6 replies to this topic

[kopi]

Hi guys, i have a silly question. I want to perform a simple voltage drop test to rule out excessive resistance. 

 

Symptoms on my 92 Spi: Crank getting weaker and weaker. Battery 12.65V after 8hours, 14V at engine idle. Battery earth strap checked and cleaned, extra earth strap added from alternator mount to (paint free) body. Solenoid/Battery terminal to Solenoid case 12.65V. Shorted Solenoid/Battery terminal with Solenoid/Starter terminal and the motor seems to whir smoothly/freely. 

 

I guess I'm just missing a voltage drop test for a definitive answer. In my mind, + lead to + battery post, - lead to starter battery feed terminal. If there's a >0.5V then at least i know the wiring is suspect. But is there anything else i could test? And how do i get test leads that are long enough!

 

Kopi

 

 

[mini13]

Test from the battery to the body, then from the solenoid to the body, you can assume this not much drop though the body.

Normally the culprits are the straps from the batt new to the body, or the block to the body, also worth disconnecting them and cleaning the joint and copper greasing it.

[tiger99]

Don't assume, do the earth return test too, from battery negative terminal to engine block with starter operating, and then from the head of the earth bolt at the battery to the other at the inner wing. You may be surprised.

 

Assume: make an ass of u and me

 

Can't remember where I first heard that.

 

The current density around the earthing bolts at battery and engine earth strap is high and it is only steel which is not the best conductor. I was thinking about this a few days ago and considered that a circular patch of at least 1.2mm steel, fully welded around the edge, would make an improvement. The greater the diameter the better, starting at about 2 inches. A large repair washer welded on would be about right.

 

If you don't want to do anything so drastic, please ensure that the contact area between earth strap and shell is as large as possible, It is the length of the outer edge in contact that matters. A large concave washer, edge down, would help. And use a proper earth strap at each end with a heavy, properly crimped lug. The type made from flat copper braid and soldered at both ends are utterly useless as the solder always cold flows and allows the contact pressure to relax. Never, ever clamp or crimp any tinned (with solder) copper wire, only bare copper, as the same thing always happens. The thin tin plate on bare wires does not matter.

[kopi]

Thanks guys. So that's my question, I don't have test leads that stretch from the battery to the block!

[Swift_General]

To measure the drop on the earth return measure the drop from battery negative terminal to body, and then body to starter motor, and add the figures. As above there will be a negligible drop through the body itself. Obviously tests need to be done whilst cranking. Just as another thought have you considered the condition of your battery itself? Whilt battery voltage appear healthy, as it ages and fails the internal resistance rises (think of a resistor wired in series within the battery). With the high current at cranking this then becomes significant. Either get someone with a load tester to test the battery for you, or measure the voltage at the battery terminals with a multimeter, ideally one the saves a minimum reading.

[tiger99]

If you do that, which is a good idea, measure between battery terminal and a point on the body well away from it, and preferably away from the current path, to get a good reading. A rear bumper bolt or tail light fixing would do. At the engine end, measure between block and somewhere on the opposite side of the car to the earth strap.

I am very interested in what figures you obtain.

[kopi]

Hey guys. Just to close this one off. 

 

The voltage drop readings were negligible (or was it my multimeter?)

 

So ... I did the next best thing. Cleaned by battery earth strap terminals. Added two extra earth straps. 1) alternator to fender. 2) clutch house to fender. And i made sure there was bare metal contact with lots of WD40 electrical contact cleaning juice. 

 

My starter is going super strong now!!