Renix sensor ground points and TPS ???s

My TPS input voltage is 4.66 volts whereas it should be 5. I know most people's Renixs over time get bad grounds where the sensors ground so they run auxillary ground wires. My question is where do the sensors ground in the first place? Is it somewhere that isn't accessible? If it is I would like to just try cleaning the ground and seeing if that works first. The other thing is since my input voltage is 4.66 I have my output at 3.85, am I in the ballpark here, or did I go the opposite way I should have? The last time I set it was about a year ago and I was thinking it was .82% or .83% somewhere in there. I think I have a TPS spec page bookmarked somewhere I just have to find it. Also a bad TPS can cause anything from rough idle to backfiring right? Any help would be 'preciated.
Thanks,
Collin

Oh BTW its a...1990 so no C101 connector or whatever.

1) The main sensor ground for RENIX is on the passenger side of the engine block, back by the dipstick anchor screw. Check and clean.

The RENIX "ground loop" also goes to the chassis by way of the engine block (battery to passenger side of the engine block, driver's side of the engine block to the firewall by way of the open braid ground strap.) Check that ground strap as well - it's probably contaminated (don't bother trying to clean it, it's not worth the effort...) and clean a spot of the firewall, centred on the screw holding down the ground strap (and about the size of a postage stamp,) right down to bare metal. Coat with Ox-Gard before reassembling.

It might also be helpful to run a secondary ground from the battery negative to the fenderwell sheet metal (clean another spot for that one...) using at least 10AWG wire.

And, the ECU should know what the reference voltage is, which is why you set the TPS to a percentage of the input/reference voltage. With five RENIX XJ's, I don't have one of them that gives me a solid five volts (I seem to recall them running from 4.1-4.8VDC, but I'd have to check...) As long as you can accurately read the input voltage and crunch numbers accurately (a calculator helps...) you should be able to adjust the thing properly...

5-90

5-90 said:

1) The main sensor ground for RENIX is on the passenger side of the engine block, back by the dipstick anchor screw. Check and clean.

The RENIX "ground loop" also goes to the chassis by way of the engine block (battery to passenger side of the engine block, driver's side of the engine block to the firewall by way of the open braid ground strap.) Check that ground strap as well - it's probably contaminated (don't bother trying to clean it, it's not worth the effort...) and clean a spot of the firewall, centred on the screw holding down the ground strap (and about the size of a postage stamp,) right down to bare metal. Coat with Ox-Gard before reassembling.

It might also be helpful to run a secondary ground from the battery negative to the fenderwell sheet metal (clean another spot for that one...) using at least 10AWG wire.

And, the ECU should know what the reference voltage is, which is why you set the TPS to a percentage of the input/reference voltage. With five RENIX XJ's, I don't have one of them that gives me a solid five volts (I seem to recall them running from 4.1-4.8VDC, but I'd have to check...) As long as you can accurately read the input voltage and crunch numbers accurately (a calculator helps...) you should be able to adjust the thing properly...

5-90

Click to expand...

Quick response as always 5-90, thanks. I've actually already got your full battery cable set so the connection on the block is clean and that stupid ground braid is now a 1 ga cable so those should be good. I suppose over time that 5v is just unreasonable to expect. I may try to run an aux. wire from the negative terminal to some bare sheetmetal just for fun. As long as it's not hurting to have the input voltage lower like that, that's fine with me. Reason I was asking about the symptoms is because I have a rough idle and backfiring right now and I've replaced every other sensor so I'm starting to think it's a faulty TPS again, even thought the voltage is within specs, but I'm not sure. I actually just got to reading another one of your posts and realized that I peeled most of that gold "tin" stuff off my knock sensor wire so I'm wondering if thats screwing up my timing as well. Guess I'll rewrap that in tinfoil tomorrow for some fun.

The key is to measure from the "ground" on the tps and the ground point on the ECU. It should be less than .1v. The +5v is developed by a 3 terminal regulator in the ECU and can vary quite a bit. Measure this voltage from the ground on the ECU to the sensor power on the ECU. If it is different than what you find on the TPS, you have a wiring issue.

BigBlackHeep said:

Quick response as always 5-90, thanks. I've actually already got your full battery cable set so the connection on the block is clean and that stupid ground braid is now a 1 ga cable so those should be good. I suppose over time that 5v is just unreasonable to expect. I may try to run an aux. wire from the negative terminal to some bare sheetmetal just for fun. As long as it's not hurting to have the input voltage lower like that, that's fine with me. Reason I was asking about the symptoms is because I have a rough idle and backfiring right now and I've replaced every other sensor so I'm starting to think it's a faulty TPS again, even thought the voltage is within specs, but I'm not sure. I actually just got to reading another one of your posts and realized that I peeled most of that gold "tin" stuff off my knock sensor wire so I'm wondering if thats screwing up my timing as well. Guess I'll rewrap that in tinfoil tomorrow for some fun.

Click to expand...

Your TPS may just be getting old and getting some flat spots. I went through this a couple months ago on my 88, it had a brutal flat spot. Turned out to be the TPS.

You do need that foil cover on the knock sensor wiring - it prevents "falsing" of signals. Since the knock sensor retards timing to make up for spark knock, you should be able to guess what happens... Check an amateur radio supply house if you have one handy, or a better electronics house (Radio Shack does not usually carry shielding tape or foil. You can use aluminum foil in a pinch, but I don't recommend it - that "gold" colour was due to a high copper content.) You'll probably see a stripped wire that the foil was connected to - that's a drain. Reconnect it. If it wasn't connected to an earth/drain lead, set one up (run it to any convenient chassis ground.)

I've worked with a number of low-power DC voltage regulators in various projects (78XX +VDC and 79XX -VDC fixed voltage regulators in particular,) and they are NOT precision devices - variation of 10% or so is NORMAL, right out of the box. If you want precision regulation, you'll have to pay about four times as much (and special order the thing...) or use a variable voltage regulator, install an output meter, and WATCH IT CAREFULLY. Or, just do the easy thing - set up the voltage comparator with a "feedback" loop, so you give it the "reference" source signal and the "sensor signal" - the voltage as divided by the TPS. Bendix likes to do the latter, and it's really good design (even a precision regulator can drift...) Voltage comparators drift A LOT less than voltage regulators - they don't have to do as much work.

(Looks like Tom beat me to the salient points - but I do tend to explain...)

5-90

I'll try to check around for that gold foil-"ish" stuff tomorrow. I don't think I pulled it all off it was just what flaked off when I unwrapped that harness I think. I'm also going to warranty my TPS tomorrow and then get the new one adjusted so I'll let you guys know what happens. Thanks to both of you fellas for the help.

You're going to be looking for "shielding foil" or "shielding tape" - it's the stuff that radiomen and electronics techs use to shield sensitive circuits against crosstalk and falsing.

5-90

i just spliced into the sensors ground and went right to the battery with them.

90xj06 said:

i just spliced into the sensors ground and went right to the battery with them.

Click to expand...

So, there's no need to cut the wires, just strip off part of the insulation and splice right? I think that's what I may do. I'm going to test some more voltages today and see what the other sensors input voltages are looking like.

Replaced the TPS today and that wasn't my problem apparently. Also did some voltage checking and my MAT sensor and CTS are both at exactly 5v. It must just be the TPS input that drops over time. Still have to get some shielding tape like 5-90 suggested to rewrap that knock sensor wire. Thanks for the help fellas.