And,...the 'new' hog works! (On the end of my 'lectrics, earthed to a valve cover bolt) This one's made by Texas Instruments, VAG p/n 037 129 893, is that the right number? I think this one was off a Polo, same shape/size but the lid's a bit better attached, so I'm inclined to do the swap-out and then dismantle the dead one. Don't suppose any clever person can tell me the part numbers of the o-ring and gasket...?
inspired by this thread I checked my hog too (I do have a bit of roughness on a cold start) and guess what??!! broken anyway, I too have 12v at the plug for the heater and all is well at the relay. I get no resistance between pin 87 (of the relay) and the heater connection (ie through the fb) but masses of resistance through the heater connector to earth. The inlet manifold is earthed ok but if I measure resistance between the cover of the heater to earth then I get open circuit. My assumption therefore is that either the heater is AWOL or that the earth between the heater and the inlet manifold has failed. Pete, you got any insight as to how the heater is earthed? is it just through the three bolts that attach it to the bottom of the manifold? I am assuming that there is a rubber gasket in there somewhere...
Funny you should ask, I've been looking into this a good deal lately! It appears not to be a completely brilliant design (!) The bulk of the hedgehog is an alloy casting which is definitely earthed to the heater element ground side. Then the cover part is steel and riveted onto the dissimilar metal of the main part (the one on my car has three brass rivets, separate to the mounting bolt holes IIRC; the spare I have acquired has the 'rivets' integral to the lid part). The three fixing bolts are again steel (obviously), but going into alloy of the manifold. So lots of scope for age related, and galvanic corrosion leading to poor contacts. The one on my car currently measures 500 ohms, feed wire to manifold, hence the pathetic current. One possibility, which I think I've discounted, is that the intended ground is from the alloy of the hedgehog main body directly to the alloy of the manifold, it is a snug fit, but not what I'd choose as an electrical contact. I think the three mounting bolts are supposed to do that job, and on the spare hog, when I bolt up tight onto the three 'rivets' the resistance from feed-wire to main body goes down from a handful of ohms to the 'correct' 0.5 ohms approx. Undoing one of the mounting bolts on the one in the car, cleaning both bits and retightening doesn't change my 500 ohm measurement though, so I think my dodgy ground is from the heater element itself, to the main hog alloy body, or the element is U/S, but from what I've read this is unlikely. There's an o-ring at the top of the alloy bit to stop vac/air leaks into the manifold, and a gasket (thin, not ever-so-compliant) between the flanges of manifold and hog. Just purchased new ones of these, and another intake manifold gasket for my forthcoming fun getting it all off and back together...Wish me luck . I'll be totally dismantling the old hog to hopefully shed some light on what may go wrong here.
hmm, thanks Pete. I'm not sure I can be bothered to take the manifold off just to fix this pesky heater. Having had the manifold off to swap the head I'm not sure that I want to tackle removing the manifold with the head in situ. I take it that the heater won't come off without removing either the intake or ex manifold though, not enough room?? my other plan was to run an earthing cable from the one bolt I can get at easily enough back to somewhere and see if that helped!! no much optimism given what you found tbh but worth a shot....
Correct; unfortunately, you can't even get the forward-most bolt right out, let alone the hog. Well worth a try, brand new bolt, clean up the cover in the area the underside of the bolt head/earthwire will go, and hope for the best .
well I had another go over my lunch break and cleaned up the contacts on the engine bay connector and now I have the on spec 0.5Ohm reistance from the relay pin 87. So it would appear that the connector was guilty of unduly high resistance. that said, I tried turning the ignition on and there's still no heat to be felt... that was after a minute or so
Sorry, I've confused matters (as usual). The 0.5 ohms is from the wire coming out of the hedgehog, to the ground of the hog, i.e. the resistance of the heater element itself (if 'sound'). Should be virtually no resistance from the relay socket pin to the loom end of the feed, but most multimeters are pretty inaccurate below 1 ohm so I'd say your supply side is fine, as mine is.
that's right Pete, I should have been more specific... I measured 0.5 Ohms between pin 87 of the relay terminal to the -ve of the battery, ie measuring the resistance of the hog all the way from the relay back to the battery (so the heater resistance plus a bit of wiring) 3 days later this is now at 20 Ohms so I guess the corossion in the connector is causing problems again :-( cheers
I have new information on this issue, and hopefully some pictures once I've mastered the whole hosting/posting thing. I got the intake manifold off again yesterday, and managed to fully dismantle the old dodgy hog. It's very similar to the throttle body heater, except there's 4 of the little PTC elements all connected in parallel. There's a sprung set of 4 contacts that takes the 12V feed wire onto the high side of each of these, and a conductive paste under each element that grounds the low side to the casting. The contacts between the 4 sprung bits and the high side of the heaters looks like where the corrosion has been feckin things up. More to follow...
