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Davewillb

Rover V8 Hotwire TPS - Throttle Position Potentiometer

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Posted (edited)

Hi,

I came up with this (bodge) while I was fitting Megasquirt and found my TPS was open circuit, working replacements seemed hard to find and the other options beyond my pocket so I came up with what you see in the picture. Basically its a 5K ohms linear potentiometer mounted on a bracket I bent up which is bolted to the original bracketry holding the throttle spindle. I 'extended' the end of the throttle spindle slightly with a piece of 6mm mild steel rod and a bit of careful work with a mig (off the plenum!!). This then goes to a flexible shaft coupler which takes a 6mm shaft in and fits the 6mm shaft on the potentiometer and also allows a bit of movement if things aren't perfectly aligned.

After that it was just a case of calibrating the settings for TPS in Megasquirt, I dont see why this wouldn't work with the original 14CUX system, it would just need a bit of work with a multi-meter to get the potentiometer in the correct position for closed throttle, the colour coding of the TPS connector info is out there - but I haven't tried  this on a 14CUX.

Oh, just remembered, with my arrangement the shaped pipe rubber pipe that goes from the plenum inlet to the rear of the plenum (fuel pressure regulator is it, not sure) was too tight across the back of the potentiometer so I swapped it for a bit of ordinary hose to gain some space.

Hope this helps someone, I don't know how long it will last, I don't see why it shouldn't as there is no vacuum to suck muck in to it with it mounted like this, but time will tell. its a cheap solution anyway, as it happens I had the bits available, but Ebay is your friend if not and a fiver should do it and get you running.

I did find that I could get quite a good fixing on the end of the original throttle shaft with the flexible coupling without extending the shaft  by taking off the end clip, but I was 100% convinced it would stay on indefinitely - it was like that for a few days without a problem though.

I am going to mod my setup when I get a minute, it needs a plug and socket fitted and some sort of strain relief on the cable as it leaves the potentiometer.

Dave 

IMG_20180625_091300.jpg

Edited by Davewillb
addition of text

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Nice to see someone doing this, the TPS isn't magic; MS can use basically *any* 3-wire TPS (and probably others of you can work out the wiring), Lucas may well *need* it to be 5k linear but I'm sure thre's other less awful TPS's on modern cars that could be used.

The P38 one is much nicer.

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Posted (edited)

I'm going to take the original TPS apart when I have a minute to see if I could use the stub shaft from it to link into a replacement potentiometer in the original position.

Think you are100% right FF about it not needing to be a 5k potentiometer, its just working as a potential divider so prob best not to go lower than 5k, but 10k worked fine for my MS install as well, it just needs the increasing voltage against ground as the pedal goes down I think.

 

Edited by Davewillb

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My only concern is how waterproof is that Pot (normal ones are not very waterproof)

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For 14 CUX you will probably need another 5K car TPS POT as most normal POT's would be 5K over around 270 degrees where are a TPS is probably 5K over 90 degrees

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The Lucas system setup manual is very specific about getting just the right voltages at closed/open so much more of a pain. MS only uses the TPS for acceleration enrichment, unless you use MAPdot mode, then it doesn't even do that.

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On Tue Jun 26 2018 at 12:23 AM, zardos said:

For 14 CUX you will probably need another 5K car TPS POT as most normal POT's would be 5K over around 270 degrees where are a TPS is probably 5K over 90 degrees

That's a good point for sure. On the waterproofing issue I've used a decent quality potentiometer and I think it will be ok in normal circumstances (time will tell :) ) but automotive grade items are probably a better option for sure.

Interestingly, I didn't know my TPS was open circuit until I tried to calibrate MS to it, I remember checking it a few years ago and it seemed to be working then so when it failed I don't know...

Dave

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On 26/06/2018 at 12:23 AM, zardos said:

For 14 CUX you will probably need another 5K car TPS POT as most normal POT's would be 5K over around 270 degrees where are a TPS is probably 5K over 90 degrees

Or use a 15K pot...

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I would just watch how well the pot is mounted to the bracket, if it slips you might have no throttle or WOT when you dont want it.

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My Lucas one wasn't exactly waterproof, sometimes you had to pump the throttle a few times to bring it back to normal, generally only if I dunked it though it resisted splashes fine. 

The wires failed on mine where they came out the housing but I managed to repair it. I did intend to 3D print a housing for a regular POT but sold the range rover before I needed another. If anyone wants to try this I'm happy to 3D print the part for them. I don't have a car to use as a template anymore. 

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On 28/06/2018 at 1:04 PM, HoSS said:

I would just watch how well the pot is mounted to the bracket, if it slips you might have no throttle or WOT when you dont want it.

That's a fair point and thanks - I'm looking at various pots to improve the mechanical strength and location of the potentiometer.

Dave 

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