420S Rainy Season Build in Western Washington

[CBuff]

https://www.caterhamlotus7.club/forums/topic/266498-boot-floor-removal-please-help/
 

I don’t think you need to cut the boot floor. Some people say they do it to make it easier. 

 

https://caterham420detailedbuildblog.co.uk/2020/02/05/throttle-cable/
 

I was lucky and didn’t have to adjust my pedal or throttle cable. My 420 idles at about 1100rpm indicated (can bus data says it’s 945rpm but don’t know which to believe). 
 

my throttle cable end (the barrel that slides into the throttle body) came out when I was reving the engine with the throttle body once.  When I got it back it my throttle wouldn’t rest on the idle screw and was reving higher.  I turned the barrel a couple of times to tighten the twist on the cable (it was “unwinding a bit at the barrel). This allowed it to seat better in the throttle body and gained the slack to return to a “normal” idle. 

[JohnCh]

8 hours ago, Timberline said:

Where should I check for the sensor resistance?

You could back probe the plug on the top of the fuel assembly.  The two thinner inner wires are for the sender (the thicker outer wires are for the pump): Green/black handles the signal and black is ground.  

 

 

 

[JohnCh]

Work got in the way, and I accidentally hit submit before I had finished my post above or corrected a confusing portion   When I wrote "back probe the plug", that was poor word choice on my part.  Pull the plug off the fuel assembly and put the multimeter on those two inner prongs on the male end of the plug.  I just tested the one I removed from my car and it ranges from 53 ohms empty to 462 ohms full.  Keep in mind that's the range in bench testing, not the range it can reach in the confines of the tank.  If you do need to pull the fuel assembly out of the tank for further checking, I can recommend this removal tool: https://www.amazon.com/gp/product/B0079GPYA4/

 

Let me know if you need me to take any other measurements.

 

-John

 

 

[Timberline]

Thanks for the wires! It’s so odd how they skipped pins. Six pin connector and they use 2,3,5,6.

 

 Anyways, got resistance of 220 ohms and lifted one side of the car and got 225 ohms. Mostly just a check for the float to be working.

 

 Working from there to the dash, where should I chase it next?

[JohnCh]

I believe that gauge has three wires at the plug: signal, 12v+, and ground.  If that's the case, I'd make sure the ground and power are working and that there is continuity to the green/black wire at the sender plug.  I have seen a bad oil pressure gauge on a new kit, so don't assume it's not the gauge.  Have you asked Bruce if he has one you could swap in as a test?

[Timberline]

Anyone know what the signal wire is supposed to show at the gauge? Also, I've got four wires at the gauge. 12V and ground are obvious. Trying to figure out what the other two are supposed to be. Not getting any obvious resistance or voltage on them.

[JohnCh]

The fourth wire is probably for the light. I forgot about that earlier: ground, signal, 12v+ gauge, 12v+ light when side/headlights are on (probably white/red).  Is one of the wires green/black? According to my notes, that is the signal wire.  As for what you should see at the gauge end, it should be the same ohms reading you took off the sender, but it might require the ignition on to register.  I repurposed the signal wire for my setup, but it is a voltage based (0-5v) sender.  That signal was not changed along the way.  This tells me there isn't any Caterham magic happening between the sender and gauge, but @CatManDo could confirm based on the work he and Bruce did for the fuel gauge fix.  

 

[hyper7]

4 minutes ago, JohnCh said:

The fourth wire is probably for the light. I forgot about that earlier: ground, signal, 12v+ gauge, 12v+ light when side/headlights are on (probably white/red).  Is one of the wires green/black? According to my notes, that is the signal wire.  As for what you should see at the gauge end, it should be the same ohms reading you took off the sender, but it might require the ignition on to register.  I repurposed the signal wire for my setup, but it is a voltage based (0-5v) sender.  That signal was not changed along the way.  This tells me there isn't any Caterham magic happening between the sender and gauge, but @CatManDo could confirm based on the work he and Bruce did for the fuel gauge fix.  

 

I bought one of those fuel gauge correction units..it didn't work, so this is helpful info so I can troubleshoot it.

[CatManDo]

The wire from the gauge to the sender is Green/Black one. Red/White is the light. Green- 12V, Black- Ground. the sender is just a potentiometer to ground. An ohm meter between Green/Black and ground will read about 70 ohms when empty and 460 ohms when full. We have seen wiring harnesses come from the factory with pins swapped. One had the Green/black tank sender wired to the alternator. @hyper7 if gauge correction unit isn't working please provide us with what it is doing. Or send it back and I will check it so we know if it is the unit or the car.

[hyper7]

6 minutes ago, CatManDo said:

The wire from the gauge to the sender is Green/Black one. Red/White is the light. Green- 12V, Black- Ground. the sender is just a potentiometer to ground. An ohm meter between Green/Black and ground will read about 70 ohms when empty and 460 ohms when full. We have seen wiring harnesses come from the factory with pins swapped. One had the Green/black tank sender wired to the alternator. @hyper7 if gauge correction unit isn't working please provide us with what it is doing. Or send it back and I will check it so we know if it is the unit or the car.

I'll DM so as not to clog up this thread.

[Austin David]

23 minutes ago, hyper7 said:

I'll DM so as not to clog up this thread.

 

OK but please do post a summary when you sort out the wiring and your troubleshooting.  It does serve as a valuable future reference -- I haven't broken my sender YET, but "replacing the whole assembly" is on my to-do list ...

[hyper7]

Well in that case, here's the two pictures I sent to catmando in regards to the gauge correction unit.

 

 

[Timberline]

So, I checked the green/black to ground at the gauge and got a massive resistance of 1.2 Mohms. When I checked it at the track it was the expected 220 ohms for a partially full tank. I think the wire got mixed up somewhere. Any stops along the way? I know it goes into a mystery unit back by the tank (fuel pump relay?)

[CatManDo]

If you are getting 1.M ohms it is definitely not connected to the tank. Try this: Check voltage between Green/black and ground at gauge connector with the gauge disconnected. Try it with key off and on. Voltage should be zero in both cases. If you read voltage let me know what you get.

 

The Green/Black wire goes from the gauge connector through a large connector up under the dash in about the middle. It then goes straight back to the tank sender. We had one where two of the pins were swapped on the connector in the middle of the dash.

 

The mystery unit by the tank is a fuel pump controller and has nothing to do with the gauge. It is straight out of Ford Focus and can be used to control fuel pressure but Caterham does not use this feature and leaves it at a constant pressure.

 

You can also check continuity between Green/Black at the gauge and Green/Black at the connector under the left rear of the car that goes up to the controller. You should have continuity(zero ohms) there.

[Timberline]

@CatManDo With the ignition off, checking at the dash between green/black and ground, I am seeing 0 V and actually getting 470 Ohms (tank is full now, so that might be right).

 

With the ignition on, I am seeing 0.001 V (its repeatable positive and negative flipping the leads) and am seeing the 1.3 Mohms one way and no resistance with the leads flipped.

 

I undid the zip ties at the back of the car to loosen the harnesses and connector at the tank. Struggling to get the connector apart because it's got a locking tooth on 3 sides  So, no readings, but all the wires and colors are correct.