?v=oo1SRIFIkhs&t=1682s

I bought this low amp clamp:
Amazon.com: ESI 695 80 Amps DC/AC Low Current Probe: Automotive
after doing some research for checking for parasitic draws and any other car current testing. But I’m thinking I can also use it to check into my hot engine start issue. In theory, I should be able to get my clamp around the starter cable before the starter somewhere, get a current reading during a cold start, then try it again after engine warmed up.
I have been too busy and it’s too hot outside to try this out but this idea should work right? If voltage stays the same and if I have high resistance at the starter or starter cable, then my current reading should drop?

80 amps won’t cut it; starter will draw 2x that minimum. You need something that will measure up to 300 amps or more.
Best way to check without the current probe is to measure voltage drop; connect one lead of voltmeter to the positive post of battery and the other to the positive terminal of starter. Measure DC voltage while cranking (ideally below .5v). Then connect one lead of meter to the battery negative post and the other to the starter case. Measure DC voltage while cranking (again should be below.5v). If above voltage drops are exceeded, test each connection in circuit individually to find problem.
If voltage drop on each is less than .5v, and battery is good, you need starter repair (or engine has issues such as over-advanced timing).

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Ok Robert, thanks for letting me know!