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2023-11: Warosu is now out of extended maintenance.

/diy/ - Do It Yourself

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1962071 No.1962071 [Reply] [Original]

Hey

anyone here familiar with low amp probe relative compression testing on cars? I bought myself an osciliscope and a low amp probe but when I disconnected the fuel injector fuse and hooked up my probe to the negative battery output I didnt get any readings what so ever except for the initial crank over when I turned the key, I have the probe facing the right way, also If anyone is good at explaining how to use this shit I'd be very grateful

>> No.1962081

>>1962071
You'll have better luck posting on /o/ again

>> No.1962087

>>1962081
Yea, I dont think anyone on /o/ has even looked under their fucking hood before

>> No.1962089

>>1962087
There were people in that thread posting their fancy setups for exactly what you're trying to do though

>> No.1962094

>>1962089
none of them actually answered my question as to how the fuck do I use this

>> No.1962109

Cant help but I’ll give you a bumo in the form of a question.

Why use this instead of a regular compression test kit? Is there any benefit?

>> No.1962111

>>1962109
Quicker and more simple

>> No.1962115

>>1962111
>>1962109
basically what he said. Lotta modern cars make it a pain in the ass to access the plugs so its just easier to do this instead of having to rip everything apart

>> No.1962119

>>1962111
>>1962115
Something for me to consider. The best I’ve done is use an analog multimeter to check the rpm on small engines. Neat stuff. I’ll be monitoring this thread.

>> No.1962121

>>1962071
The starter motor takes 150 Amps.
Once the car is running, and the battery is on the charging circuit, the battery won't show much current.

If you disconnect the alternator and run it off of the battery total loss, you will get a better understanding of how much your system is requiring.

>> No.1962124

>>1962121
Could you elaborate more anon? this is what im trying to do at 3:30 https://www.amazon.ca/gp/product/B07WJXHV26/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

>> No.1962149

>>1962121
>Once the car is running
Yeah. But you actually don't want the engine to fire while performing this test. Because even an intermittent bump will mess up the reading.

What you are looking at is the relative height of each current peak that the starter draws as it pulls each piston through TDC. If one peak is lower than the others, that cylinder has low compression (less starter power needed to turn through TDC).

But here's a thought: Skilled mechanics can crank an engine and hear a low compression cylinder by the uneven cranking speed.

>> No.1962193

>>1962071
Depending on the size of your engine, the starter can draw between 100 - 200amps. What you have will not work for what you’re trying to do.

>> No.1962314
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1962314

>>1962149
>But here's a thought: Skilled mechanics can crank an engine and hear a low compression cylinder by the uneven cranking speed.

When this test is done properly with a scope and you also show pulse from a cylinder, this test tells you which cylinder has low compression. I'm guessing your skilled mechanic is not so good at that.

>>1962193
Correct. People tend to assume everyone has a newish car. Pic related is my 84 ford F150 300 I6 which surges to about 500 amps to get the engine rotating, then is about 170 amps for the peaks later on. When I did this test I had never heard of the relative compression test, but it looks like mine are ok. (Ignore the voltage scale which is the arduino ADC 0-5 volts). This is a home made current probe using an analog hall sensor, arduino nano with bluetooth to send the trace to my phone.

>> No.1962325

>>1962071
Check South Main Auto on youtube, he's done several videos on relative compression testing

>> No.1962377
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1962377

>>1962071

>> No.1963038

>>1962314
>this test tells you which cylinder has low compression
And what will you do with that information? If it's your car, more serious diagnostics will be performed. If its a shitbox you are considering buying, just walk away.

>> No.1963045

>>1962071
https://www.ebay.com/p/1223066157?iid=393002794762
So here is the correct tool for diagnosing a bad cylinder on an engine with a distributor.
It'll even tell you what cylinder it is depending on the firing order.
Low amp probe. my sides.