Author Topic: Oil pressure light. Help! (Read 7814 times)

Staubig · « **on:** April 22, 2015, 10:17:55 AM »

First, I know this is a SOHC forum, wo i apologize, but I'm in desperate need of help. Okay, so I own a DOHC 79 Cb750 K/limited edition. I spent the past 2 months replacing the entire charging system because it was all bad. Last night I received my regulator/rectifier and wahla! it was fixed. So I got 4 quarts of Pennzoil 10w-40 and did an oil and filter change, excited to take it for a ride. After the oil change my Oil/Stop light came on and won't shut off. I'm demoralized. I have no idea what went wrong but the light stays on even when the motor is running (5-10 second tests). What could possibly have happened and how do I fix it? This is the last straw.

1979 CB750 Limited Edition

wjustinleigh · « **Reply #1 on:** April 22, 2015, 10:39:42 AM »

Did you check the switch and the wires to it? On my cb500 it's on unless the switch turns it off. I had a wire that had worn through resulting in an always on oil light.

Staubig · « **Reply #2 on:** April 22, 2015, 10:42:41 AM »

I spot checked the wire. I don't really know how to test the switch itself.

1979 CB750 Limited Edition

martin_uk · « **Reply #3 on:** April 22, 2015, 11:14:49 AM »

Not sure about DOHC, but If there is one wire going to switch, it is usually grounded with no oil pressure and live feed to other connection on warning light.

If you disconnect lead at switch light should go out, if it stays on the lead is touching ground/frame/engine somewhere.

The risk damage by of running engine with no oil pressure is not worth risking, so extreme caution recommended.

Sometimes, systems need to be primed after oil/filter change.

I always fill cannister type filters with oil before fitting to reduce time until pressure develops.

You should get oil pressure up by cranking with kill switch on and plugs out if necessary, to allow it to spin quicker, but if light still on after running sounds like you need to rule out a faulty switch.

Either swap with a known good one or see if you can borrow an oil pressure gauge to connect at switch position.

There may be DOHC specific advice, but above is general to most systems.

calj737 · « **Reply #4 on:** April 22, 2015, 02:48:00 PM »

It's part 18 on the attached fiche- http://www.cmsnl.com/honda-cb750k-750-four-k-1979-usa_model496/partslist/E++10.html#results

The switch is exactly like our SOHCs, just in the other side it seems...
Staubig - you can use the method described by Martin to test it. Replace if defective, easy. Don't over-torque it upon replacement.

Staubig · « **Reply #5 on:** April 22, 2015, 06:26:50 PM »

Hm, okay. so some tidbits I did today and left out earlier. Because I'm a dumbass I rode the bike 2-3 miles after the oil change last nighy, with the light on.. No blown motor or seizure. Am I lucky? Or is it functional? I unplugged the lead to the sensor/switch and the light still came on, but it was delayed by a second or two. Also, I tried "burping" the filter by loosening the housing and turning the bike over. It is DEFINITELY pumping oil to the filter. From the sounds of it you're saying now that I have a grounded wire to the switch? I ordered a new switch today, but o guess I'll check for ground exposure on the wire to the sensor.

1979 CB750 Limited Edition

mystic_1 · « **Reply #6 on:** April 23, 2015, 02:40:00 AM »

Quote from: Staubig on April 22, 2015, 06:26:50 PM

I unplugged the lead to the sensor/switch and the light still came on

If the oil pressure light comes on even after disconnecting the oil pressure switch, then you have a short circuit somewhere. Ordinarily +12v is supplied to the bulb and then it is switched to ground via the oil pressure switch.

mystic_1

Staubig · « **Reply #7 on:** April 23, 2015, 05:44:36 AM »

Quote from: mystic_1 on April 23, 2015, 02:40:00 AM

Quote from: Staubig on April 22, 2015, 06:26:50 PM
I unplugged the lead to the sensor/switch and the light still came on

If the oil pressure light comes on even after disconnecting the oil pressure switch, then you have a short circuit somewhere. Ordinarily +12v is supplied to the bulb and then it is switched to ground via the oil pressure switch.

mystic_1

Yarg, thanks mystic_1. Anybody have any advice for finding grounds? I've got a multimeter and a test light and some word leads. Think it will be as simple as exposed wire?

1979 CB750 Limited Edition

mystic_1 · « **Reply #8 on:** April 23, 2015, 06:00:16 AM »

IIRC the oil pressure sensor lead runs from the switch into the starter motor cavity and then across to where it joins with the alternator sub-harness. from there it runs up out of the motor and up the left side frame member next to the airbox where it joins an 8-position molex. Disconnect this circuit one step at a time, working from the end, and test for ground at each step.

BUT...

First thing I would personally do is to clip one of your meter leads to the frame ground, the other to the oil pressure switch itself, set for resistance, then fire up the bike to see if it ACTUALLY has low pressure or not. Then get the wiring sorted.

mystic_1

Staubig · « **Reply #9 on:** April 23, 2015, 07:53:01 AM »

Thanks! Multimeter work is new to me. I'll report my findings.

1979 CB750 Limited Edition

Staubig · « **Reply #10 on:** April 23, 2015, 08:00:47 AM »

Bike off:
Frame ground to switch: 1.5 Ω

Bike switched on (not running):
131.6 Ω

1979 CB750 Limited Edition

Staubig · « **Reply #11 on:** April 23, 2015, 08:04:45 AM »

Resistance between switch and first connected .5 Ω

1979 CB750 Limited Edition

mystic_1 · « **Reply #12 on:** April 23, 2015, 08:06:02 AM »

That.. is a little odd. With the wiring disconnected, there should be no difference in the electrical resistance of the pressure switch when the bike is switched on.

That said, 130 ohms isn't really all that big. When you start the bike, what you want to see is that the switch resistance should jump to Infinity.

This means that the switch opened due to oil pressure, and it's "telling the light to turn off".

Remember this switch provides the ground path for the indicator light, so low resistance = On, high to infinite resistance = Off.

mystic_)1

Staubig · « **Reply #13 on:** April 23, 2015, 08:08:47 AM »

With the bike running it displayed a "-1." Which I don't think is necessarily a reading because it displays "1." By default in resistance setting.

1979 CB750 Limited Edition

Staubig · « **Reply #14 on:** April 23, 2015, 08:10:37 AM »

I didn't disconnect anything for those tests. I had one lead on the switch, which was in the bike with the wire attached to it, and the other on the frame ground.

1979 CB750 Limited Edition

mystic_1 · « **Reply #15 on:** April 23, 2015, 08:16:24 AM »

Ah ok, I was assuming the switch lead was still disconnected from the switch.

The -1 is actually a pretty good sign as this means "Infinite Resistance" most of the time, the same reading you get when the test leads aren't touching anything.

So here's what I would do:

Disconnect the switch lead, then measure resistance between the switch's screw post and the frame ground. Should be low resistance until you start the bike, then you want to see it switch to high or infinite resistance.

Next shut off the bike and test between the frame ground and the still-disconnected switch lead. Should be high to infinite resistance.
If you get low resistance between the switch lead and frame ground, you've got a short. Move up the circuit to the next junction, disconnect it, and then test the "upstream" side i.e. the one still connected to the light. Repeat this process until the path to ground goes away, and that means the problem is in the segment that you just disconnected.

mystic_1

Staubig · « **Reply #16 on:** April 23, 2015, 08:25:00 AM »

3.6 Ω between ground and switch-- bike off

With the bike running I get no response between switch and ground.

Now to test segments!

1979 CB750 Limited Edition

mystic_1 · « **Reply #17 on:** April 23, 2015, 08:44:09 AM »

Yup that is a very good sign as far as your motor is concerned! Sounds like you have good oil pressure.

The rest is just circuit tracing. Just for the sake of it, try disconnecting the blue+red wire as close to the oil pressure warning light as possible. If the light stays on, the problem may be in the bulb socket itself. This may save you some time tracing wires through the harness.

mystic_1

Staubig · « **Reply #18 on:** April 23, 2015, 09:02:13 AM »

Quote from: mystic_1 on April 23, 2015, 08:44:09 AM

Yup that is a very good sign as far as your motor is concerned! Sounds like you have good oil pressure.

The rest is just circuit tracing. Just for the sake of it, try disconnecting the blue+red wire as close to the oil pressure warning light as possible. If the light stays on, the problem may be in the bulb socket itself. This may save you some time tracing wires through the harness.

mystic_1

Dreading opening up the headlight. I'll dig and dig and see what I can find in the circuit. My girlfriend is making me leave the garage for now though. She works in a few hours though

thanks! I'll follow up.

1979 CB750 Limited Edition

mystic_1 · « **Reply #19 on:** April 23, 2015, 09:07:00 AM »

By definition, SWMBO must be obeyed

Sounds like you're on the right track, good luck!

mystic_1

LesterPiglet · « **Reply #20 on:** April 23, 2015, 09:59:20 AM »

What's this trepidation about opening the headlight? Everything is colour coded, hard to get wrong. Making it neat is the hardest part.

Staubig · « **Reply #21 on:** April 23, 2015, 11:35:48 AM »

Quote from: LesterPiglet on April 23, 2015, 09:59:20 AM

What's this trepidation about opening the headlight? Everything is colour coded, hard to get wrong. Making it neat is the hardest part.

I've just had the headlight on and off for 3 months. Put in new front turns and found two bad wires in the mix. I just don't want to see or dig through the rats nest again after "fixing" what I thought to be all my electrical problems lol

1979 CB750 Limited Edition

calj737 · « **Reply #22 on:** April 23, 2015, 11:41:06 AM »

This previous work is a pretty good indication of where you problem likely lies. The bulb is on the bar clamp, yes? The indicators are in a small harness and it's a common place for shorts to occur.

Staubig · « **Reply #23 on:** April 23, 2015, 01:05:33 PM »

Quote from: calj737 on April 23, 2015, 11:41:06 AM

This previous work is a pretty good indication of where you problem likely lies. The bulb is on the bar clamp, yes? The indicators are in a small harness and it's a common place for shorts to occur.

Yeah, betwixt my speedo and tach. About to pull the headlight off and get to the nitty gritty.

1979 CB750 Limited Edition

Staubig · « **Reply #24 on:** April 24, 2015, 12:34:34 PM »

I followed the wire start to finish and everywhere in between. I appreciate everybody's input, but I really think its the sensor. I'll let you guys know how it pans out when I get it.

1979 CB750 Limited Edition

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Author Topic: Oil pressure light. Help! (Read 7814 times)