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Big hesitation on small or constant throttle

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Old Jul 4th, 2019, 14:33   #21
sleek lemur
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Cleaned the contacts in the TPS and that has all but cured the "cough". I can get the car to do it "on purpose, by fethering the throttle at 40mph in 4th for very gentle acceleration, but it's a thousand times better, plus the car generally runs more smoothly.

C1800, I partially followed your instructions for idle speed. I was too scared to dismantle the TPS. Those wires look fragile! Plus the head of the lower screw which you loosen to allow the unit to be revolved had been damaged, so I didn't want to remove it without having a new one handy. I had previously used the stop screw to adjust the idle speed after fitting electronic ignition (red face), so I backed it right off and used the correct adjuster.

Thank you, thank you, thank you.
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Old Jul 5th, 2019, 00:12   #22
c1800
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SL, good to hear that you’ve got it running much better.
Yes those wires are fragile, I was very cautious and managed not to damage them.

I didn’t touch the large screw inside the TPS, (not sure what it’s for, maybe mine is loose?) . I loosened the upper and lower screws (visible with the cover on) that clamp against the graduated slot that allows the TPS to be rotated. This is how I got the TPS properly adjusted so the shut off switch would close and the wiper contact on the zig-zag was in the correct position.

In regard to your stumble, this may be the cause(from the SWEM site)

“PC board foils (especially the area where the slider sits almost forever at cruise throttle openings) if edges of conductors show excessive wear at the edeges, that's the classic cause of hiccups at cruise”
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Old Jul 5th, 2019, 09:38   #23
sleek lemur
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Thanks C1800, yes, I have a feeling that I'll end up changing the TPS. Car has only done 70k miles (I believe), but it's 49 years old.....
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Old Jul 9th, 2019, 18:39   #24
sleek lemur
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Have been unable to adjust TPS, as lower screw head is damaged (why is it always the inaccessible one...?!). Am considering buying a restored unit from Scandcar on an exchange basis. Assumingg I eventually manage to extract it, will the TPS simply slide off the throttle axis?
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Old Jul 10th, 2019, 16:11   #25
Ron Kwas
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SL; I'm happy to hear of significant improvement...remember, TPS adjustment and Idle Adjustment are two separate adjustments! Do not mix the two up! See: https://vcoa.org/forum/9-P1800-1800S...onic-FI-system (shown here is the older style TPS, with internal wires, but function, tests, and adjustment are substantially similar to that of the later)
From that link, here is picture of Idle Adjustment Screw:

If your Idle Adjustment Screw is damaged, I'd simply loosen jamnut to allow its removal and replacement, then proceed with idle adjustment.

Age does not cause wear, it causes surface contamination...and thanks to complete internal access, TPS can be checked functionally and surface contamination can be cleaned away and contacts can be cleaned and treated with Deoxit as I have outlined to fully restore proper function...age alone is NO REASON to replace TPS! I'd take an old, crusty TPS any day over a worn out one!!

TPS does indeed simply slide off Throttle shaft after removing retaining screws.

C1800; "...from the SWEM site)

PC board foils (especially the area where the slider sits almost forever at cruise throttle openings) if edges of conductors show excessive wear at the edges, that's the classic cause of hiccups at cruise”

This is the old (underinformed!) position I had before I undertook my indepth examination of the TPS and MaC...I have since corrected that position to note that due to the design of the supporting circuit (an edge triggered Flip-flop), wear at the edges of the alternating comb conductors is NOT/CANNOT be the cause of MaCs. From: https://www.sw-em.com/bosch_d-jetron...nformation_tps

Also from my notes:
Bad MaC Explanation No 1. Detail of the PCB dispels Bad MaC explanation No 1. and presumed cause of the MaC...that the Idle switch is somehow closing and changing fuel delivery to Idle Level while at cruise. It is clear from the picture that this in absolutely impossible...Drag Switch Idle contact may indeed close, but electrical path is open anywhere but at angular Idle position because of Slider on PCB being in series and only able to be closed when at angular Idle also...so: Idle Continuity is ONLY possible at Idle position!

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Old Jul 10th, 2019, 16:52   #26
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Ron, I think he’s referring to the flat head slotted screw in the TPS as the one that’s damaged, not the idle adjustment screw.
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Old Jul 10th, 2019, 17:42   #27
sleek lemur
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Ron, C1800, thank you both again. Your patience is greatly appreciated! The damaged screw was the lower one you loosen to allow the TPS to revolve for adjustment as described in the article you attached. I'm lucky to live in Thorndon, Suffolk where a lovely fellow called Rob, who restored Amazon's has his workshop (his father, Tony is also a big Volvo man I hear), and he managed to extract the screw, so I'm away to the races! Let's see if I can get this puppy running perfectly!

Thanks for the advice on the TPS, Rob thought it looked pretty good inside, so I'll give it another clean and see how we go.
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Old Jul 10th, 2019, 19:52   #28
Ron Kwas
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C1800; I understood the Idle Adjustment Screw to be damaged, and that's the straight blade adjustment screw, so that's info I included...I guess I misunderstood....

SL; ...in rereading, the statement "fethering the throttle at 40mph in 4th for very gentle acceleration" stands out...if it coughs with quicker calls for acceleration, that suggests the TPS may not be supplying contacts to enable enrichment pulses, (MAP sensor is able to keep up without them)...I suggest you check for and verify those 20 clicks with Ign ON. Also, have you checked Fuel Rail Pressure (under demand!)...if it is collapsing then there exists a supply problem...

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Old Jul 11th, 2019, 10:39   #29
sleek lemur
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RK, re-cleaned TPS, went through the adjustment process, which I understood fine, until the part where you turn the unit back until meter reads 0 ohms. All fine until you need to then further revolve it 1/2 a mark, when I then got an infinity reading. So I left out that second bit and went on to checking the throttle stop clearance. All good. And.....

Test drive gives no hesitation!

To answer your and re the problem I was having, in fact accelerating cured the problem, so it appears that all other units are functioning correctly.

Thank you and C1800 again! Now, what's the next job? 😁
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Old Jul 18th, 2019, 17:09   #30
sleek lemur
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A further clean of the TPS and some adjustments, following C1800's link and I am now hesitation-free!
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